Every article I read about “Energy Programmable” thermostats say only positive things about them. They will cut your energy costs by ?% and take all the guesswork out of when you should down the thermo up or down. Most articles tell a homeowner to set the thermo back a few degrees at night and at times when the occupants are not at home.
That sounds great but! How much energy is used when the thermo clicks the furnace on . Now it has to work a little longer to bring the house back up to the occupant’s comfort level and maintain it.
I never see the articles address this aspect of setback thermos. I lean towards leaving the thermo at one setting and let the furnace maintain that setting all the time. Seems to me it would be steady as opposed to up and down, up and down.
What does everyone think about this issue?
Replies
You may never hear the "warm-up from a cooler setting" extra cost except from some uneducated persons from the fuel supplier. Yes, it does take a bit more fuel/heat to bring it back up to temp but the heat system may only run 10-15 minutes more. While the temp was set back for 8-10 hours, it may have saved 30-45 minutes of run time so there is a net benefit of 15-30 minutes of fuel saved. (for oil using .85 -1.2 US gal per hour nozzle maybe 1/2 gallon @ $2.50 .....guessing at retail price). The savings are generally considered approx. 3-4% for every degree F of fulltime setback. Variations can be calculated such as 16 hours set back per day saves maybe 2/3 of 4% (best case) or 2.8% per degree.
With that logic why dont you leave your stove on all the time ready to cook at a moments notice.The digital thermostats due make a difference. And you can fine tune the programing to save even more $.
The information that "experienced' has given is RIGHT on the money..
Set back T stats in most cases, do save in the long run. The actual amount of savings will depend on the home itself as to insulation ,tightness,location, etc, and the amount of set-back.
Also the heating unit will play a big part in this. If sized right.
If you have the time, record the actual minutes of burner "on " time in a 24 hour period. Then (with a similar outside temperature) record the amount of time with the 'set-back' settings.
Set-back times are usually 3-6 degrees at the most. but even if the difference in burner running time is only 15 minutes a day that will be a fairly good savings over a month. And it will not be as hard on the equipment either.
Edited 10/16/2005 5:09 pm ET by Hubedube
Isn't it also a function of the type of heat, and the thermal mass of the home? Would forced air result in different savings than hot water? A year ago we installed a high-efficiency modulating boiler in our old brick farmhouse. The contractor cautioned us about expecting much savings from thermostat setbacks, mostly I think due to long recovery times.
I installed a fancy digital thermostat with the schedules and "smart" recovery time, mostly because I like the house cooler at night regardless of savings. It's impossible to know if I'm saving fuel, but I like to think I'm saving something. I do know that I'm saving a ton of dough because of the new boiler.
Marc
marc, yes, the type of system,construction, location,sizing, R values,etc, does enter into how much, if any, a set- back will save.
and it is POSSIBLE to figure out how much you actually are saving with your present units set-back setting.
As I mentioned in my previous post, you can take notes over a 24 hour period WITHOUT a set-back, and then compare them with a similar 24 hour period WITH a set-back. (it may be time consuming but then you will know for sure)
The burners' total running time over a 24 hour period is the key to this calculation.
well, that again depends on your heating system. If you have a responsive system, burner time may not be as critical as things like water temperature or stack temperature.Setback savings depend heavily on a lot of factors. I strongly disagree with the 'extra 15 minutes' assessment of coming out of setback. That might be how fast it first satisfies the AIR temperature requirements, but the newly cooled mass of the home is drawing heat out of that heated air... and you.. faster. You will increase cycling during a long period of time in the morning and cycling is not your friend.Also, you are much better off if you let the system run constantly for a period of time before beginning setbacks, IF setbacks will even help you in the first place. At least then your system stands a chance of getting you up to a good mean radiant temperature. Otherwise, you don't quite get there, cool down, don't quite get there again... so on and so on. Especially if you have something like forced hot air it can take *days* to get the mass of the home actually up to temperature, regardless of what your thermostat is reading. And when that mass isn't up to temp, you are not comfortable. If you've ever felt chilly when your thermostat reads the temp you set it at, you know what I'm talking about. Imagine your shoulder season demands, where the system wasn't running all day and then just as it gets in the swing of things in the evening, you turn it down... you might not get a decent MRT until later in the winter!!As a rough rule of thumb we discussed this over at heatinghelp.com, and basically all the energy you let bleed out of the mass (the "coasting down" period) must be recharged later (energy is conserved). Your only savings is after you bottom out at the lower temp, to the point where you turn the thermostat back up. If you have low mass and high heat loss, that might be a good chunk of time, but then I'd say you have greater concerns than a programmable thermostat will solve... fix your envelope. As loss drops or mass rises, your savings fall.I suggest setting your heating system to a temperature and *leaving it alone* for at least a week during the beginning of the season. See if you can actually get comfortable after that at a lower temperature (MRT does make a difference!). When you have the "steady state" temperature you are *truly* comfortable with, THEN think about setbacks IF you have low mass and/or high loads.Then again, if you don't care about comfort, just turn the heating system to 40 and wear sweaters and mittens. You'll save a ton.-=Northeast Radiant Technology=-
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To advocate that no one turn back their t'stats to save energy is irresponsible!!! Most of the population live in older houses that are essentially low mass with relatively high rates of uncontrolled air exchange. These folks will benefit from setback, some more than others due to house/heating system characteristics. But all will save something and that's what we're up to here.
Are the studies done by government and funded/private energy consultants all crap??? I don't think so!! After being in another forum and kicked off three times for bringing "Research" to the forum to back my recommendations about air barriers vs vapor barriers, air quality, etc (even fighting a posting moderator!!!), I'm beginning to feel the web is too much of place for people to feign knowledge by "baffling them with BS"!!!! And a bit of this may be going on here.
Mean Radiant Temperature
Even though not in direct contact with the body, hot or cold objects still greatly affect our perception of temperature. This is because they emit and absorb radiant energy which activates the same sensory organs as conducted or convected heat. The net exchange of radiant energy between two objects is proportional to their temperature difference multiplied by their ability to emit and absorb heat. Mean radiant temperature (MRT) is simply the area weighted mean temperature of all the objects surrounding the body. It will be positive when surrounding objects are warmer than the average skin temperature and negative when they are colder.
MRT is only one of 4 factors that affect human comfort. The others are: (1) air temperature, (2) air velocity, (3) relative humidity.
In a house during the winter, most people are not dressed in summer apparel with a lot of skin exposed. This reduces the MRT effect to begin with as 90% of the skin is not "seeing" the cooler walls. This "effect" will not be permanent but slowly disappears as the materials warm up. Setback MRT effect from the inner wall surfaces is small anyway since they are only 5-10 degrees cooler than they used to be when the t'stat was set at 68-72 degrees. The inner window surfaces, on the other hand, are going to be much cooler and if there are no window covering, the skin "sees" the full outdoor temps through the transparent glass unless it's "Low E". So if you're sitting by a window you will feel cool.
When we use programmable t'stats, the air temperature is already back up to our desired level when we enter the space, so again the MRT effect is a smaller part of the comfort equation.
One thing not mentioned was natural air exchange. Most older houses have quite a bit. If you leave the temp. up to 72, rather than setting back to 62, and then lose 1/3, 1/4, 1/5 of that heated air every hour at 72 and its replaced with cold outdoor air to be heated to 72, you have to heat that air an extra 10 degrees and that costs more the leakier your house is.
No, programmable t'stats are not the big solution to energy savings. They may only save 3-4 to 10-12% but if you do this (save) in 4-5-6 areas of the home by airsealing, weatherstripping older window/doors, upgrading the attic/walls/basement after airsealing, installing high efficiency heating equipment, installing low flow shower heads , turning water temp down, cold water washes.....savings will be really noticeable.
So you just agreed with me. As mass decreases or load increases, savings increase, and vice versa.I suppose one point here is most of the people I work with are building new or renovating for energy efficiency, and generally with higher mass homes... this forum is a broader spectrum of buildings and I didn't take that into account. Please Pardon me if I sounded too sweeping there...Of course MRT is not the only indicator of comfort. But it is important and most people don't think about it; they notice drafts, humidity and air temperature so I don't call that out specifically ;) Letting a space, not just its air, get up to temperature before you start crippling it's ability to heat it can make quite a difference in comfort, ESPECIALLY with forced air or baseboard systems. I've seen the difference even in efficient buildings.Clothing is *very* subjective. Personally, I wear wool socks and heavy clothes in winter.. grew up in a house with a wood stove and that was it. I've had roommates that were personally offended if I mentioned that perhaps shorts and a tank top was not appropriate wear indoors in january here in maine. You can't make assumptions about clothing without knowing the people you are talking to.If you use a programmable stat, all you are doing vs a manual setback is coming out of setback earlier. That reduces your savings but does increase your comfort. It does not, however, eliminate the reduced MRT in the morning, just when most people are getting wet and naked in their bathrooms!This is a bit controversial, but basically you just listed about a dozen other things you can do to improve efficiency by much more substantial margins than by using programmable thermostats, and most of them improve your comfort, not reduce it. Heck, if setback is so great, how about you setback all night, come out for a couple hours in the morning, and setback all day while you're gone, come out for a few hours at night. Some people do this anyway. Then let the system run smoothly for a couple of weeks and see what the difference is. perhaps, in your home, with your lifestyle, it's not a big deal. But for many people, it is. So how about both of us refrain from making blanket statements :D-=Northeast Radiant Technology=-
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You say these set-backs may only save ...3-4 to 10-12 %..... hey, i'll only be too glad even if its only a paltry 3 %, That small bit of savings is enough to give me few do-nuts and coffee at Tims... With 12 % maybe even take in the odd movie or two.
One item you overlooked in the "human body " comfort is the human body itself. Some humans need more heat within to feel comfortable 74-75 F is good to some, others only require 68-69 F to be comfy.
Setbacks ,normally, do SAVE..... its up to the individual (human body) to decide just how much.
OK, I have a question about programmable thermostats. I am currently building a cabin in northern MN. We have radiant slab heat using an electric Thermolec boiler that is hooked up to off-peak. The off peak in this case can only be shut off between 5:00 PM and 9:00 PM. And when talking to the power company, it is very very rare that it is ever shut off in winter.
The cabin will be used on weekends only for the most part. I would like to install a programable thermostat for this. I would program the thermostat to warm the place up to 70 degrees starting every friday morning at about 8:00 AM. Then I would program it to cool off back to 55-60 degrees at about 10:00 PM. The theory being that if we were going up there that weekend we'd just override the program for the weekend. And if we didn't go up for the weekend, the cabin would cool right back down. Does this make sense to everybody? Am I allowing enough time to warm the place up? Its a small slab, 24'x36', and has 2" insulation all around and under it.
Final question, any suggestions on a good thermostat to use? The boiler only has connections for 2 wires.
Thanks!
You'd be better served with a phone switch. Call before you head up there, and turn the system up remotely.-=Northeast Radiant Technology=-
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Slab heat is a whole 'nother issue. The thermal mass is large, including the soil below the slab to a degree (depending on how much insulation is present below the slab). (Frankly, it would not be my choice for a weekend cabin, due to this thermal mass.)What you propose is probably basically what you'd like to do, but the time required to warm things up can vary enormously, so you'll have to experiment.It may be that you can get a phone or internet-operated thermostat, so that you can control things better.You probably should also invest in a few space heaters, to provide heat for a few hours until the slab is up to temp.There's nothing special about the way a programmable thermostat hooks up vs a regular one. Ignoring heat pumps, there are three basic variants of thermostat hookups -- standard low voltage (24V), line voltage (120V), and millivolt. The vast majority are low voltage, and thermostats are pretty much interchangeable between low voltage systems. Even if your unit requires line voltage, a simple relay setup can convert it to use a low-voltage thermostat.
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Well, we do actually have a couple of baseboard heaters that are going to be installed in the living area just for the reason that a slab is slow to heat up. Also they are there for back up heat in case something goes wrong with the boiler, etc...
I've thought about the phone or internet choices but there currently isn't a phone line on my road and the cost would be over $2000 to get one run up there.
I'll have to do some experimenting this winter to see how much time I need to warm it up and what the set back temp should be.
One item you overlooked in the "human body " comfort is the human body itself. Some humans need more heat within to feel comfortable 74-75 F is good to some, others only require 68-69 F to be comfy.
Also...the same people (mostly all of my kids) who need 75 degrees to feel warm in the winter, seem to also want the AC set at 62 degrees in the summer, go figure.
Personally.....68 in winter .....70 or so w/ac
I do set back at night to 60 degrees, but this is because I love our down comforter, I seem to sleep better (no sweaty pillow in the am), and.......the #1 reason to set back the termostat at night.........DW gets very cold (he he he)
Hey, are you in Buffalo?
No, but i'm approx 250 miles NW
Why? do you ask?
Tims.....
Tims?????
Are you referring to Tim Horton's coffee/do-nuts?????
What else!!!
and just what was the purpose of your post to me with this one word only;... 'Tims' ????
If you keep a 24 hour record , starting in the morning and ending in the morning to just before the "make-up"time, that will include this 'mass' loss you speak of.
I'm not sure what you're saying. The mass may or may not even be up to temperature by the time you start your setback. For instance, the last home I rented had a monitor air heater, built on an insulated slab, open floorplan. It was three days after I stopped my fiance from turning it back at night before it finally felt comfortable in there. Also the whole idea of a 24 hour record makes assumptions about the type of heating you have. a simple FHA furnace.. sure. log run time. Also note comfort. If you have anything with outdoor reset though, that won't tell you much as such a system is supposed to operate more frequently.Just putting that out there for clarification as I'd hate to see someone read this with a weather responsive system and think that it was behaving badly because it runs more often!-=Northeast Radiant Technology=-
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I would think that after a couple of weeks in a home with the heating system operating normally that the home would be under a normal moderation of temperature.(no cold spots)
That depends. As I just noted, you can have a situation where because of night setbacks, the system does not ever actually get the MRT up, especially during shoulder seasons. If you let the system get to steady state and then start setbacks, that's better.-=Northeast Radiant Technology=-
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"you can have a situation where because of night setbacks, the system does not ever actually get the MRT up, especially during shoulder seasons."
NRT: describe this type of situation in a regular insulated house not designed for thermal mass storage or concrete radiant floors (which are special situations in my mind)
I just did. Slab floor house, monitor FHA unit in the center of the open first floor. Read: high mass house, low mass heating, low loss as well the home was very efficiently constructed w/slab insulation and I believe double wall construction, though it did have some large windows.The main room was *never* comfortable when it was set back regularly. Once we stopped setting back, it took several days for the room to truly feel warm. After that, it was much better. I did not, however, go back to setback after that so I can't comment on whether it would have maintained it after achieving a good MRT prior to beginning setback.I can, however, say with absolute confidence that the home did NOT achieve a good MRT while it was being set back from the get go (read, shoulder season). Theory; setback kept the heating down at night. During the day, solar gain and warmer outside temperatures kept the air temp up without really heating the whole space, and the heater didn't run enough to overcome all the mass with the puny ability of FHA to do that particular job. Then when it started getting colder, and the heater ran more often, it would get put back into setback before it had a chance to really let some BTUs soak into the mass.This was a particular situation, yes, but it certainly illustrates why blanket statements about setback are not helpful, just like blanket codes requiring setback with no regard to the building itself or its heating system are not helpful.I'll be putting Tekmar 500 series thermostats in my new (to me) home shortly as a first step upgrade on the current baseboard heating system before I tear it all out to put in radiant (of course :D) and I'll be sure to use the programmables and get some sensors and datalogging in there so I can experiment further. It's a common set up for this area so my fun should be applicable to a larger cross-section of houses and I'll be sure to let you know how it goes!-=Northeast Radiant Technology=-
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Sorry, but this will only work if weather is identical on the two test days.
that's right They have to be similar days. And there are quite a few of those that happen in succession, aspecially in Nov.
Perhaps, if a person "tuned" in to a weather forecast they would know whats going to occur.A degree or two difference in temperature over 2 days will have no significant bearing on this test.
It will at least give a general idea of the savings.
Generally the local weather service will report "degree days", allowing you to compare days and decide how similar they are. You could in fact collect info for several days both ways, and then use degree days to match up pairs for comparison.--------------
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Many people don't seem to understand what drives heat flow to the outdoors and the equations that describe the heat flow. They are actually parallel to electric current flow equations!
Electrons (current) are driven/pushed by voltages. The greater the voltage difference (Delta V) between the ends of the wire ,the higher the current flow. The more resistance (R) in the wire, the lower the current flow.
BTU's (heat) are driven/pushed from the house (look at the wall as the wire) by the inside-outside temperature difference (Delta T). The larger the Delta T, the higher the heat flow. The more insulation (R) in the wall, the lower the heat flow.
These equations allow us to size heating equipment more accurately in the same way we size wire that will be safe for the current flow. And they are actually derived/governed through the Laws of Physics which we don't seem to be able to resist/change to suit ourselves.
If you use setback, from the moment setback kicks in and the house temperature starts to lower, you've lowered the delta T (the driving force) and heat flow becomes less thus saving you $$$$. There is no disputing this natural phenomenon. The longer the temps stays down, the lower the total heat lost and the less that has to be replaced by heater run time. It's all pretty simple.
Let's look at uncontrolled air exchange that is part of all homes especially older ones. The worst of these looser, uninsulated homes can heat and lose up to 1/3-1/2 the volume of the house air every hour. This causes cool drafty homes with high heating bills and too dry air in the coldest parts of winter. If the air that is entering (infiltration) only has to be heated to 60 degrees rather than 72 before it is lost (exfiltration) again, then it is easy to see why heat loss will be less because we're not heating it the extra 12 degrees. Pretty simple so I don't know why people don't believe that setback will not save $$$$.
The more efficient and massive your house is ,the harder it is to get large savings but scientifically it can be measured with enough equipment. And we should not have to tread over old ground again and measure- we did it all in the 1970's-80's. But history seems to repeat itself- all on this planet know war is bad and we already did it a number of times but we continue to do it.
One building scientist has said that a lot of the stuff we already know and is in the texts is keeping him "ungainfully employed" fighting popular energy misconceptions like the High R value of tin foil!! I have a list approaching 60-70 popular energy misconseptions myself!!!
you do no one a service if you imply that a simple delta-T calculation is the end all of heat transfer equations.Qualify your statements please. Mass matters. The rate at which the mass cools (heat load) matters. There are a lot of people reading this in ICF homes that won't see a lick of benefit from setting back for a night.-------------------------------------
-=Northeast Radiant Technology=-
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Thermal mass is only a factor because it affects delta-T.--------------
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This isn't a course in thermodynamics. I'm just trying to say it's not as simple as outside temp vs inside temp (which is what many people think of when you say "Delta-T") in all cases and when making statements it helps to make sure the exceptions are noted.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
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My quote from the post you mention:"
"The more efficient and massive your house is, the harder it is to get large savings but scientifically it can be measured with enough equipment"
I just reread what I posted, and not only did I not read what you wrote very carefully (apparently), I really jumped down your throat. This is very unlike me and I'm a little disturbed by it.Please accept my apologies. I will do my best to avoid knee-jerk posting in the future.-------------------------------------
-=Northeast Radiant Technology=-
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I'm enjoying your discourse here...can you, carrying on the electrical analogy & describe the difference in heat loss between a High thermal mass wall and drafty one like mine?the relative mass doesn't change the delta T, contrary to DANHs' postassume steady state temperature outside, no t-stat setback, and ignore the inside fluctuation in temp at the t-stat as the furnace cycles...
high or low thermal mass, the system goes into equilibrium (actually a bad choice of words, the furnace puts out heat, which dissapates through the wall at some steady rateintroduce a setback t-stat, and we get a phase shift...one "equilibrium" at one delta T, and one at the other delta T, with an intermediate state between them
the thermal mass affects the duration of the phase shift
just a guess here, but if the duration of the phase shift is shorter than the set back duration, then it's a simple system...if it's longer, we introduce another, longer cycle in which the residual heat stored in the thermal mass when it's at it's highest point gradually lowers to a new levelI think short cycling (mentioned in an earlier post as a likely bad consequence of the morning warm up period) in an HVAC system is a bigger issue in cooling mode than in heating mode from an overall "comfort level/indoor environment" perspective
From an energy perspective, unless it kicks on the backup resistance heat, it's not a problem, unless you want to worry about the operating efficiency of the furnace, and if it short cycles during ####warm up period it seems to me it probably short cycles at equilibrium as well.and just a note to the radiant heating poster, all the thermal mass in china won't help a drafty house, and anyway the trend in general is to add insulation, not thermal mass...even in a house with a lot of radiant heat, air exchange is probably more of a factor in both comfort level and energy efficiency than thermal mass (please don't flame me on this...a warm tile floor is nothing to scoff at...)
I didn't say that mass CHANGES delta-T, I said it AFFECTS delta-T (at least that's what I recall saying).The delta-T of interest for heat transfer is the difference in temps measured on opposite sides of the wall. The temperature transfer through the wall (ie, the heat loss) is in direct proportion to that difference.The rate at which the temperature drops inside (with no heat input) is basically determined by the temperature transfer rate divided by the thermal mass. Of course, temps will never drop evenly in real life, so you'd have to use some sort of discrete model, or just be content to deal with the "average" inside temp. Unlike with weather models and the like, since the processes involved are fairly linear (with the exception of certain convection processes), an average computation will be accurate.Note that, assuming a constant outside temp, and sufficient thermal mass (or heat input) such that the inside temp remains relatively constant, thermal loss is constant and independent of thermal mass.The issue is whether lowering the thermostat setting during periods when the structure is unoccupied (or the residents are snug in bed) will save energy. If the thermal mass is essentially infinite (compared to heat transfer rate), then the temperature inside will not drop, delta-T will remain unchanged, and there will be no reduction of heat loss.Of course, the mass is never actually infinite, and there will always be some lowering of temp with a setback, but quite possibly not enough to be worth worrying about.
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Yeah, "phase shift" is a good way to look at the pattern that setback produces. As thermal mass is increased (or insulation is increased), there is more and more phase shift. In addition, there's a potential that the setback temp will never be reached (or at least not for very long), reducing the effectiveness of doing a setback.The phase shift can be compensated for, to a degree, by having the setback times "lead" the desired temp pattern, but too much of this produces unpleasantly cool temps prior to setback (while too little produces unpleasantly cool temps after the setback).I don't understand why you imply that setback will lead to short cycling, though. When the setback goes into effect the furnace will not run at all for a relatively long period, and then will resume more or less it's normal pattern. Then the setback ends, the furnace will have a LONG cycle, as it brings things up to normal temp, then once again resume it's normal pattern. This generally will improve furnace efficiency.
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DanH, your sum up is exactly on target, I couldn't have said it as good.
Well...
thermal mass would indeed change the rate at which Delta T changes, but that's a second order effect...it's reasonable in this discussion to assume a two state delta T, IMHONot that I'm an expert, so I'll ask...does thermal mass include permeability? (i.e. is the airtightness factored in). "the rate at which the temperature drops inside (with no heat input) is basically determined by the temperature transfer rate divided by the thermal mass" temperature divided by time = temperature divided by time divided by thermal mass?
the units have to agree for the statement to make sense....not trying to bust your nut here, just trying to understand....and introduce a little scientific rigour into what is otherwise a he said she said conversation....LOL thermal mass and air exchange rate will make a big difference in how fast a system cycles
my gut feeling is that, during warm up periods, the furnace will cycle more frequently than at equilibrium, just as, during cool down, it will cycle slower (i.e. not at all)"there's a potential that the setback temp will never be reached"....man, I'd like to live in that house!
thermal mass and air exchange rate will make a big difference in how fast a system cycles
my gut feeling is that, during warm up periods, the furnace will cycle more frequently than at equilibrium, just as, during cool down, it will cycle slower (i.e. not at all)
During warm-up, the furnace is on. One cycle until the temp reaches the setpoint temp.
During cool-down, the furnace is off. Again, one cycle until the temp goes below the set-back temp.
During your steady state is when the cycling occurs. How fast the house loses heat to the outside (affected by thermal mass and air exchange rate), and how fast the house gains heat from the furnace (affected by the BTUH rate and efficiency of the furnace, as well as what stage is being used in a multi-stage furnace), and how low below the setpoint temp the thermostat is programmed to fire up the heat, determine how fast the system will cycle.
Ah, light in the tunnel....absolutely correct, thanks....
I've experienced some systems which seem to cycle a lot due to t-stat locations, drafts, etc. & I think sometimes it just takes a while for the whole house to sort of stabilize at the higher temp...
but in the main, you are 100% correct
Yeah, air exchange rate is a wild card. It's a function of (at least) the wind outside, whether the furnace fan is running, and the temperature differential. Generally the temperature differential factor is going to be small, but in some situations a higher delta-T will increase convection-driven leakage significantly.For a reasonably "tight" home it can probably be modeled as a constant heat loss (relatively independent of temperature), for a given set of wind conditions, and it likely won't upset the calculations much. A draftier home is more apt to delta-T dependent. But a strong north wind can throw the whole equation out, if the home isn't pretty tight.(I suspect that most of the fancy heat loss calculators go out the window when you have a strong wind and a drafty home.)While trying to figure out your units, remember we're not talking about TEMPERATURE flow (degrees) but rather HEAT flow (calories or joules or whatever). A given thermal mass will store a certain number of calories per degree temperature (to the accuracy we need here), so the calories absorbed or released is in direct proportion to the temperature change of the mass.Re furnace run time, during warmup the thermal mass is being "charged". If you view the system as having a single thermal mass, the furnace will run continuously until the thermal mass is up to the setpoint. In practice, however, there are at least two thermal masses -- the air and the "real" thermal mass -- and the heat flow between them isn't instantaneous (but can be modeled as a resistor between two capacitors). What will happen is that the air will warm up to the setpoint, the furnace will go off, and then the heat will flow from air to "real" thermal mass until the air temp drops to the furnace-on temp. However, the "on" time of the furnace during this warmup period will be longer than the "on" time at equilibrium, since the thermal mass is drawing heat out of the air while the furnace runs, making it take longer to reach setpoint. It is true that the "off" time of the furnace will be shorter than normal, but overall average cycle time will be longer.
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I think I probably erred with regard to some of my assumptions here about air exchange heat losses. For the simplest case, a "tight" home will have a roughly constant air exchange rate (ignoring wind conditions) as activities in the house "pump" air in and out. Since the heat content of the air is a function of its temp, the heat lost due to this constant air exchange will be proportional to delta-T (and, as such, can be lumped in with conductive heat loss in any computation).In a draftier home the "chimney effect" will come into play, and the rate of air exchange due to chimney effect is going to be roughly proportional to delta-T. Since the net heat content of the air is also proportional to delta-T, this means that chimney effect losses will be roughly proportional to delta-T SQUARED.A very good reason to try to seal a drafty house.
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"However, the "on" time of the furnace during this warmup period will be longer than the "on" time at equilibrium, since the thermal mass is drawing heat out of the air while the furnace runs, making it take longer to reach setpoint. It is true that the "off" time of the furnace will be shorter than normal, but overall average cycle time will be longer."It's like Alice in Wonderland...... I would have guessed that the on time per cycle would be shorter (air heats up fast in a forced air system), and the cycle time sorter, too...the air gives up the heat faster until the real thermal mass gets to steady state....anybody else dizzy yet?
In fact, if you want to carry the electrical analogy to its extreme, the system can be modeled as an RC circuit. The furnace is essentially a switchable constant-current supply that charges the capacitor (which is the thermal mass). Heat transfer is a resistor connected to the capacitor on one end and a variable-voltage supply (representing outside temp) on the other. The total charge that flows through the resistor represents heat loss.So long as the difference between the capacitor voltage and the outside temp voltage is relatively large, switching the furnace on and off will produce a fairly linear sawtooth pattern. If the voltage differential is small (ie, on the same order as the sawtooth voltage) then some "decay" will be apparent in the sawtooth.Note that part of the modeling involves switching the "furnace" on and off with a "thermostat" (the one non-linearity in the system). Alternatively one could vary the current of the "furnace", based on the average output of the furnace at a given temp setting, but this would actually complicate the analysis.
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ouch...gotta think about that for a while!
the system can be modeled as an RC circuit
Interesting thought. Might could make the case for an RF circuit instead, the house being an antenna, both radiating, and recieving energy, that has to be "rectified" to achieve some sort of control.
It's an imperfect analogy, but better from some of us in more widely-swinging climates.Occupational hazard of my occupation not being around (sorry Bubba)
You are misunderstanding what I said. I am in no way saying mass equals efficiency (though, in some climates, it sure does help). And what I am saying has nothing to do with radiant heat (though, now that you mention it, it will re-heat mass faster than any other kind of heat).If you have a high mass wall... ICF, perhaps... or a slab floor... that mass draws heat from the space until you hit equilibrium between indoor temperature and the heat flow through the medium, as has been noted ad nauseum. When it does, if you then set back the temperature, you lose heat from the mass to the space until they again reach equilibrium. That's heating the space, the only difference is, you put the heat into the mass previously, and are now releasing it in a reverse flow.The mass effect is to store heat, based on the density and specific heat of the mass, after all.If you don't then ever get to the lower equilibrium point, (it doesn't reach the lower setback temperature) and coast there for awhile, you aren't really saving much energy at all, as all the time your heating system was off, it was drawing heat out of your mass, heat which you will recharge when you turn the system back up again. Energy is conserved. So your system will cycle more during the warm up until the mass is recharged, and you can repeat the cycle again.In a standard stick built home, this is not such a big deal.In a slab on grade home, or an ICF home, or a Brick home that has been insulated or sealed against infiltration (that is, has a lower load than just an old brick uninsulated home), or even just a superinsulated home, then it IS a big deal, and setback is not going to help you unless you can leave it at a lower equilibrium state for some extended period of time, because it will re-radiate stored heat for so long into your set back space. Those examples are for various reasons; either high mass, or low heat loss to the outside. As said before, as mass goes up, or loss goes down, setback becomes less attractive if energy savings are your goal. I've certainly seen superinsulated stick builts that you could turn down at night and it wouldn't lose the setback temperature in the course of an overnight.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
> If you don't then ever get to the lower equilibrium point, (it doesn't
> reach the lower setback temperature) and coast there for awhile, you
> aren't really saving much energy at all, as all the time your heating
> system was off, it was drawing heat out of your mass, heat which you
> will recharge when you turn the system back up again. Energy is
> conserved. So your system will cycle more during the warm up until the
> mass is recharged, and you can repeat the cycle again.There are, I think, a few misconceptions here. Since (as we've been discussing) you ALWAYS have less heat loss at a lower temp, even if the inside temp doesn't make it down to the setback point, some energy is saved (not lost to the outside) because of the reduced delta-T.The heat that comes and goes in the thermal mass is, of course, conserved. But the heat lost to the outside due to delta-T is lost forever.And the system will "cycle" LESS (though for longer periods) during the warmup phase. For virtually all heating systems fewer, longer cycles means better efficiency.
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If the charged mass is losing heat to the outside, you don't reduce the delta-T until the mass cools down. Your indoor temperature vs outdoor temperature delta-T calculation here is ONLY applicable for a steady-state calculation.If it only cools down a few degrees over the course of an entire evening (quite normal for high mass, or low loss/superinsulated homes), you can do the math; you aren't saving much, and I would put forth the additional load on the heating system coming out of setback will offset whatever savings to some degree; enough so that you're not really saving much, if anything.Cycling depends on the heating system. If you're heating with FHA or baseboard especially, it will cycle more. Air temperature will rise quickly (low specific heat, plus it's being heated directly unlike the mass of the space) at first, shutting itself down after the initial longer burn getting the air temperature back up to "normal", but then it will cool down faster than it would at equilibrium as the mass then draws heat from the air faster that it did at steady state, triggerring another cycle. And this can happen quite a bit depending on your output, amount of mass, etc... you could very, very easily see increased cycling for many hours. This would be less pronounced but still a possible factor with low-mass radiant systems as well.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Well, you're pretty much wrong on all counts, but I'm tired of explaining it.
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"When it does, if you then set back the temperature, you lose heat from the mass to the space until they again reach equilibrium. That's heating the space, the only difference is, you put the heat into the mass previously, and are now releasing it in a reverse flow."on the whole, as already noted, in a high mass low exchange house, the flow isn't reversing much, it's still delta T driven to the outside....
Now, if we could make a wall that polarized to IR wavelenghts, we'd be in business....
"..you lose heat from the mass to the space until they again reach equilibrium..and are now releasing it in a reverse flow.."
Rob,
This statement shows the weakness in your logic.
Lets say inside air temp is 70 degF, outside air temp is 10 degF. Setback is initiated and heat input is stopped. Does the heated wall release heat to the 70 deg space or the 10 deg space? You chose the 70 deg space, which is not the way things work. 70 degrees inside, 10 degrees outside, heat is tranfered through the wall from high energy to low energy. At the onset of setback, assuming the system was at or near equilibrium, the main heat sink is outside, and the wall (and other envelope elements) continues to tranfer heat from inside to outside. There is no "reverse flow". The envleope continues to be a heat sink, as when the system was in equilibrium.
No, see, your logic requires the mass to be AT or below room temperature to hold true. Immediately, or quickly. Neither are the case for charged-mass situations (though it's close enough for stick-framed suspended floors, I suppose). In the case of a charged mass presence though, heat will radiant BOTH to the cooling room, and outside because the MASS is the high-energy point. That presumes room temp drops quicker than the mass, through windows/ceilings/floors/doors or infiltration, which should normally be the case.Slab Houses.. decently built ones, anyway... can often coast for several days with no heat before they freeze, even in the middle of winter specifically because of this re-radiation of stored heat into the cooling rooms.Furthermore, the heat you are radiating outside or back into the room FROM YOUR MASS will need to be recharged later. This is not savings, this is offsetting load from one time period to another.There is *some* savings when the wall or floor cools down and lowers the actual delta-T between wall/floor and outside, and *some* savings of a lower delta T through windows/doors/infiltration, but I speculate this is offset by increased system strain afterwards.. unless you have a grossly oversized system, then the "additional strain" may in fact be better for your efficiency!and that savings amount is very little to begin with. and you're less comfortable in the morning since the mass is not recharged that quickly. all presuming high mass is present of course!-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Slab Houses
Will also radiate heat from ground contact, just to complicate "simple" modeling. It can take months to change ground temperatures, undisturbed soil being a decent insulator.
If I remember the empirical testing process correctly, you have to use a thermograph to measure the indoor & outdoor temperatures of the structure after it reaches equalibrium after stopping all mechanical heating or cooling. That gives you the baseline thermal lag/perfomance for the one structure in question. After that, you can then postulate/prognosticate on what effect mechanical HVAC will have on the structure. But, that really only "counts" if one is being scientific.
"Scientific" is not how most people approach HVAC 'comfort' (at least in my experience). They can be even less "rigorous" when it comes to paying the bills for that comfort.
So, "proving" a setback stat is good, bad, or otherwise can be a tad problematic. Occupational hazard of my occupation not being around (sorry Bubba)
apparently ;)-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
"..your logic requires the mass to be AT or below room temperature to hold true."
Yes, the mass of the walls, ceilings, floors, etc., in the vast majority of buildings will be at or below the room air temperature during the heating season. I can think of no situations in which this would not be the case, except the obvious of infloor heating. In the case of an interanally heated mass, the discussion of response and discharge of absorbed heat does not apply. If I am missunderstanding your statements, please explain them differently.
Think Mass.Once you hit equilibrium, the mass may be very, very slightly cooler (surface temp) than the room, but not much.. if it is, you're not in equilibrium and you shouldn't be setting back at all, your system hasn't even finished heating your space yet!Now turn down the stat. Room air temperature drops... most likely, faster than the mass temperature does. Air doesn't contain much heat... mass does.As the air temperature drops, there is a period... potentially, quite a Looooooong period depending on the amount of heated mass, heat loads, etc.... where the mass temp is *higher* than the room temp. Dropping room temp then drops its Delta-T relative to the outside through the non-mass elements in the room, but accelerates its gain from the mass objects. heat transfer is driven by Delta-T, right?And all "heat charge" lost by the mass, will be recharged eventually when the system comes out of setback. That includes heat "lost" to the room during the cool down period, as well as heat lost to the outside during that period... any heat drawn from the mass lowering its heat content below what it was at equilibrium will have to be replaced. You may lower the delta T of the mass to the outside, slowly, but that would be such a fractional measure over the course of a night that it's not even worth thinking about.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
If you have a lot of low R value glass (say R4) in a massive house, lessening the delta T by setback, will save energy. The heat won't get out of the mass quick enough (due to the R8-10 inner wall insulation) to stop the t'stats from coming on as the room temp drops. It will be leaving by the windows quicker than entering the room if it enters at all. The temperature of the sandwiched concrete will not be room temperature but somewhere in between the exterior temps of the past few to 7-8 days and the standard interior temp, say 72. If the sandwiched concrete is at 55-60 degrees, the heat will not move back in until room temps are below the concrete temp......simple physics.
I don't have the advanced calculus and programming skills to do the projected #'s on it but a good friend may know. I replaced him as introductory building science lecturer in the local Architectural faculty while he did his Phd. in building science at MIT in 2001-2002.
Here is a link to setback and setforward. Tests were done on two identical test houses.
Once again, the correct answer to how much will you save is.................
it depends.
http://irc.nrc-cnrc.gc.ca/fulltext/rr/rr191/
Edited 11/5/2005 11:56 pm ET by rich1
The way I read it (in a quick scan) a reasonable scenario produced a 5-10% savings during heating season and a somewhat larger savings during AC season. This was a medium efficiency furnace in a medium-well insulated house (ie, latest standards but not state of the art). I missed where the houses are located but I presume somewhere in SE Canada, not too different from the US northeast and upper midwest.
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They are r-2000 houses, doesn't get much better. Basically, the colder it is outside, the greater the savings. In a mild climate, not much savings for setback.
What I found interesting was that setforward doesn't give the same savings. The greater the setforward, the longer it took to cool down, up to seven hours. Not real practical.
These R2000 house are located in our capital- Ottawa, Ontario. Since they are the most efficient houses being built as a class (the other being ICF's), to try and get big setback savings is harder since they don't cool down quickly as would regular housing. And most of our housing stock is older, so the setbacks will really help in 80-90% of homes.
The executive summary of said that savings of 13% were achieved with 2 periods of setback to 16 Deg C (approx 61 F)- not too shabby
Pretty simple so I don't know why people don't believe that setback will not save $$$$
My suspicion is that there are just too many cases where the stat is meddled/fiddled with/adjusted, and those cases mean the stat is "chasing" a moving target, which will be as wobbly a standard as a round stat will provide for $75 less.
I've noticed that there remain a frighteningly large number of people who remain convinced that their HVAC systems vary how hot or cold the air produced is, per the stat setting. (You can substitue "Sultanate of Oohmpapa Maomo" for "Delta T" and get the same "I see, I see" nod . . . <weary sigh>).Occupational hazard of my occupation not being around (sorry Bubba)
The only common case where you won't save energy is with a heat pump, if the thermostat causes the emergency heat to come on. (This is because the emergency resistance heat is less efficent than the heat pump.) But a properly designed system could prevent this.
It's valid, however, to question HOW MUCH you'll save. As was explained, the better the insulation and the greater the thermal mass, the less you'll save. The units are most effective in your classic old drafty house.
In any event, most people find it more comfortable to turn the heat down a tad overnight, and (except for the heat pump case) it definitely won't cost you money to do so.
No electrons were harmed in the making of this post.
An addition to Dan's comments would be that if you have a HP and a power company (PC) that has differential on and off peak rates, it may be beneficial to not set the thermo back even though the HP is less eficient during the colder night temps (but at lower power rates) and even though the heat loss without setback overall is greater, you still may be a few $$ ahead. Very rate specific.
Local PC had differential rates last year (now discontinued) and own calcs for house were to turn the HP on to Higher temp at 6-8 AM just before the rates went up, also let the house cool down from 6 PM to 10 PM when rates were highest. - lotta factors, gotta use math, guessing does not work in such situations. .
Edit PS: In temperate climates (few if any < 10F days) it pays to not even have a resistance backup in a HP. Have never turned on the elec resistance heat in own HP except to test, controlled by a separate 'emergency cold day' thermostat that has to be manually activated. .
Edited 10/28/2005 1:32 am ET by junkhound
In response to your edit PS:
Unless you've got other heat like wood, gas, you may want to have that electric heat backup installed just for the day that your compressor goes and the HVAC co. can't get a replacement in for a day or so on a cold weekend.
I lean towards leaving the thermo at one setting and let the furnace maintain that setting all the time. Seems to me it would be steady as opposed to up and down, up and down.
That depends on the structure, and the settings used.
If the structure will retain heat, heating early (or off peak hours, in certain energy markets) to create a "bubble" of warmth is very economical.
The "cost" of turning the heat "on" is fixed, it happens every time the heat kicks in. If the heater comes on 3 times an hour at one, even setting, the whole day through, that's 72 times a day. If the heat does not turn on due to a reduced stat setting, every time it doesn't saves that cost.
Say that changing the stat means the heat comes on only once an hour for eight hours. That's sixteen "starts" saved on the energy bill.
For residences, there's two times that can be targeted for reduced heating need, the work day, and the sleeping night. Even if these are rediced to only six hours each, that's 12 hours per day of reduced need. Reduced need usually equals reduced cost. Now, that also sort of mandates that the occupants aren't continuously fiddling with the stat, too.
For larger buildings, the changes can get very interesting. Timing the zoning of a couple of hundred thousand cubic feet of office space can generater some cost savings with a lot of decimal places in them (if, of course, the occupants are not monkeying with the stats . . . )
Just a note from a redneck who used to have one of these - The last house we lived in had a miserable steam heat setup when we moved in. From the time the furnace kicked on until the radiators warmed up was about an hour. I got a programable thermostat and set it up to kick the furnace up a few degrees about an hour before I got up. That way I got up to warm, steaming radiators instead of a cold house. Then the thermostat set the thing back a few degrees until an hour before I got home from work. Again, I came home to a warm house and hissing radiators instead of freezing and waiting for the house to warm up. So programmable thermostats have a place besides energy savings. I'd definitely use one again if I had hot water, steam, or radiant heat.
Anybody going into boxing already has brain damage. [George Foreman]
I advocate the use of programmable thermostats for several reasons.
One is that the comfort level of the space can be adjusted automatically to the occupants preferences. Most folks I've asked about this, in winter, like the house warmer in the morning and cooler at night. IF, like in my DINK household, most of the day the place is empty, setback can save the cost of heating an empty house all day. Never quatified the numbers, though, and never seen anything other than opinions on the actual savings.
Another great reason for the use of programmable thermostats is that during mild, humid cooling seasons, if the house is allowed to let the load buildup and then run the AC, the system runs longer and dehumidifies better.
In commercial applications, where ventilation is mandated during occupied times, the programmable thermostats, albeit commercial versions, are one of the most practical ways to open and close outside dampers, start and stop exhaust fans, etc., based on regular occupancy times.
I will offer that unless you have a well insulated house set backs are brutal both physically and financially. I own a 55 year old cape cod in PA. Our walls are plaster and there is no insulation. Last winter was our first winter in the house. I changed the thermostats to programmable what the system vaccilated back and forth between cycling up or down. It cycled down much more quickly then it cycled up. I eventually just set the thermostat at 70 and let things be. We will probably go another winter w/o decent insulation.
The effectiveness of a programmable is as much a function of the house and its ability to retain heat as it is a function of the heating system cycling down.
The one on DW never has worked.