Hydronic Radiant and/(vs?) Ventilation
I’ve noticed a huge disconnect between information supporting the installation of Hydronic Radiant systems and the need to Ventilate. It seems that all of the ‘pros’ of hydronic systems do not support the need for whole house ventilation systems (ie. hydronic systems do not stir up dust/allergens, have heat loss through ducts, require filter/duct cleaning ….).
Even emerging ‘green’ standards speak of the energy efficiency of hydronic systems, and then speak of the need to address ‘indoor air quality’ yet never seem to speak of how these two specific concepts work together.
If a homeowner was installing in-slab hydronic throughout a house of 3500 sq.ft., I would still think that a whole house ventilation system would be prudent with an HRV/ERV, and air handler with filtration to ensure indoor air quality and buffer incoming air.
Can anyone provide any input, suggesstions, or ideas on this issue?
Replies
Yes you do still need ventilation (7.5 cfm per occupant) but you don't need an ERV or an HVAC system to do it. There are many free standing ventilation systems that will meet this need. And the only way radiant floor is more efficient is if you compare it to a badly sealed forced air system. It gains it's efficiency by not sucking bad air from the crawlspace and pushing it through the house and out the master bedroom windows and ceiling. Otherwise a BTU is a BTU and the only way to make free BTUs is to get themfrom the sun with a solar water heater linked to the radiant floor and / or a passive solar design dumping solar BTU's into a thermal mass.
Flame me for saying this asyou wish.
------------------
"You cannot work hard enough to make up for a sloppy estimate."
Shelternerd,
There's more to it than that.
One big item is that a low temperature heating system only needs a low temperature heat source to drive it. For instance, if you have an oil or gas burner as your heat source, the exhaust temperature needs to be only a few degrees above the output temp of the heated circulating fluid.
If you're running baseboard convectors or a hot air system, that might be 150 F or higher. If you have a heated floor, that might be 90F. Less energy thrown away out the chimney with a hydronic floor system.
There. Is that flame hot enough for you?
Ron
Low temp flue gas means high efficiency furnace designed for low temp flue gas ... You cannot run a standard boiler/water heater at low flue temp regardless of the temp of water you supply your baseboards or whatever ... else you will condense moisture which has LOTS of acid in it and destroy your flue. Standard water heaters need a minimum flue gas temp to operate properly.
clewless,
Of course!
My point is that you can only run a boiler with a low exhaust temp in a system with a low temp output demand like a hydronic floor. I was trying to tell shelternerd that there are possible efficiency gains with hydronic that he was not aware of.
ron
I think you can have e.g. a higher temp system (e.g. baseboards/radiators) w/ a low temp flue ... that is a high efficiency boiler providing 150 degF water and a 120 degF flue gas temp (not sure of that flue gas temp, but you get the idea). Low temp flue is not characteristic of only low temp heating systems ... or did I miss another point you were trying to make?
The flue temp is limited by the return temp of the circulating fluid.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
Hey y'all I'm not trying to knock radiant. I'm a plumber as well as a builder and I install radiant in most of the houses I work on. I just wanted to say that I think it's efficiency is somewhat over hyped in the market unless it's tied in to solar thermal and that expectations need to be brought into line with reality somewhat. I have it in my own house and love it and would have it in the next house I build if I ever move again. m------------------
"You cannot work hard enough to make up for a sloppy estimate."
I agree. Radiant is often oversold as some magical energy saving thing ... like I will save energy just because I have radiant heat. While we may argue all day long about the nuances of forced air vs. radiant ... generally if I set my space temp at 70 degF for heating ... forced air or radiant ... I will use/consume X Btus to heat that space.
It's like people saying the shear existence of thermal mass some how magically 'saves me energy' ... when in fact, there are many complex issues that have to occur for that to be the case.
Again ... there are some nuances we could discuss at great length and maybe agree or disagree on ... the basics aren't any different.
Personally, I think that I can turn down my thermostat w/ radiant heat and therefore save some energy ... for that is the basic premise of radiant heating.
Clewless 1
You are forgetting basic human anatomy. The feet tend to tell you you are cold and they do so because they are the farthest from your heart and thus the blood is coldest by the time it reaches them.. Forced air blows that warm air up and it quickly arrives at the ceiling.. cold air sinks, warm air rises..
The floor is coldest in a forced air home and warmest in a radiant floor home..
In fact temp graduations tend to be fairly even in a radiant floor home and highly striated in a forced air home.. get a accurate thermometer and check yourself this winter if youi doubt high school physics.
Thus you feel warmer with less BTU's used because you are able to set the thermostat lower while you remain comfortable..
You are preaching to the choire, dude. That is exactly what I said. Many people set their thermostats the same even though they have radiant heating. Thus, no savings. The fundamental problem w/ radiant heat is that we don't have radiant thermostats ... just air temp sensors (although I've heard of special stats) ... so people don't think of setting their stat lower.
Radiant heat ... heats the only part of a building that people actually have regular/continuous contact with ... I've taken baths in hot springs at 20 below F and been comfortable standing up ... as long as my feet are in the pool, I'm good to go.
Clewless 1
That's the problem with the internet. Two people can be in agreement and still develope an arguement over individual words. If we were sitting here discussing things I could tell instantly through your body language and facial expressions we were in agreement.
You mean it actually takes skill to create text that is clear? :) It ain't easy. Like writing a 'how to' book ... and I suspect most of us couldn't do that even though we have the knowledge and skills in the topic. Knowing it and 'splaining it to someone is two different animals.
Totally agree ... sometimes I've seen guys disagree w/ me, yet I READ that they are saying the exact same thing as me.
Edited 8/9/2008 9:40 am ET by Clewless1
you're oversimplifying heavily. and most people who have radiant don't, in reality, turn down their thermostats; they just enjoy the extra comfort they didn't know they could have. that arguement for radiant is a nice one, but it just doesn't happen.-------------------------------------
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NRTRob,
I can't answer for most. I can only speak for myself and I wound up turning down the temps by 6 degrees. I still walk barefoot most of the winter.. Love those floors!
this was actually studied, by some outfit in canada ( I forget who now), and generally it doesn't happen.not that it couldn't, and with fuel prices being what they have been, maybe that will change, but ten years ago most people who got radiant chose to soak up the extra comfort than to turn down their thermostats.I'm interested that you still have warm floors if you turn the stat down to 62 to 64. do you get "real warm" floors, or just "comfortable"?-------------------------------------
-=Northeast Radiant Technology=-
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NRTRob
The old furnace I used to keep up to 72 degrees because I felt so cold all the time.. (cold floors syndrome) now it's at 66 and I'd be willing to drop it another degree or two but she who must be obeyed wouldn't like that. <grin>
I really do walk around bare foot all winter but to be fair some of the house doesn't have finished floors yet and the heat comes up nicely between the boards.
Also to be fair I used electric water heater/s to heat this place and when the temp gets around 20 below or so I kick on my forced air furnace with duct work under the big picture windows to keep from freezing as I walk by those.. They are Andersen Low E with argon but no way will they insulate as well as my SIP's will.
The whole trick to lowering energy costs is build a well insulated house.. Stick building won't cut it anymore as far as I'm concerned.. SIP's or ICF's
I used my old furnace untill I got this sytem fired up and that furnace in the old house cost me $500 a month during Jan & Dec.
Once this house is 2 times as large as that one was and has 3 times as many windows yet costs me about $220 in Dec.& Jan to heat! I don't have a good comparison yet on the electric bill to see if I've saved much more with this system. I expect so but don't have enough back to back comparisons..
One more point.. my ceiling is 28 feet above my feet in the great room.. temp variation between the floor and ceiling is 1/2 degree measured on a digital infrared guage..
If you see value in SIPs, maybe you see value in better windows. Anderson and 'plain' low-E glass is 'standard fair' in my book ... there are better glazings out there with much better performance. Also consider redesigning your windows ... 'big' picture windows may or may not be good, necessary, or desireable (in the big picture). I advocate CAREFUL consideration for each sqft of window during design to maximize the benefits (ALL of the benefits). I designed a sunspace using Heat Mirror glass ... in the cold of winter, I never felt the 'cold draft' coming off my 10 ft high glass wall.
A 28 ft ceiling ... wow ... that seems like a LOT ... knowing not your specific situation. Comment on your temp measurement ... for reasonable accuracy, you can't do what you said ... your device measures the inside surface temp of your ceiling, not the temp of the air up there ... admitedly maybe close ... but to get the air temp, you have to get a ladder out or air probe on a stick. I'm assuming you are saying air temp NEAR the floor (and protected from the direct radiant heat of the floor). You did this on a cold day?
I'm guessing that radiant floors should normally end up with reverse temperature stratification ... lower at the ceiling and higher near the floor ... or like what you are saying ... no variation. Most residences won't see much variation if the supply air is at the floor (and I've measure this) ... as the warm air rises, it loses its heat ... resulting in little temp variation floor to ceiling. You get variation/stratification when heat is supplied up higher (or from the ceiling).
Clewless1
Thanks I'm enjoying this discussion.
Glass's R value simply cannot be as great as the 6 inches of foam, 10 inches of wood and 4 to 6 inches of stone in my walls! I don't care how efficent it is!
With forced air furnaces aimed at windows you should never have cold coming off windows. With infloor radiant under cold enough (minus 20 and below) you will feel cold unless massive amounts of tubing are installed to offset the heat loss.
Please remember I thought long and hard about both windows and heating and carefully considered all my options..
No windows meant the house would lose the absolute minimum of heat but be the least pleasant to live in. Due to the fantastic views all windows would be the most pleasant to live in but be the most expensive to heat.
I sized my windows carefulluy to maximise the view while minimizing the size.. that still meant realitively large windows in some locations..
If I attempted to offset the heat loss where those large windows were with in floor radiant I would need a much more expensive boiler and far more heat zones in order to keep run lengths to the proper size..
By using forced air furnace under certain limited conditions I didn't need to use a boiler at all (I could replace it with a much more efficent {and cheaper} water heater) and I could heat the house with a minimum of zones. The result means my home is comfortable at the minimum cost.. Since I had a near new high efficency forced air furnace and managed to save all the ductwork the cost of reinstalling a forced air furnace was absolutely nominal. I doubt I have $100.00 in the whole systems cost.
Why make a system work that much harder when it's only required for relatively short periods? Why not exploit all the best of any system?
Finally with forced air heat pumped out at even floor vents the floors will remain colder than in floor radiant and since your feet are your true temp guage a room will seem chillier and thus require higher heat levels with forced air than with in floor radiant. Cold air will always continue to settle.
I agree w/ what you are saying. Sounds like you gave a fair amount of thought to during the design process. You are generally right ... windows don't match the R-value of walls ... but that doesn't relegate you to the cold wall affect ... as evidenced by my own experience. If you want, you can get windows down to 0.10 U-value (R-10) ... and even lower ... if you want to spend the $.
The small forced air system is perfect for those areas requiring a bit more than the radiant floor is designed for (which is not the cold wall affect of large window areas).
It sounds like you have a well balanced system. Hats off to you!
See, there are those out there that will do it ... he turned his down AND he walks around barefoot ....
Yes, oversimplifing ... to make/illustrate the point. I don't disagree w/ you ... but there are plenty of instances where people will turn down the stat because they are more than comfortable ... and save a few bucks. It's difficult because there is no opportunity for 'side by side' comparison of otherwise identical situations.
There are people that understand what I said and do take action on it ... you are overgeneralizing the propensity of the population to NEVER turn down their stats ... and while I fully respect that point of view ... you make it sound so absolute ... which I'm sure you have to agree that that is not the case.
all I'm doing is relaying information an actual study on the matter found. while this claim is a big marketing tool for radiant, and I have every reason in the world to want to promote radiant, I do not believe the "turn down the stat and stay comfortable" arguement is one of them."more comfortable at any temp" is true. but most people who care enough about heat to put in radiant do not then keep their thermostats at 60. there are exceptions, but the exceptions are not the norm. I object to promoting what is, in effect, a false expectation in people that if they put in radiant they won't WANT to keep a 68 to 70 degree room. it's simply not true in most cases.-------------------------------------
-=Northeast Radiant Technology=-
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I'm not disagreeing with you ... but you found one person very readily that actually does do it ... without any searching ... although I admit ... that this forum MAY have tendency to find that person more readily than a cross section of the population as a whole. But I think there will be a natural tendency for people to adjust their stat to their comfort and as their comfort improves, they may have a tendency to ease the stat down rather than live at the higher temp [comfort temperature].
I totally agree, though, I'm also against promoting something without the caveat that to achieve savings will require the stat be set lower. Few radiant heat reps will state that as a condition of saving the 'big energy' they make claims to ... though most will readily admit that is what has to happen.
NRTRob
Sometimes we get so caught up in semantics and word play that we miss the forest for the trees..
More comfortable at any temp is a classic example.. Isn't another way of saying that statement, "you can be comfortable at a differant temp (lower) with radiant"?
Phsycology plays a role here too. When you first come home from being outside in the cold for an extended period of time the natural tendancy is to turn the heat up.. If you are inside and active the tendancy is to turn the heat down. However if you are sitting watching TV the same room that felt stuffy earlier might seem chilly now..
If you are starring out the window at a blizzard you'll want it warmer than if you are looking at a fireplace..
Since physcology plays such a critical part and most people "feel" their relative warmth thru the extremety furthist from the heart (the feet) radiant which does such a magnificent job of warming the feet is a way to have the thermostat at a lower temp than you would have with forced air.
Lower temp means less BTU's consumed and thus the potential for energy savings..
I guess if I were going to market a product fairly I would say it has the potential for lower energy bills. Then if you feel you absolutely must elaborate you can give the explaination.. Since marketing should never be about technical data. Since they serve a differant goal, you can sell without being deceptive.
the difference is in saying "if you kept two rooms at 60, one is radiant, the radiant room will be more comfortable" versus "if you have radiant, you suddenly won't want to keep your thermostat at 68".The first is true but doesn't mean someone will LIKE either room. the other carries with it a bunch of expectations of the person you are talking to that they have no way to disprove before they occupy the room you're talking about. Fact is, most people don't turn down their thermostats with radiant, so my acting like they will would be, at best, an optimistic statement and at worst a shady lie.I am not saying that radiant can't be more efficient than other forms of heat. I am simply saying most people don't hit a 68 degree radiant room, say "wow, this is more comfortable than I've ever been", and then turn it down so they are as UNcomfortable as they were before, but now at a lower temperature. You are a notable exception. That is why anecdotal evidence does not a study make.-------------------------------------
-=Northeast Radiant Technology=-
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NRTRob
I read what you said and realized you were talking rather large extremes.. I then placed it in more realistic context and looked at it.. in one room with forced air at 72 degrees and another room with in floor radiant heat at 68 degrees. ..............
I don't honestly believe that the occupants would be uncomfortable in either room. I think you can agree honestly with that statement.
That 4 degrees would make a differance in the cost of heating over a heating season. That is something that is fair and honest without being extreme..
If you expect heating systems to last for a year before replacing then that is the period of time that the savings potential should be discussed for.
If you believe the heating system will remain in that house as long as it's around then that period of time should be discussed..
The currant buyers may only occupy that home for less than a decade but if that house remains standing for 100 years shouldn't the potential savings over a 100 years be discussed? Aren't the currant owners in fact passing on that potential savings?
I'll use some numbers now to attempt to clarify my case.. I don't know the accuracy of these numbers but I'm not trying to be wild here so if I am off by a significant amount please correct me..
First let's deal with inflation.. It's going to happen..cost of things will go up.. however hopefully we'll get pay raises to deal with those increases and since that number really is unknown I'll use todays money so we won't get confused..
Let's assume a 2500 sq.ft. house in a northern state. the differance between 72 and 68 degrees for a full heating season could be say $400.00 a year? times the 100 year life time of this heating method means $40,000 over that 100 years.
Sure things will need to be serviced and replaced periodically but any system will have those needs so let's keep things simple..
While that large number may seem daunting remember no adjustment for inflation was used in calculating it so once you put even a moderate level of inflation into that number it gets really large..
If you are uncomfortable with $400 savings. Please realize that my savings were much, much, greater than that but admittedly I turned down my temps by a bigger number.. and more than double the size of house while trippling the numbers of windows..
frenchy, you are predicating everything on behaviour you still can't predict. I can make up all the numbers I like too, but at the end of the day, if the owner does NOT turn down the thermostat, this source of savings does not exist. and MOST people do not turn down the thermostat. that is all.and I know lots of people in uncomfortable FHA rooms even when it's 72 degrees ;)-------------------------------------
-=Northeast Radiant Technology=-
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NRTRob
I guess I can understand your point however I still don't think it's decepetive to state that the Potential savings could be ####
True enough they may not choose to use that but if the potential is there it's fair to discuss it.. You aren't being deceptive!
PS how far off do you think my numbers are assuming they would turn down the thermostat?
I have no idea what the numbers would be: depends on the system, the house, the climate, the cost of fuel, etc etc etc. as a percentage, and not a cost figure, it's still very variable based on the house and climate. and your own experience can be very wildly different compared to your old system, depending on the quality and type of construction on your old house, quality of the heating system, site conditions, etc. there is no doubt that turning down the thermostat saves money. saves money with FHA or radiant. But you weren't comfortable at 68 with FHA.. you just lived with it, and thought maybe it was ok. when you get 68 with radiant, you're not likely to want to go back to being chilly.Unless you're a wicked tightwad (whistles innocently) ;)-------------------------------------
-=Northeast Radiant Technology=-
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I'm not an expert, but it seems to me a radiant system would be generally more efficient than a forced air system b/c there is less heat loss from water circulating through PEX rather than hot air circulating through ducts which I suspect in many houses (including mine) is not completely sealed. With a radiant system, if you had a water leak, it would be pretty obvious and you could fix it, whereas with a forced air system you could have a leak and never know it.
In addition I am sure that outside (ie. cold) air is probably drawn into return ducts running through non-condition attics, crawl spaces, etc. It also seems that many forced air systems are not optimally designed and may create positive or negative pressures in rooms resulting in warm air escaping or cold air being drawn in. I would think that these factors would make radiant system at least slightly inherently more efficient than forced air even if the thermostat was set the same.
Depends. Around here heating ductwork is almost always inside the "envelope" so heat loss through leaky ductwork is no big deal.
Too much sanity may be madness. And maddest of all, to see life as it is and not as it should be! --Miguel de Cervantes
Other than the fact that leaking ductwork can lead to pressure imbalances in the house and actually foster unwanted air leakage. While benign in concept, the results can and have proven to be significant w/ blower and duct blast testing. It's actually fairly simple science, Dan.
as dan said, it depends.it is not fair to compare a badly installed FHA system to a well installed hydronic system. You can also run hot water pipes in stupid ways and lose lots of heat.-------------------------------------
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Lets say we compare properly installed versions of each type of system.
Heat loss through PEX, not in the floor, would be no less than the heat loss through a standard insulated galvanized duct. Not a significant difference.
If your house is leaky, it will leak air into and out of its exterior walls regardless of how you deliver heat to the spaces. This is a function of wind and construction details, nothing else. In other words no difference from system to system.
Residential forced air systems operate at very low pressures (most systems will not produce more that 1/2" of static across the fan in cooling/high speed ops, and heating is always at lower fan speeds), not a significant pressure inducer. I.E another insignificant, negligible difference.
The primary differences in system efficiency is due to fan power vs pump power required to deliver the heat to the space. While these costs are typically less than 10% of the systems' overall operating energy consuption, pumped water moves heat more economically than blown air, probably 20 to 25% more effectively. Another usual source of increased system energy efficiency is the fact that boilers utilized in modern residential hydronic in-floor heating systems are modulating/condensing boilers operating with return temperature below 120 degrees F (the temperature at which mod/con boilers jump from the 90% AFUE range to the 97-98% AFUE area) and the best condensing furnaces operate around 94% AFUE.
So barring any poor equipment/selection/hack installation, and all other things being the same, a hydronic system could respresent a 3 to 7% overall system energy consumption improvement.
Bottom line, the systems are not significantly different. Comparing well designed and installed version of both systems you will find very little difference in comfort either. I do believe that (what I would consider) good forced air systems in the present era of the "hack" i.e. low cost rules, is a rarity. Most any monkey can manage push hot water around semi-effectively (there are those that are highly skilled at hydronics and I do not mean to impune their excellent work).
Edited 8/14/2008 2:25 pm by Tim
Our GFA furnace claims 96.6% efficiency.Since air temps rarely exceed 80F, in theory it's possible to approach that as a flue temp in a forced-air system (with a "perfect" heat exchanger). Hydronic systems benefit from a lower real-world temperature gradient across the heat exchanger (for roughly equal construction "quality"), but are held back by the additional temperature gradient in the radiator structure. The total temperature gradient effectively limits efficiency.Bottom line, as you say, there's really very little difference in the potential (or realized) efficiencies of the two systems. I would, however, question whether a "monkey" can as effectively install a hydronic system as a FA system. And there's also the matter of installed cost.
Too much sanity may be madness. And maddest of all, to see life as it is and not as it should be! --Miguel de Cervantes
No matter what name you give them, unskilled and unexperienced installers, in my personal opinion, have a better chance of making a hydronic system work than a forced air system.
In other words, a hydronic system is more forgiving. Installed cost was a subject I intentionally left out of the equation, but forced air systems are by far a greater value at roughly half the installed cost. The value of hydronics has to be in the level of comfort and bragging rights. The dollars do not make sense.
I'm using rule of thumb/ square foot numbers in this comparison, but comparing apples to apples (i.e. high end, full features well desgign and properly installed with full complement of applicable controls and various bells and whistles) a full featured infloor hydronic system will run in the $8 to 10 /sf without cooling and a forced air sytsem, with air conditioning will run on the order of $5/sf.
You make a good point in that the mod/con boiler for a hydronic system will be more efficient than a furnace for a FHA system. And you also make a good point on how water is more efficient way to transport the heat around the house. I know that a well-designed FHA system would be comfortable, but I still think hydronic is a better, more elegant way to heat a house (though I am sure more expensive!).
I have limited in-floor heating in my house. Walking barefoot on a tile floor that is warm to the touch in the middle of a midwestern winter is a wonderful experience. The best forced air systems out there (and mine is very good) cannot provide that.
Almost all of the common drawback, complaints, so-called failings and short comings attributed to forced-air heating systems are either pure fiction or due to poor equipment selection and/or inadequate duct design/installation. My favorite is "all that dust blowing around". If your house is filthy enough to have "all that dust" around, whether it moves or just sits is an insignificant detail, in my opinion. Clean the house, don't blame the furnace! Furnaces do not create or destroy matter. That includes dust and water. They do not dry out the air. As many plumbers-turned-heating-experts will decry, they "scorch" the air. These are people that ought to sitck with DWV work as allowed by their local.
When it comes to the basic need of providing comfort heat at a reasonable cost, FA can't be beat. For a higher level of comfort, at a significantly greater cost, infloor hydronics cannot be beat. And then you have to address air treatment separately.
again, a poorly installed FHA system can have significant issues with leaky ducts and heat loss and such. absolutely.But a poorly installed hydronic system can have all those issues as well. The heat loss through pipes in cold space might be smaller than through ducts in cold space, ok, the FHA system can be botched WORSE, but it's not fair to compare bad FHA to good radiant. that's all I'm saying. I can point to good FHA systems that are likely more efficient and more comfortable than MANY radiant systems I've seen botched too.-------------------------------------
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>>I agree. Radiant is often oversold as some magical energy saving thing ... like I will save energy just because I have radiant heat. While we may argue all day long about the nuances of forced air vs. radiant ... generally if I set my space temp at 70 degF for heating ... forced air or radiant ... I will use/consume X Btus to heat that space.Two questions:My understanding is that temp of the surrounding surfaces will have a significant effect on the perception of warmth: for 2 rooms with the same air temp, if one has cool walls and the other has warm walls, the latter will feel warmer.Also, it is my understanding the the actual air temp gradient in a forced air system is significantly different that that of a radiant heat system.
In Xanadu did Kubla Khan
A stately pleasure-dome decree :
Where Alph, the sacred river, ran
Through caverns measureless to man
Down to a sunless sea.
Coleridge
1) Right, they definately can. Generally under any static outside air condition, the temperature of the interior surface of a wall (e.g. a very cold day) will be about the same temperature regardless of system type. While there WILL be SOME variation, my guess is it will only be a degree or two or even a few ... which in the radiant heat world (my body radiating heat to that surface) will mean little.
Radiant floor systems won't appreciably increase wall surface temps, I don't think. And two rooms at the same air temp with the same outside air temp will have identical wall surface temps. A thermal mass wall that has stored some Btus in it can certainly provide some 'perception' of warmth.
Also, FYI, it is not PERCEPTION ... it is real heat transfer. My body will lose heat to a cold window because of radiant heat transfer. Btus will transfer from one place to the other ... just like the sun heats the earth.
2) It depends on lots of things. A radiant floor vs. a forced air system supplying air at the floor ... you may see little thermal stratification from floor to ceiling (I'm assuming this is what your are referring to). IMO stratification can be a funny thing and is often misunderstood. It doesn't always or automatically occur (significantly anyway). If you supply heat at the floor and the heated warm air begins to rise ... through cooler air ... the warm air loses heat as it rises (to the surrounding air molecules) ... leaving you with very little stratification.
Since there is very little air flow in radiant floor systems, my guess is that reverse stratification is more likely (cooler at the ceiling). But I've also seen reverse stratification in forced air systems ... i.e. cooler upstairs even though there is an open stairway. A forced air system with high velocity may very well produce stratification by blowing warm air up to the ceiling level before it gets a chance to lose heat closer to the floor.
Radiant ceilings will almost always have BIG stratification ... I'm not much of a proponent of them even though the concept is theoretically good. Forced air systems supplying heat at the ceiling CAN have stratification if the air velocity and pattern are such that good mixing in the entire room does not occur (which is often the case).
But again ... stratification, like moisture can be a funny (i.e. mysterious) thing and can occur when you might not expect it (i.e. the cause isn't always easily identified).
you may have more stratification at the ceiling level itself using a radiant ceiling, but the temperature gradiant below that is very even, much like radiant floors are.there isn't much that can make a FHA temperature profile even along the height of a normal body. the room is cool to hot from floor to ceiling in most cases.both radiant floor and radiant ceiling have warmer floors and ceilings. Radiant ceilings have higher temps at the ceiling, even down, then warmer again at the floor, typically, due to direct radiation absorption. this is a blanket statement but still it is pretty typical.radiant floor is higher temps at the floor, even up, then warmer again at the ceiling due to stratification... less stratification than FHA, and not enough to throw off the temperature profile on the way up, but still some increased upward heat loss. just less than most other methods.Radiant ceiling may be less optimal for a cold ceiling in the same way that radiant floor is less optimal than a radiant floor over a cold space (from an energy usage standpoint ONLY, for comfort these are both good things), but the stratification effect you talk about is not the same for radiant ceiling as it is for a typical FHA setup, and does not have the same comfort implications.-------------------------------------
-=Northeast Radiant Technology=-
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I was thinking the same thing. Using thermal solar is what can really pay off. Where I live there is alot of sun just begging to be used.
clewless,
Like Dan said, I don't think it's possible to have the output temp higher than the exhaust temp. Or at least, it isn't done. I suppose it might be possible with an opposed-feed heat exchanger.
ron
I believe you are incorrect ... if I have a high efficiency boiler running say 120-150 degF flue gas ... I should be able to generate all the way up to 180 degF water temp (e.g. for that cold Frostbite Falls winter) for my baseboard heating. I've never heard of the two temps being particularly directly related or otherwise dependent on each other. The efficiency w/ which I combust fuel ... and thereby lower my flue gas temp has little to do with the temp that I heat my water to inside the boiler or ... the hot water supply temp.
Maybe I got something wrong, here ... maybe a boiler pro can jump in here and straighten us out or confirm one or the other. Maybe I'm missunderstanding what you are saying.
clewless,
You are right. We need a boiler pro.
Here's the limits of my knowledge: I had lengthy discussions about heating options three years ago when I was designing my own house. At that time, I was told clearly that, with the boilers I could select from, the temp of the flue gas would always be higher than the temp of the output to the circulator. This was simply the nature of the heat exchange in the boiler.
It follows that the right boiler supplying a low temp system and having a low temp exhaust will waste less fuel on exhaust than the same boiler supplying a high temp system.
The exhaust temp does not depend on the combustion efficiency as much as it does on the efficiency of the heat transfer to the circulating fluid. That heat transfer is more or less efficient according to the required temperature of the output to the circulator.
If your heat source has no exhaust, like an electric boiler, none of this applies.
The person who told me this might have been wrong, might have been misinformed, might not have been telling me the whole story. I am ready to be corrected. (I hope to see him today.)
In my case, it's not an efficiency I have realized. I designed a mixed system with two floors of in-floor and one with baseboard convectors, so my boiler output is consistently somewhere around 140 or 150 F and feeds the in-floor with a mixing valve to reduce the temperature. Next winter, I'll measure the exhaust temp.
And, Dan, I don't think I misunderstood you. I think we agree.
ron
Maybe I'll try to clarify .... rereading my words leaves ME confused.
Umm ... The exhaust (i.e. flue) temp depends on the combustion efficiency of the furnace/boiler. A high efficiency boiler will always have a low flue temp. Heat transfer of the water to the circulating side has little or nothing to do with the COMBUSTION efficiency of the unit. If you choose not to circulate any water at all, you still have a combustion efficiency based on the design of the burner and heat exchange between the combustion gases and the water in the boiler/water heater.
Since the circulating water heat exchanger is immersed in the boiler water, assuming a reasonably sized/designed unit, will have very good efficiency (which is normally not a point of discussion amongst us users).
Maybe I'm not understanding you right?
Edited 8/9/2008 9:38 am ET by Clewless1
The way you circulate the water can have an effect on efficiency. You get the highest efficiency, especially in a counterflow heat exchanger, when the incoming water is relatively cool. A circulating system that, eg, first sends the hot water to radiators and then sends the somewhat cooler water to underfloor heating will produce cooler water coming back to the boiler, especially if flow rates are kept relatively low.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
"Heat transfer of the water to the circulating side has little or nothing to do with the COMBUSTION efficiency of the unit. If you choose not to circulate any water at all, you still have a combustion efficiency based on the design of the burner and heat exchange between the combustion gases and the water in the boiler/water heater."Heat transfer to the water has EVERTYING to do with the efficiency of the unit.Basically efficiency is the ratio of USEFUL energy out of a system divided by the energy in.In this case the USEFUL energy is what is heating the water.And with a heat exhanger you can't transfer to a HOTTER SURFACE.So the cooler the water the cooler the exhaust end of the combustion process can be and still transfer heat to the water.If you have high mass boiler and a heating system that requires say 160 degree water then the exhaust temp will be higher than 160.If you have a low mass heater running a radiant system with say a 120 water temp on the discharge and 100 on the return the return water can be go through a heat exchanger that is on the exhaust end and get more heat out of the exhaust. You will end up with a much cooler exhaust and higher eff boiler..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Are you talking about a tankless water heater based system?? The relationship of a heat exchanger (used for circulating water to e.g. a radiant floor) inside a boiler/water heater has little relationship to the general combustion efficiency of the ability of e.g. gas to heat the hot water. This is different from the heat exchanger between combustion gases and the water in the storage tank. I think I'm just missing something from the conversation, here.
Define efficiency..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Define Efficiency:That's easy, Bill - it's the boiler's AFUE (Annual Fuel Utilization Efficiency) which in the case of most direct vent boilers is in the range of 89-98% - higher, generally-speaking, than other boiler types.
Jeff
That is good, but meaningless in this discussion.AFUE is defined under a specific set of operating conditions. So any 90% (for example) unit operating under that specific test conditions will have over the cycle 90%. IIRC it is test over a specific cycle of startup, run, shut down. I am not sure, but I don't think it is over a series of operating conditions that simulate real life. IE, having different loads that require different runtimes over the year.However, the discussion is on running under different operating conditions.Specifically different water tempatures..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Bill
What you are saying is exactly the way I understood things to be when I began reading in this discussion. DanH and NRT Rob have clarified things for me.
With a counterflow heat exchanger in place, the flue gas temp can be as low as the temp of the cooler returning fluid in a hydronic system. It is not limited to the temp of the outgoing fluid.
And, yes, the only efficiency rating that counts is the ratio of BTU's delivered to the house to the BTU's in the fuel consumed.
Ron
No, I said the input temp can't be higher than the exhaust temp.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
seems at this point, you need to be more specific if you want someone to understand what it is you are talking about. Input temp of what??? Exhaust temp ... you mean the flue gas temp, right?
The flue exhaust temp can be no lower than the temp of the returning circulating fluid. There is no practical limit on the temp of the outgoing circulating fluid.I don't know how I can make it any clearer.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
DanH
I do hope you can make it a bit clearer. I thought you were saying, as I have been, that the exhaust gas temp had to be higher than the temp of the output to the circulation loop.
Now, I see that you are saying the exhaust temp must be higher than the temp of the return side.
I must admit I don't see a connection. The temp difference between the output side and the return side depends on a lot of things that have no logical connection to the boiler: the length of the loop, the circulator speed, the heat demand of the rooms and the efficiency of heat transfer to the rooms.
Maybe I'm dim, but I don't get it.
ron
The exhaust can be no cooler than the hottest fluid it exchanges heat with. That is the incoming circulating fluid (water or air). High efficiency heat exchangers use a "counterflow" design where the flue gasses and circulating fluid move in opposite directions relative too each other so that the hottest flue gasses are in contact with the hottest fluid and vice-versa.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
Dan
OK thanks. Makes perfect sense.
Much of what I understood still stands. In floor is still more efficient because the temp of the flue gas is going to be lower than the flue gas temp with a baseboard or radiator system or a hot air system. Therefore, less fuel out the chimney.
Ron
Actually, hot air has the lowest temp for the incoming fluid, but gas-to-gas heat exchange is less efficient (in terms of heat transfer per volume of heat exchanger) than gas-to-liquid, so the theoretical efficiency of hot air isn't obtained.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
DanH
No, you can't put your hand on the exhaust stack of a running hot air furnace, can you? I can put my hand on the exhaust stack of my boiler when it is running. I'm not comfortable, but I'm not suffering either. It isn't throwing away a lot of oil.
Ron
I can put my hand on the exhaust stack of our forced air furnace. It's barely even warm.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
DanH
What about when it's running?
modern condensing furnances, much like condensing boilers, can have very low exhaust temperatures. as in, vented with plastic low.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
NRTRob
I have seen that. Do you kow what sort of temp the flue gas might be? One machine I worked around last year had a PVC vent. PVC ought to be able to take a temperature up to 200 F or so, shouldn't it?
Ron
up to, yes. which is a very low exhaust temperature. however, not nearly as low as it can be, and with condensing appliances, flue gas can be as low as just a few degrees higher than return water temps (or something like that in the furnace world).so imagine you have 90 degree return water. That's pretty low exhaust temperature potential with a condensing boiler.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
NRT Rob
Yes I got that now. DanH was perfectly clear.
I was thinking that the exhaust temp of the hot air furnace was still above the exhaust temp of a boiler supplying in-floor heat circuits directly, instead of through a mixing valve. What do youo think?
Ron
depends on the type of furnace, type of boiler, and temperature of the heating circuits.Presence or lack of a mixing valve wouldn't change exhaust temp though. If a BTU generated is not emitted, it goes back to the boiler. can't fake your way around that.I don't know what determines furnace exhaust temps. I imagine it's related to return air temperature, but I don't know what the relationship is.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
NRTRob
I was thinking that if a boiler supplied an in-floor system directly then there would be less temp change in the circulating fluid and that that would probably mean greater efficiency.
There are so many variables that I'm sure exhaust temps are unique to the installation and conditions at the time of measurement, but every pro will have some general idea of what's going on, no?
Ron
less temp change in the circulating fluid? I don't know what you mean. I spit out a half formed thought in that last post though that isn't accurate the way I wrote it, sorry.-if you are using a mod/con boiler and can turn it down to the radiant temperature, and the radiant temperature is low, you're doing great. that's the way to go.If you use that same boiler and turn it up and use a mixing valve, that's a shame. Sometimes it's necessary, but whatever you can do to avoid it is worth it, in all likelihood.if you have a conventional boiler you have to run at a minimum temperature that is likely hotter than the radiant can take, and mixing is required. But this doesn't lower the return water temperature, because you have to maintain a minimum return temp with a conventional boiler anyway.I have no idea how much a given pro will understand or not understand. Some pros have never installed a mod/con boiler. some do nothing but.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
NRTRob,
I may be mistaken but isn't the upper practical limit of water temp in pex for prolonged period of time 130 degrees? Doesn't water boil at 212? So a boiler would need to operate at 212 degrees to be a boiler (up to that point it's simply a water heater)..
If the returning water plus the boiler water exceed 130 degrees aren't you required to cool it to 130 degrees? Cooling heated water seems inefficent to me..
Ah, but they pull a vacuum. ;)"Boiler" is a term of jargon that lost it's original connotations when folks switched from steam to hot water.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
yup, you are mistaken. you can run 180 through pex.
And for practical purposes, boilers are built to one standard, water heaters to a different standard. Which is why in many jurisdictions, water heaters can't be used as boilers. Besides which the effecincy of a standard water heater is ####.
And in a closed system, I can have the water at 220 or hotter without it boiling.
In another thread, someone stated that they keep their water heater at 160. When it cools down to about 140 it will heat back up to 160. Definitely a hot water heater.
Water heater relief valves can be installed with the spindle horizontal. Boiler relief valves are installed with the spindle vertical.
2 weeks of holidays start tommorow. I need to stop thinking about this stuff.
rich1
"Boiler relief valves are installed with the spindle vertical."
Can you explain the reason for that? I'd like to know because mine is installed horizontally.
Ron
ASME standard? Gotta do some research on that.
rich 1
I know 180 can be run thru pex however, as I stated for prolonged periods of time.. I can rev my engine to 6000 RPM but if I want it to last I don't. I minimise the revs I use.
As for efficency,
Again we need to set up some standard.
I use simple standards like money.. how much money does it cost me to heat my house with a boiler and with a water heater..
Combine the purchase price plus the cost of using and as we all know highly efficent units cost more intitally but less over a long term.. the point where those two curves meet are the optimum replacement point..
It's wonderful if that point is reached as a new breaktroough in technology becomes commonly available. It's also wonderful if that point coinsides with a personas ability to replace the units in a timely manner..
I use electric water heaters which as we all know are 99.95% efficent at transfering the cost of electricity into BTU's True Gas water heaters appraoch that if the most efficent ones are selected but the boilers I looked at didn't meet those same leels of efficency..
Now here's where it get's interesting.. the practical limit of life of a great boiler approaches 30 + years.. while the practical limit of a good water heater is only 10 years.
In a decade will I purchase a boiler or another water heater? If I choose a boiler I couldn't justify that purchase for another 2 decades beyond that point..
What are the chances of significant efficency gains in three decades?
Since all that we have is at best a guess I'm in favor of keeping my options open..
I know you like water heaters, and I won't change your mind. A lot depends on the system and your location. If I need 200f water, a water heater won't cut it. My electricity costs 3x my gas. A modulating condensing boiler with outdoor reset can give you 95%. For me that beats electricity.
Pex can take 180 for a prolonged time. Wirsbo has been tested at 203f/175psi with a life expectancy of a 100 years.
rich 1
I guess with costs like those I'd make a differant choice as well. I don't say that my selection is the only correct one.. it was correct for me but might not be for others..
What does a great boiler cost now days? $3500? $5000? My water heater system (and it's a system) cost me a shade over a grand. But admittedly few homes up here in the arctic tundra of Minnesota require as little heat as mine does.
That means I can replace the system with a more efficent system every decade compared to that of a boiler..
I'm sure that wirsbo's testing is good solid testing but I'm absolutely sure they didn't actaully test it for 100 years.. Nor did they test it in real life with maybe less than perfect workmanship or flaws..
When I put my wirsbo up I was carefull to ensure that it wouldn't catch on the egdge of the plates used to transfer heat. I carefully radiused them. A lot of homes the installers don't take that level of care and as the pex moves abraiding will happen.. extreme temp variations will cause more movement than modest variations.. and thus greater abrasion potential..
yeah, I'll concur with the other posters and wonder where you got this info from.you can't safely run higher than 140 in concrete, but that's not a limitation of PEX.There are/have been some really substandard pipes out there in the past with lower ratings than the typical 180 for PEX, but the vast majority can handle any normal temperature a closed hydronic system would throw at it, including high temp baseboard (but not steam).-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Actually, we do some ice melt in car wash slabs where the water is 180. 5 years and no problem. The trick must be once the system starts, it doesn't shut off until it gets warm out side, so a run time of 5 or 6 months.
-30 and people still feel compelled to wash their cars.
Not positive, but I think that most radiant floor slabs operate well below 140 degF. No need for higher temp, I don't think.
Boiler is the common industry term used whether you are boiling water (creating steam) or simply heating it up (to 180 degF). ... even though generally a domestic water heater is still often called a water heater.
A high efficiency furnace uses a PVC flue ... and flue gas in the 120+ range ... you should be able to easily put your hand on it and keep it on it.
"A high efficiency furnace uses a PVC flue ... and flue gas in the 120+ range ... you should be able to easily put your hand on it and keep it on it."That does not guarantee high eff unit. I am not sure of all of the different types of boilers and furnaces.But forced vent water heaters are about the same eff as standard WH's. They just use a blower to mix in more air to dilute the exhaust to lower it discharge temp. .
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Forced or induced draft directly affects the combustion and standby efficiency of the water heater. Usually it guarantees a higher efficiency unit than e.g. standard atmospheric and/or standing pilot systems ... also assuming forced draft also means electronic ignition (which isn't req'd, but usually part of a forced draft unit). A better heat exchanger bumps it up into a high efficiency (90+%) unit w/ low flue gas temps using a PVC flue.
Not disagreeing w/ you I don't think ... just not sure we understand each other.
What I am saying is that low flue temps, by themself, don't indicate a more efficient systems.IE the more efficient system the lower the flue temp.But a lower flue temp does not guarantee a higher efficient system..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
I think a more efficient system will 'automatically' mean a lower flue gas temp. To take more heat out of the combustion gas, you will have to reduce your flue temp. That is simple thermo. I combust 100,000 btu of gas and I either have a high flue gas temp and low efficiency or a low flue gas temp and a high efficiency (assuming the same flue/boiler). This is somewhat simplified, admitedly.
Efficency - instantaneous combustion efficiency. Not necessarily referring to AFUE ... which was created to mimic the efficiency of equipment over a theoretical range of operating conditions (i.e. actual weather imparted affect on efficiency ... a given or specified range of weather conditions set by ASHRAE or whoever sets that standard).
Not referring at all to system efficiency overall as most people never discuss this nebulous topic. They often assign some degree of inefficiency for e.g. heat exchange, pipe loss, boiler loss, etc.
"I think a more efficient system will 'automatically' mean a lower flue gas temp. To take more heat out of the combustion gas, you will have to reduce your flue temp."But taking heat out of the conbustion gas is not the only way to reduce flue temps. Mixing it within ambient air will also reduce the exhaustion exhaust tempature.For a given unit lower exhaust temp does indicate that that unit is operating more efficient then under some other conditions.But to compare different brands and styles of boiler then one would need to measure both temp and FLOW RATES to compute the energy that is wasted in the exhaust.But even that is not complete. It does not account for that portion of the fuel that is not combusted or only partially burned. "Efficency - instantaneous combustion efficiency.Efficency - instantaneous combustion efficiency.Efficency - instantaneous combustion efficiency."That is meaningless for several reason. Combustion efficiency on tells how well the fuel is burned. It does not tell what happens to the heat energy.And the unit will go through way range of "instantaneous conditions when run a cycle.To use an extreme example start up a high mass boiler without any water in it. For several minutes the CI will be cold and lot of energy is transfer to it and the exhaust temp is low. Eff will be high.But after s short period of time the CI will become very hot and much less energy is transfer to it and the eff will down way down.The only meaningfull meausrement is the steady state energy that the system puts out. That is either hot water or hot air.The water or air is what the heat exchanger transfers heat from the combustion to the media. And the cooler that media is the more heat that can be transfered to it and less to the exhaust..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Generally, I've really never heard of mixing ambient/mech room temp w/ flue gas for the sole purpose of lowering the temp ... I suppose that has an advantage if you want to use PVC w/out providing a high efficiency boiler ... maybe that is exactly what I have ... it is a 85% efficient system, but not condensing style and it does use PVC for venting.
"But to compare different brands and styles of boiler then one would need to measure both temp and FLOW RATES to compute the energy that is wasted in the exhaust."
Not sure what you are saying. What flow rates? In the flue gas?
Instantaneous combustion efficiency is not really meaningless. It allows a basis for comparisons of equipment ... like comparing the gas mileage rates for cars. The combustion efficiency does give you how well the fuel is burned and transferred to the water (or air). What you do with it after that is another matter of efficiency, but I can't market boilers based on what they may or may not be used for. And while you are right about starting up a cold boiler, that is not what the combustion efficiency is about ... it's about the efficiency measured under what might be considered 'normal' conditions ... hot stored water ... or normal mixed air temps, not starting up under unusual circumstances. The AFUE rating method attempts to iron out combustion efficiency across a theoretical range of operating conditions (the climate). While not perfect, it is better than simple instantaneous efficiency at very specific steady state conditions.
Under operating conditions, there is no such thing as steady state ... as the load is constantly changing. And trying to determine efficiency of your overall system is VERY difficult to do, so we usually don't attempt to compare my overall system efficiency with yours ... it's too difficult to determine. Ideally and theoretically, yes, that would be the way to compare things, but it just isn't done.
Actually, flue gas temp is very closely related to efficiency. High efficiency means that you remove more of the heat from the flue gas, and that obviously means lower flue temps.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
As I said you can reduce flue temp by mixing it with air.That is what is done with forced air WH so that they can use PVC for discharge.And I am not sure, but I think that some oil burners might do the same..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Yeah, you can reduce flue temp by mixing with air or reducing the efficiency of the combustion somehow. But except in special cases (such as the forced air WH)) this is a disadvantage as is causes condensation, creating corrosion problems that otherwise wouldn't exist. For this reason you don't see FA furnaces with efficiencies in the roughly 85-90 range, as this is the cutover range for condensation, and designs will either go one way or the other and not dangle on the edge.As you suggested (but never specifically spelled out), heating system efficiency is the ratio of energy out (in the form of useful heat) to energy in (in the form of gas, oil, etc). Assuming a heating system that is completely enclosed in the building envelope (no external ductwork, etc), the only energy loss in a combustion heating system is the heat that escapes through the flue. That is, you can accurately calculate the system efficiency by simply measuring the fuel input rate and the temperature and volume/second of the flue gas (along with a slight adjustment for combustion air temperature/volume).And, since a well-designed, properly tuned combustion process (good burner configuration, just enough air for complete combustion, etc) will produce virtually the same combustion temp and flow rate per BTU, regardless of the specific design, you can come pretty close calculating efficiency based on flue temp alone.And even when the system is not optimized (as in the water heater example), flue temp establishes an upper bound on efficiency -- you can't have better than about 86% efficiency unless the flue temp is below 212F.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
>>No, you can't put your hand on the exhaust stack of a running hot air furnace, can you? Depends on the type. a "90+" will run about 110 +/- Fahrenheit
In Xanadu did Kubla Khan
A stately pleasure-dome decree
Where Alph, the sacred river, ran
Through caverns measureless to man
Down to a sunless sea.
Coleridge
rjw,
I had no idea there were such things.
Ron
Maybe it would help by instead of simply stating your 'facts' you explain WHY. If I have a high efficiency water heater (storage tank) exhausting say 130 degF flue gas ... I can still turn my water heater up to 150 degF if I choose too ... as you said ... the temp of the supply water (to the heating system) has no limits. The supply of the return water is dependent on the supply water temp and the load on the system (i.e. how much heat that loop loses before it comes back). If I supply 150 degF water during cold weather, my return loop will be lower than if I supply the same temp during mild weather. The supply and return water temp therefore little or nothing to do with the temp of the flue gas and the efficiency of the heat exchange between combustion gases and the water heater.
Are you talking about an instantaneous water heater?
Efficiency is fairly tightly linked to flue gas temperature. The temperature of the flue gas can be no lower than the incoming water. Thermodynamics 101.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
Are you speaking of a tankless or instantaneous water heater applied to a radiant floor system?? Thermodynamics 101 will also require you to be more specific about what you are talking about ... Am I the only one confused about what you are trying to explain ... even if I am ... I am still confused and I hope you can respect that. Maybe I got caught into the middle of conversations or forgot where they started.
I'm talking about the limits imposed by thermodynamics -- doesn't matter what style boiler you use. The flue gas can be no cooler than the coolest water in the boiler.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
Because you simply cannot exchange heat FROM a 140 degF flue gas and 160 degF boiler water ... as heat flows from hot to cold. Is that what you have been trying to say?
So how does a condensing boiler with low flue gas temps provide higher temperature water (e.g. 150-170 degF)?
1) Yep, that's what I've been saying.2) Counterflow heat exchanger, with relatively low returning water temp.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
No, that is NOT what you've been saying. I said it. You may have been thinking it or may have MEANT to say it, or maybe you wanted me to read your mind and think it, but you did not say it. You simply kept repeating yourself; I had to figure it out for myself (finally); I was just making sure it was what you were TRYING to say.
I don't see how your inability to understand plain English is my fault.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
Didn't think I was placing or implying fault at all. We are simply having a discussion. Just because you are unable to explain what it is you are trying to say to someone other than yourself, doesn't make your point clear ... nor does simply repeating yourself. It takes two to communicate and in this media, it's not always as easy as we might think (or like it to be). If I am having difficulty understanding what someone is saying, I hope others have the respect to simply try a different approach.
"Just because you are unable to explain..." isn't implying fault?
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
I don't fault you for your inabilities.
Nor I for your denseness.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
Them's fightin' words. I'm not dense or afraid enough to ask dumb questions if it will foster less denseness. Only dense people repeat themselves thinking that makes them somehow more clearly understood and thinking that only their way of explaining things is always 'crystal clear'.
clewless, you need to drop this. DanH was clear as clear can be, repeatedly. You should thank him for attempting to take the time to educate you in spite of your own confusion, instead of lambasting him for it. I don't see any way he could have been any clearer from the get go.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
NRTRob,
With all due respect. DanH doesn't seem to know much about hot water heaters, it seems. I've been doing some research. I'm an idiot thinking he actually had something relevant to say. He clearly doesn't know how to say it/explain it. Knowing something and knowing how to explain it to someone else are two different things. I'm far from being convinced he knows anything and I know he doesn't know how to explain what he thinks he knows. Repeating yourself doesn't make anything more clear.
A hot water heater or boiler can be set e.g. at a setpoint of 160 degF while having a flue temp that is below that (and I'm not referring to the typical draft hood that allows ambient room temp to mix with the flue gas). While the temp of the water in the tank may raise the temp of the flue gas somewhat as the flue gas leaves the combustion chamber/tank, the flue gas can be clearly lower than the temperature in the tank.
And supply and return water temperature to a coil, baseboard, or radiant floor really have little to do with the flue temperature (at least within the simple context of this discussion). They are dependent on the load and the system design. A typical delta T of roughly 20 Fdeg is about normal for many systems (at or near design loads). A very light load and a high supply temp will give you a very low delta T.
For instantaneous water heaters this discussion probably doesn't apply.
Unless you're mixing ambient air, the flue gas temp must be higher than the coolest water in the heat exchanger. Like I said, simple thermodynamics -- the flue gas can only get cooler by giving up its heat to some other medium (the water), and that can only occur if the water is cooler than the flue gas.The coolest water in the system, in steady state, will be the water returning from the circulating loop. That temp therefore establishes the lowest temperature of the flue gas.
Too much sanity may be madness. And maddest of all, to see life as it is and not as it should be! --Miguel de Cervantes
Think what you want. Just because you say 'it is simple thermodynamics doesn't meen squat. I'm not saying thermo has nothing to do w/ it ... it has everything to do with it. It's just that you can heat and store hot water in a storage tank at a higher temperature than the leaving flue gas temperature.
At the bottom of the combustion chamber, you are heating water with say 800 degF combustion gases ... potentially heating the water to very high temps. As the flue gas loses its heat and leaves the water heater ... it is coolest and can then absorb some heat from the water heater. That doesn't mean it will suck all the stored heat out of the water tank. If you are referring to the very small amount of water directly adjacent the exit point of the flue, you may be right, the flue temp will always be higher than that miniscule volume of water. But that isn't what this discussion is about. Also regardless of what heat exchanger design you have, the bulk of the stored water in a water heater tank can be much higher temp than the combustion flue gases leaving the equipment. And yes ... it is simple thermodynamics ... I think you might be simply thinking of one thing and I another.
Yes, you can get the water hotter than the exiting flue temp. This is easily accomplished with a counterflow heat exchanger. But you can't get the flue temp any cooler than the coolest water. Heat doesn't flow "uphill".
Too much sanity may be madness. And maddest of all, to see life as it is and not as it should be! --Miguel de Cervantes
You must be a salesman ... can't talk straight talk. You think whatever you want, dude. Go to a rep and ask him if you can buy e.g. a condensing water heater or boiler and set it at say 180 degF and have the flue gas discharge way below that temp. Like I say ... you seem to be talking about one thing and I another ... so enough of the twisting of my words to make your point ... which I'm not even sure what your point is (I just know you are going to say something like 'its simple thermo' or 'your flue can't be any cooler than something'). I think you've removed the original discussion completely out of context.
I don't think you necessarily need a counterflow heat exchanger. The coolest water comprises a miniscule volume of water in the water tank, so your statement doesn't mean much. I can still set the setpoint of my water heater significantly higher than the combustion flue gas discharge temp. While heat flow cannot go uphill, you also don't instantaneously balance the stored water temp at with the flue temp at the fraction of an inch that the flue gas discharges from the tank (or even the last couple of inches). So while the hot stored water (e.g. 160 degF) will flow to the flue gas as it exits the water heater, not ALL the energy stored will do this. The combustion gas starts out at a VERY high temp (as does the water directly adjacent the combustion chamber ... but the AVERAGE STORED water temp can be significantly above the exiting combustion flue gas temp. Its simple thermo, too.
Yes, the AVERAGE stored water temp can be above the flue temp, if the tank is sufficiently stratified. But there needs to be a counterflow arrangement so that the outgoing flue gas is last in contact with the coolest water (which will normally be at the bottom of the tank).
Too much sanity may be madness. And maddest of all, to see life as it is and not as it should be! --Miguel de Cervantes
Except that most conventional water heaters don't have this counterflow capability and they can have their setpoint above the flue gas temp.
Doubt it. Not without mixing air. And if by "conventional" you mean non-condensing, then they all have flue temps above 212F.
Too much sanity may be madness. And maddest of all, to see life as it is and not as it should be! --Miguel de Cervantes
water heaters don't measure their water temperature at the bottom of the tank where the burner is either, and when you set a boiler temp you are setting exiting temp, not return temp to the boiler.bottom water and/or return water to the boiler are the coldest temps dan is talking about. NOT setpoint in either case. You are confusing yourself.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Finally some logic here. Boiler temp ... is storage temp. Then there is the supply temp that may be set independently of the boiler temp as with the one I just finished. I need a certain boiler temp for CAPACITY (energy) and then a supply temp to accomodate my specific load at any point in time (using a 3-way valve).
I thought mr H was referring to the setpoint of the water heater/boiler ... in the context of our discussion, I was not really concerned about the details of water in different parts of the boiler ... just the basic setpoint ... temperature of the water available for our use. I didn't think we were having a detailed discussion of the internal thermodynamics of the equipment ... not even sure why that has any meaning to our discussion.
The design of the equipment will dictate flue temperature and that temp has nothing to do with the return temp of a circulating system (e.g. radiant floor) ... they essentially function independently of each other. I'm not saying they don't influence each other, just that my choice of setpoint for the storage temp is largely independent of the return temp ... which is more dependent on load and supply water temp. A low return temp will cause the boiler to fire more often, but if you set it at a certain temperature, it will maintain that temperature ... regardless of the supply/return temps (assuming the boiler is sized to meet the load).
Respectfully please don't say that I'm confusing myself ... it takes two to communicate and while I may not 'get what you're saying' as quickly as another, does not mean I don't deserve respect ... nor does it mean that every thing I say is incorrect. It seems that much of this discussion was about two very different concepts.
I never mentioned the setpoint. I tried several different ways to explain that I was referring to the coldest water in the system.
Too much sanity may be madness. And maddest of all, to see life as it is and not as it should be! --Miguel de Cervantes
I thought I did ... which is what started the conversation. But I may have lost the original point, maybe ... so I'm bowing out of this chat.
when you are confused, acting like a jerk is not the solution. YOU did not understand dan, who was very, very clear to me and others in this thread. YOU should probably apologize for your treatment of him.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Whatever, dude. I was just having a discussion with obviously either differing opinions ... or simply completely different points of view. I've lost the original point and right now I'm not going to try to sort it out, so I'm outta here.
Actually, in a boiler, return temps DO affect flue temps. A lower return temp in a boiler means a lower supply temp which means a lower flue temp. If one goes up, so do the others.
Which is why on a standard cast iron boiler you don't want return temps below 135f. Flue gases condense and start to eat away the cast iron until you end up with a leaker.
I understand. What if you have a 3-way valve? Then it seems like supply/return is somewhat more independent of boiler temp.
Doesn't matter if you have 3way, injection,p/s, p/s/p or any other configuration, at design temps, you use the same btu's and the return is going to be "cold". (keeping in mind that cold is a relative term.)
And assuming the system is properly designed and installed.
What leaves the system must be put back into the system. That is why half of the discussion is irrelevant. Ignoring the system is like buy a Maserati or Ferrari (sp), and then driving it only on gravel roads. Kinda of takes the fun out of it.
Edited 8/24/2008 9:32 pm ET by rich1
I think I agree ... not sure about what discussion you thought irrelevant as it appears my discussion and others' digressed into different specifics. I understand that what is taken out must be put back in. Under a static load ... if I supply 180 deg water ... the return temp will be higher than if I supply water at 150 degF. The delta T will be the same, though. If my boiler has a minimum return temp, a 3-way will ensure proper temps (I think).
If I keep my boiler at 180 degF, I can supply water at any temp I choose ... e.g. a reset controller ... to reduce supply temp based on load. If I want to be a little more efficient, I can reset my boiler temp w/in the limits of the [boiler] equipment design.
In some ways, this is a hard topic to discuss like this. There are numerous components and concepts to keep straight. Not sure I get your analogy (danh and what's his name would tell you I'm a little dense here). I lost track of the point a ways back, maybe.
Man, I really need to get a life. I'm sitting in Havre, Montana, on holidays and I'm checking breaktime.
Anyway, If the return water temp is above about 135, the flue gas will be above 135 and the boiler no longer condenses. Basic physics.
In Canada, we can no longer use abs for venting. Only certified pvc. If the flue gases are going to be high temp, we have to use certified cpvc.
So, why use a modcon boiler on a high temp system? Because you only need the highest water temps 10% of the heating season. The other thing that makes a modcon shine is it modulates. When combined with outdoor reset, think of it as cruise control for the heating system.
>>I think you can have e.g. a higher temp system (e.g. baseboards/radiators) w/ a low temp flue ... that is a high efficiency boiler providing 150 degF water and a 120 degF flue gas tempAs I understand it, heat will move from warmer to cooler: if my supply medium is 150 and my heat source is producing flue temps of 120 - heat will move from the supply medium to the flue gases (as the pass by the heat exchanger)IOW: a once the flue gases heating the supply medium and the supply medium itself reach the same temp (which, I think, they'll never do) no more heat will move one way or the other.So the supply can't get hotter than the flue temps.
In Xanadu did Kubla Khan
A stately pleasure-dome decree :
Where Alph, the sacred river, ran
Through caverns measureless to man
Down to a sunless sea.
Coleridge
Right, I think. DanH was struggling to point that out to me (which involved some mind reading on my part); I finally got it ... Makes sense, but I still am not quite 'getting it' ... thought you could set the water temp to 'anything you wanted' ... even if the flue temp was e.g. below 150 degF.
Boilers that are rated to be vented in PVC pipe can be used to supply water at any temperature up to its maximum rated operating temperature, which is typically 200 degrees F.
you're ignoring low temp operational efficiencies, distribution efficiencies, and reduced stratification opportunities. compare a pump's electrical usage to a fan's sometime. you might be surprised.there can be significant efficiency gains with radiant, but it's not as simple as "radiant is more efficient" for sure, and FHA has made some big strides in recent memory as well, so it's not as bad as it used to be. it's still fans and air though... weak way to move heat. of course it also ignores any non-efficiency benefits of radiant as well.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
ShelterNerd.
Forced air systems have several disadvantages over in floor radiant heat..
first. The coldest extremity in your body is your feet because they are the furthest away from your heart.. forced air does not address that issue well at all.
Heat rises and being forced out of a vent causes it to rise quickly. Cold air settles. That's something we all learned in high school science class.
Cold air on the floor, warm air rising to the ceiling means that your feet are the coldest and you need to rise the temp of the room a lot in order to get the house feeling warm.. With my old forced air system I kept turning the temp up around 72 on really cold days (minus 20 and below)
My new in floor radiant heat I have the temp at 66 degrees and walk around in a t shirt and bare foot all winter..
What ever the cost difference is between 72 degrees and 66 degrees is the money I save.
Second, in floor radiant heat allows choices of heating medium that forced air does not..If you have a super insulated house your options are greatly increased.. For example you can use electric heat where rates are low or electric water heaters instead of boilers.
You need fresh air in any home in order to ensure that indoor air quality is good.. Using a heat recovery system should be part of any well sealed homes HVAC system no matter what type of heat is used.. remember the air drawn in by a high efficiency furnace doesn't get into the house it's used for combustion and vented outside..
Third you've mentioned dust and pollen blown around by forced air systems. While great attempts to remove dust and pollen may be employed it's impossible to regularly clean the source of such dust, the ductwork itself.. Even professional cleaning on a regular basis won't completely eliminate trapped dust as some will always remain behind stuck to the various oils, crevices ,and traps etc. inside duct work..
"My new in floor radiant heat I have the temp at 66 degrees and walk around in a t shirt and bare foot all winter.."
Put some pants on man.
Who do you think you are? Mr. T?
Impossible is an opinion.
engineerguy,
I'm a closet nudest. <grin> actaully I'm pretty fat and fat people tend to be warmer than skinny people. I'd go ahead and set the temps lower if she who must be obeyed wouldn't whine so much <grin>
We could have extensive discussions about radiant vs. forced air systems with respect to particulates, dust, etc. I'm not going to get into that.
Ventilation ... exhaust systems are generally NOT about dust and dirt in the air ... they are about moisture and other contaminates (e.g. odors, VOCs, etc.). The big item is moisture (someone may jumb in and have another point of view, here). Well insulated houses retain moisture due to their somewhat naturally reduced natural ventilating characteristics (from insulation and air sealing).
You generate moisture in a house ... people (sweat), plants, showers, toilets, cooking, aquariums, etc. Modern houses tend to be fairly well sealed ... so you need to ensure you do a good job of ventilating when necessary (e.g. after a shower or during cooking). This has nothing to do w/ the type of heating system you have.
Ventilation (controlled) is a good idea ... both for energy and IAQ. Ventilation systems often take a back seat to other high quality construction (heating and insulation systems). This CAN often result in moisture related problems.
Ventilate ... regardless of what else you do. This requires exhaust ... requires intake (i.e. fresh outside air). Do a quality job ... it will serve you well.
Edited 7/31/2008 4:35 am ET by Clewless1
As some have previously noted, hydronic heat can be more efficient for various reasons and ventilation is required. Though someone mentioned a poorly understood, code based cfm value per occupant, residences are usually ventilated based on air changes per hour. Something no one has mentioned so far is air conditioning (cooling and dehumidifying). In my opinion, low temperature, hot water based in floor heating is one of the most flexible, efficient and comfortable options available. Like any system, it can be done poorly, adequately, well or expertly.
Ventilation soley for air quality can be accomplished with one of the many energy/heat recovery ventilators. With the exhaust and supply locations well placed, a great deal of comfort and efficiency can be acheived. However, these are usually balanced systems, meaning they provide only as much air as they remove. There are things inside of house that consume air. Exhaust fans (kitchen hood, toilet exhausts) clothes dryers, water heaters, etc., remove and/or consume air that will be provided somehow. If not planned for, usually induced inleakage is the result. BTW, event the "tightest" houses constructed by conventional means still leak 0.1 to 0.2 air changes per hour (ACH) in design wind conditions.
What about those days when it is 75 degrees outside and it is 95% relative humidity? Or when its 95 degrees and 95% humidity?
I live in a predominantly heating environment, but one in which dehumidification and cooling is still a requirement for reasonable comfort. If I were starting with a clean HVAC slate, I would have the hydronic infloor heating (with a modulating, condensing boiler and an "indirect-fired" water heater) and variable speed fan coil unit(s) for air movement, cooling/dh and filtration, with high efficiency media filters and barametrically controlled outside air ducted to the return(s). Exhaust toilets and bathrooms with an energy recovery unit.
Tim
I too live where summertime heat and humidity can be dangerous..
I solved my summer time cooling requirements cheaply and efficently. I use window airconditoner units stuck thru the wall and thus haven't given up a nice view or the potential for free cooling via ventilation..
If you check the rating on the better window A/C units they are as high or higher than the best central units are. Plus they can be cheaply replaced if more efficent units come on the market.
My 5500 sq.ft. house can be cooled a full 10 degrees with one window unit or cooled another 5 degrees with two units (one in the Kitchen one in the master bedroom).. on those rare occasions when three units are called for (100+ plus 95% humidity days) I can add a suplimental A/C inside ten minutes..
It helps that my home is well sited and super efficent. Thus far this year I've run two units a total of 9 hours..4 of which wouldn't have been used at all if the humidity and heat hadn't been so extreme..
One final advantage of small window A/C units is that you can install them high up in a wall taking advantage of space that is seldom used.. plus making the unit work to your best advantage. Rather than duct work causing the A/C to blow out cold air at your feet it can come cascading down and cool you as it falls.. widow units installed in windows blow cold at your belly rather than down over your body..
Edited 8/1/2008 8:09 pm ET by frenchy
I wrestled with the indoor air quality issue also. I went to the American Lung society web site and found some good info.
We completed a major renovation/expansion in September 2007, renovating 1600 SF and adding 3500 SF. All exterior walls are foamed to R-21 +/-
We have a hydronic/radiant hybrid system using a Weil-McClain Ultra 155 boiler/indirect water heater along with an American Standard A/C system and American Standard ERV (Energy Recovery Ventilator) at about 140 CFM.
This combination is fantastic. The ERV discharges 63-65 degree fresh air when it is 30-40 degrees outside in winter.
Jeff
Edited 8/2/2008 8:31 am ET by Jeff_Clarke