Thermal mass and set back thermometers..
I have a lot of thermal mass in my home. Big white oak timbers, probably 40,000 bd.ft of hardwod inside the SIP enevelope. Plus a fair amount of marble and other mass. I’m going to in floor radiant heat and am trying to figure out if a set back thermostat will save me anything. I have 3000 feet of 1/2 tubing if that’s any help. I’ve tried it on my currant gas forced air furnace but even one degree set back when unoccupied seems to take dramatically longer to warm up.
Is it just me or am I correct in assuming that anything less than a few degrees for say 6 hours minimum simply won’t pay off? How do I calculate that cool down rate? and won’t the outside temp affect everything dramatically?
Replies
Frenchy, you don't seem to be getting much traffic.
Not a bump, I'll offer my view. Not that we have a thermostat at all here...
When you cool down the house you change the delta T (temp differential), favorably. Any heat loss calcs will give you result. Re-warming your mass is more difficult without quantifying the mass.
Likely we have a tad more mass/volume than you do. Our diurnal temp swing rarely gets to 4º. Tonight it's down toward 20º out, high delta T (for us). Plus we aren't currently active so prefer slightly warmer than 66º (our house's natural temp this time of year). Fire up the stove. Now we've got 69º, comfy. About 2 hrs for our 20k cu ft. First half hour is just warming the woodstove.
As we only run the stove for a few hours, our mass hasn't appreciatively changed temp. Pretty sure that's what'll happen in your house. I could quantify the heat stored in our mass by that but I have never bothered. I'm going on experience for the past decade.
Delta T is pretty much everything, after you account for insulation and leaks/air exchanges. Won't cost much to experiment if you want. Don't even need to buy a thermostat, just turn it down. You'll also need to track inside/outside temps, but then you'll have a clear picture. You can reasonably infer what the mass is undergoing.
Which is not what you're saying "even one degree set back when unoccupied seems to take dramatically longer to warm up." I don't think it's your mass that you're waiting to warm, mostly the air, assuming you're actually measuring air temp. That's largely related to presence/absence of convection currents. The other, very real factor, is how mass temp directly affects occupant comfort, independent of air temp.
I don't want to turn this into a tome, but warmer walls translate into comfortable cooler air temp for the occupant. Mass at work. Has to do with the various ways heat moves. And if you do manage to cool down your mass appreciably, it will require warmer than normal air temps to be comfortable. But you're not going to encounter that in your brief lowering of air temp.
Want non-computer calcs? Other Homes and Garbage, ISBN 0-87156-141-7. It's an engineering manual written for non-engineers in 1975.
PAHS Designer/Builder- Bury it!
Other Homes and Garbage
A little aside: Funny how some of us older guys into the energy efficiency of homes have this book!!!
Yup. When I first ran across it I couldn't believe my luck that somebody had actually written it. Some of the info is obviously dated, but the heat loss parts work fine.
Helped me determine what was reasonable insulation for client house, similar to this one. After reviewing both houses' performance, the calcs bore out. Our place is slightly over-insulated (due to a change of plan for the skin). And both are under-massed.
Frenchy doesn't seem to be particularly computer oriented (nor am I) and I thought something like Other might be a solution. That's assuming nobody comes along with better knowledge than I offered.
You don't have advice for him? That surprises me. PAHS Designer/Builder- Bury it!
I doubt that the book will give him much information.Bascially all he has to do it turn off the heat and record the tempature each hour for 12 hours or until it get too cold that he can't stand it.Then turn the heat back on and record the temps each hour unit it reaches setpoint.If he has that much thermal mass then he will only see very little variations..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
What the book does is show you how to determine where the heat's going and, more importantly, at what rate.
Which is pretty much what he was asking, if not for a method to calculate it.
Your conclusion is similar to mine. But it seemingly doesn't jibe with his 1º experiment.PAHS Designer/Builder- Bury it!
BillHartmann,
That's exactly what I did (to a small degree)
I started with a 5 degree set back (still have the old setback thermostat) Kicked off as we left the house and I had it set to come on two hours before I came home.
When I was working on the house it warmed up enough that it felt too warm but the girls all we complaining and kicking the temp up all the time, The house would finally warm to their satisfaction around dinner time. (me, I'd be walking around in my pajama's barefoot)
I tried for several days at each temp, would raise it a degree and note the level of complaints.
I used a digital infrared thermometer to verify the temp in the house and the temp on the thermostat were identical.. (the ceiling was within a half degree of the floor 28 feet below) Walls and floor were the exact same temp as were the beams.. (68 degrees) I don't have all my plugs in place and can check the temp inside the beams. (those 1/2 x12 stainless steel bolts are probably pretty accurite and I noted as much as a 4 degee cooler temp depending on just how soon after arriving I measured them) By the time the girls stopped complaining the temp of those bolts was 68 degrees.
It's impossible to quantify how often the furnace kicked on, outside temp seems to make the biggest change, but it fires up with less than 2 degree drop.
I'm very satisfied with the reduction in energy with the new house even though it's not properly sealed yet, in the past with this furnace, 1500 sq.ft. less, fewer and smaller windows, it would cost $500+ in December and January, currantly our worst bill is $137.00
But that satisfaction doesn't mean I can't keep looking for savings..
VaTom,
I'll look for the book, sounds right up my alley..I've got a fair number of books on the subject already and Think I'm well read.. This however seems to go against what I've read..
Is it possible that the energy load of rewarming up a mass is greater than the potental savings of the turned down thermostat?
I use two thermometers plus a third digital thermometer. One is on the wall and part of a set Thermometer, barometer, and humidistat. It's always a degree off. The other one is digital in the thermostat and of course I point the digital infrared thermometer at everything and anything.
I can actually take the temp inside of my beams.. I use 1/2 x12 inch stainless steel lag bolts and they are well countersunk so when I arrive home I point the infrared digital thermometer at them and if the house had been set back 5 degrees and rewarmed two hours before, The bolt head might be 4 degrees colder than the ouside of the beams. Later when the girls were comfortable the bolt heads would read the same as room temp.
" Is it possible that the energy load of rewarming up a mass is greater than the potental savings of the turned down thermostat?"No, but the savings might not be enough to make it worth while.But they way that you are doing it wan't save much.You need to run the test as I said.For example you don't want to turn the heat down WHEN you leave the house, but 1, 2, maybe 4 hours ealier.And depending on the mass, the details of the heating system, and the specific type of controls you might get too much overshooting or it might take way to look to get it back to temp..
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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've got a fair number of books on the subject already and Think I'm well read.. This however seems to go against what I've read..
The book does not deal with mass specifically. Only heat movement. What I wrote reflects what I think my house mass is doing. You've done more measuring than I have. And if you want to quantify your mass and its btu's, the book'll guide you through it. Shouldn't be any problem to find a used copy cheap.
We go through similarly short periods of changing the air temp, opposite of what you're interested in. I'm inferring what the mass is doing based on air temps and heat loss. Furthermore, our mass is our cooling system. It's clear how it works over a long period. Over a few hours, I may be misinterpreting, but a decade of living here makes me believe otherwise.
The book I followed, Passive Annual Heat Storage (PAHS), goes into how he measured mass temps in great detail. Very boring, little change, detail. So I didn't bother.
While I routinely opine that more mass is better, that's only generally true. I knew a guy who built a very high mass house and then allowed it to get very cold. Bad plan for a weekend place. Total heating cost wasn't the issue, comfort was. By Monday it was fine and cruised the rest of the week, only to greet him with low temps the next weekend. Set-back didn't work for him. Unrelated to your situation, just thought I'd throw it in. PAHS Designer/Builder- Bury it!
Frenchy- When the house is coming back up to temp after a setback do you feel drafts? Or, more accurately, convection currents?
This happens often in tight foamed homes I insulate. I recommend against a set back themostat for this reason to my customers.
What happens is this: 1. Thermostat sets back and air cools to that temp. 2. Mass slowly cools off to this equilibrium temp as well. 3. Thermostat calls for temp 4F higher. 4. Air temp rises quickly, mass not so much. 5. Furnace or heat added goes off. 6. Mass (walls) absorbs heat from air and air temp drops. 7. Thermostat sees air temp drop and goes back on. In step 6 the air cooling off next to the mass will be cooler relative to the middle of the room lets say. This will set up a convection current and feel like a draft. The cycling on and off of the furnace will be more frequent with a higher temperature setback. Not sure if the radient floor would help or hurt the situation.... I would think that mass heated up once would make the ramp up time less. Any other thoughts?Stu
MAsprayfoam
I noted that too, It seems I get more frequent furnace cycling than back when I had the old stick framed house with little mass. I blamed that in part because forced air tends to have more "drafts" than radiant and assumed that some combination of drafts was causing a cooling effect enough to affect air temp more so than mass temp..
I'm hoping that since radiant temps would tend to be more even the frequent cycling wouldn't happen as often..
Not sure how I could detect that with radiant. The systems I've been in with radiant seems to be extremely silent and I wouldn't have the sound of forced air to indicate when the sytem would be "on"
I'd hate to wait for months to go by to establish what efficencies I gain with radiant so I might be tempted to add a operating lite whenever the system is cycling hot water thru it..
Fr- This is why sizing the furnace is so important in a tight, foamed (or SIP), well insulated house. Often times half of what the HVAC guy guesses is needed. Don't get me wrong, some guys are great and really do their homework, but most houses have heat systems twice the required size and AC about 80% more than reqd. These facts from a couple months back Home Builders Assoc newsletter. With the limited knowledge of the situatation I would have to vote for the radient floor heat because it will be more consistent and quieter. Of course leave out the setback! The best thermostat to have on a superinsulated house like yours is one with a tighter range than normal. Meaning the difference from set point to "call for heat" point, then finally "heat off" point is tighter than normal range. Coupled with a smaller sized heating system of course.Cheers,
Stu
Frenchy:
I often wonder also if setback thermostats really save energy. One of the most sited studies on this issue was done by Oak Ridge National Laboratory (search for "ornl_con_64"). The link is too long to post. I've seen more recent work that suggests the savings touted in ORNL's work don't work the same for heat pumps.
Interestingly, I know someone that monitored energy usage and thermostat set temperature and found a non-linear relationship between the 2. That is, setting his heat 2 degreesF higher resulted in something like 50% more energy use. I'm still not sure how to explain that but wonder if its related to thermal mass.
I've got a geothermal heat pump system that we set back by 4 degreesF every night and don't set higher until we get home from work/school the next evening. 64 degrees is pretty cold in the morning but we use small space heaters for short periods of time while getting ready to go to work.
I think to really find out whether setting back a thermostat works you'd need to monitor set temp, indoor temp, outdoor temp. solar gain, use of heat sources within the house (cooking, showering) and possibly wind speed and exhaust fan use (including clothes dryer) to account for air infiltration. If this wasn't so overwhelmingly hard, I'd have already done it.
Cheers!
Ouch!, that's "cited" not "sited"...need coffee.
"The link is too long to post. "No such thing.However Prespero does a poor job of displaying long strings, most forums either automatically wrap them or replace them with a truncted version for display, but keep the whole thing.But you can use a service such as this one. http://tinyurl.com/"That is, setting his heat 2 degreesF higher resulted in something like 50% more energy use. I'm still not sure how to explain that but wonder if its related to thermal mass. "It does not take 50% more energy to MAINTAIN a house 2 degrees higher tempature.He is probably meausre how much it takes to INCREASE the tempature 2 degrees over a short period of time.And yes thermal mass gets into that.And that energy is not wasted. You get it at the other end. You don't turn down the temp when you leave, but some time earlier."The link is too long to post. I've seen more recent work that suggests the savings touted in ORNL's work don't work the same for heat pumps. "Heat pumps don't have anything to do with this. It is the controls that are used.Some system us a basic thermostat and if the heat pump has to run too long or the change in temparture setting is too high them aux heat kicks on.But a smart thermostat with adaptive recover can will know that the change in tempature and long run time is because of comming out of setback and will only run the heatpump, but will start the running it early enough to get back to temp at the desired time.With the Honeywell thermostats with adaptive recovery you don't set it for when you want to the furance to start, but rather at what time you want the house to be back at nomral temps and it figures out when to start the furnace.One qualification on heat pumps. In most case the lowest OAT is early moring during the time that when you are trying to rise the tempature. At some point the heat pump can't keep up and aux heat is needed. Thus there are some cases where aux heat would have been minimally used overnight, but would be heavily used to bring it back up to temp..
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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:
"He is probably measure how much it takes to INCREASE the temperature 2 degrees over a short period of time."
Not true. He monitored how much the heating system ran over an extended period of time (weeks) at different set point temps. The numbers may not have been 2 degrees or 50% usage but the point was that a small change in thermostat set point made a major difference in how much the heater ran. The amount (rate) of heat flux between two materials of different temperatures is a function of the difference in temp. That is, the more different the temps are, the faster you lose heat. This may explain his findings.
Thanks for the info on the URL link displaying. It was just real long, looked terrible in preview, and I didn't want to chance trashing up the post.
All:
I think we're doomed to chasing your tails with these anecdotal observations unless we consider all the other factors that affect HVAC usage (heat gain and loss from all sources as I listed earlier).
Programable thermometers don't generally work well with radiant systems with a lot of thermal mass. Regardless of the mass of bs tossed about regarding such things.
What you will get is less even temperatures as anything with thermal mass is slowly heated up. This creates environments that are less comfortable than quicker to heat materials. If radiant is used for comfortable floors and environments in the first place, why would you worry about a programable thermostat?
If you want to try it go ahead. When the high $ thermostats don't accomplish anything write it up to over thinking all this.
Beer was created so carpenters wouldn't rule the world.
Thanks Idaho Don,
I already have a programable set back thermostat, my "tests" thus far have convinced me that a set back simply doesn't work with the forced air furnace I am currantly using. So I set the temp and leave it alone. Once I have the radiant on line My gut tells me that a set back won't work either but I am certainly willing to consider everything.
I was briefly considering using the radiant heat as my primary with a back up of the forced air. Set the temp back during the day while I am away and have the forced air fire up and run briefly untill the infloor radiant can reach operating temp..
That on the other hand, would mean two heating systems on line at the same time frankly that sounds too expensive to provide any savings.
You have to design what you want to accomplish. Variable temperature thermostats work best at changing the temperature of an air mass with FAG and worst at changing the temperature of a solid heat sink with a fireplace. A radiant floor system isn't a great application either, but it somewhat depends on the heat sink mass, and you have lots of that.
With large masses, I understand that you treat the termostat more as a timing device thatn a temerature trigger device, and that you have to anticipate and plan for the time lags in your system and your temperature over-runs. You buy the thermostat with the 4+ settings per day and each day separately programable. Then if yoiu want heat for the morning shower at 6am, set the thermostat to trigger at 5am with +3Fº, a low temperature during the day and at night (treat it as a minimum setting, so you don't get too cold at night and you take full advantage of the sun during the day) and another blip for dinner/family time in the eveninig,
Phill Giles
The Unionville Woodwright
Some confusion will result from the varying definitions of mass. Some think a log is mass. Some think drywall is mass. Some think only concrete is mass. Solutions must match the materials. Mass is a continuum. The higher you are on the mass continuum--my past house had maybe 300 yards of concrete (if one wants to start a scale), and weighed over 1.25MM #--the less effective you'll be in making short term adjustments. It would have been silly for me to intend to daily lower the temperature of the main living area to save a few bucks on electricity. With truly high mass, the goal is achieving and maintaining steady-state conditions. That's where comfort is maximized, because there are not temperature increases or decreases, and no drafts, to notice...comfort being the absence of noticing the interior "weather".As the mass drops, one increases the viability of making daily adjustments. But there's not a straight line from high to low mass. For example, one could have high mass walls with a low mass roof and floor (ICF with framed roof/floor), or one could have high mass floor with low mass walls (framed walls with a thick slab). Each requires a different strategy, because it will respond differently to changing weather conditions and the introduction of btu's from a boiler/furnace/heat pump.
Cloud Hidden
I think you've said more than even you anticipated.. the variables are infinate. Here in the frozen north where temps vary from +100 to Minus 40 and sometimes below.. Comfort is difficult to achieve in the first place and to do so while using the absolute minimum of energy is a monumental task..
Solar gain during the day versis the heat loss thru windows, natural light versis energy consumption, View versis cost. Winter versis summer, mass versis heat rise. Comfort versis cost. All of those will be massively differant in virtually every home built.. Even tract homes that are carbon copies of many other homes are affected by micro climates, location and other factors. The exact house on the opposite side of the street could have dramatically differant energy needs.. The big picture window faces the sun or North? One house has little children crawling around while the next house has two career workers.
I meant to suggest all that. People talk high/low mass as though those are real choices. It's a continuum among several dimensions, and each suggests a particular strategy. In my genre I deal with too many people who draw a few circles for a footprint, draw a few straight lines for walls, and think they are done. Hell, they've barely begun. And typically they don't even have the property.Residential design, if comfort is a major goal, is a tricky proposition, and not lightly undertaken. Too many over-simplify it.
And for any given mass there is a BIG DIFFERENCE between having the mass on the walls or other non-heated items and a large mass heat transfere source..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
...and if the walls, whether they're insulated interior, insulated exterior, un-insulated, subject to solar gain, and so on...