Turn thermostat down?

What the DOE says is widely known, but incorrect. Don has explained the problem correctly. Thermal mass and insulation determine the thermal time constant, which affects how much saving results. But has no effect on whether there is a savings.
As Pete C. points out there are some heating systems that change efficiency depending on the demand. Heat pumps are one case. Another case is modulating furnaces. These will be less efficient at higher loads. But the common furnace located in a non heated area, will be somewhat more efficient as the furnace will run for a longer time before shutting off and loosing heat to the unheated area.
Dan
Hot water reset systems also operate at higher efficiency if the demand is lower, since the water temperature is reduced, resulting in a greater delta t in the boiler and more efficient heat transfer. With the right insulation and very limited air changes a steady state high efficiency heating plant without a setback might be more efficient. But very few residential systems operate like that.
Reply to
ATP*
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In the winter I have to run the furnace or it gets cold. That means heat leaks out.
If it is leaking out, don't put as much in to leak out while you are gone.
Simple ;)
Wes
Reply to
Wes
So is my hair, more or less, but I wouldn't make a big deal about it. d8-)
Reply to
Ed Huntress
:Pete C. wrote: : :> No, it isn't. It was documented on a well monitored high efficiency :> model home where the backup heat strips on the high efficiency heat pump :> were kicking in in order to provide a reasonable temp recovery time :> since the heat pump itself did not have the capacity. The electricity :> used during the temp recovery was more than would have been used on temp :> maintenance due to the switch to lower efficiency backup (100% vs. :> 300%+). : :OK, this is a killer example, where the furnace efficiency goes down the :tubes when it needs to raise the temp suddenly. It might be possible to :stage the temp rise to avoid that with a suitable thermostat. : :But, many other heating systems have no such penalty for a rise in temp :setting, such as a traditional gas forced-air furnace.
My gas furnace is 2-stage, and not particularly exotic (a common Trane model, 80,000 BTU). Presumably it's more efficient when operating on low-heat, as otherwise there's little point in the added complexity of a 2-stage burner and control. Except in really extreme weather, the only time it runs on high-heat is during recovery from night time setback.
Reply to
Robert Nichols
I'm in the midst of optimizing a brand new, Bryant 80% forced air, 2 stage, 110k Btu furnace. It sucks every BTU out of things when running in the low fire mode. On high fire it is just trying to get you comfortable enough to let it get back to the economy mode. I doubt that the difference is more than a few percent but I suspect it is measurable. If nothing else, the high fire mode requires the high speed fan and attendant increase in electrical use.
J>
Reply to
RoyJ
On Thu, 29 Oct 2009 10:54:59 -0500, the infamous Ignoramus10802 scrawled the following:
Think about what you just said, Ig. The better insulated homes can go days without an injection of heat or cooling. Poorly insulated homes can't go an hour without in exteme climes. Using a setback thermostat in a well insulated home has much less impact (if any for a day) than it does in a leaky old shack. Your comment was the untruth.
I've used setback thermometers since 1975, when I got my first home. (It was a leaky old shack, but that wasn't too bad in temperate LoCal)
Reply to
Larry Jaques
I have a programmable thermostat that go goes from 68F to 60F shortly after I leave for work till just before I arrive home, then again about 11PM till 5AM when I get up. Hard to say if it's made a significant difference, but I installed the thermostat when I had to replace my furnace in 2001 and I haven't really tracked the gas use. My house was built in 1894 and has little insulation, temperature in the winter can vary 8-10 degrees from near the heat register to the farthest corner of the living room.
David
Reply to
David R.Birch
Let the Record show that "Pete C." on or about Thu, 29 Oct 2009 13:30:44 -0500 did write/type or cause to appear in rec.crafts.metalworking the following:
OTOH., we can turn the heat down in the rest of the house, warm just the office, and double up the blankets at night. Just don't let the pipes freeze. B-) (that means no coffee.)
pyotr - pyotr filipivich We will drink no whiskey before its nine. It's eight fifty eight. Close enough!
Reply to
pyotr filipivich
Let the Record show that Lewis Hartswick on or about Thu, 29 Oct 2009 06:53:05 -0600 did write/type or cause to appear >> Please forgive me while I troll for a moment.....
Bingo. How long will you be gone, how low you set it, and how fast does it take for the inside to cool down, and warm back up. If I live where it is below freezing all the time, and the house is drafty - I may shut the heat down more when I go out, because I can't afford to heat all of Kansas! (OTOH, I might want to stand downwind of the house and enjoy the heat I am paying for.) If I have a snug warm house, that doesn't cool rapidly (lots of thermal mass) then it doesn't matter _as much_.
When you come back, what are you going to do? Cooking dinner in the oven is a good for reheating a house - or at least the kitchen. B-)
And so it goes. - pyotr filipivich We will drink no whiskey before its nine. It's eight fifty eight. Close enough!
Reply to
pyotr filipivich
Let the Record show that jeff_wisnia on or about Thu, 29 Oct 2009 15:59:36 -0400 did write/type or cause to appear in rec.crafts.metalworking the following:
Ah, but there is a difference here. As I heard it, God created Adam, and he said "Yoicks its hot! I can't wait to invent air conditioning." Shortly afterwards, God takes advantage of Adam knocking himself out trying, and creates Eve. Her first words are "Good God, it's cold in here, why not create some warm clothes and central heat!" And it has been a problem ever since. - pyotr filipivich We will drink no whiskey before its nine. It's eight fifty eight. Close enough!
Reply to
pyotr filipivich
Well, ideally you should be zoning and not heating long unoccupied rooms beyond maintenance level anyway. Certainly I have the dampers mostly closed on my spare bedroom and dining room pretty much all the time, only opening them on the rare occasion those rooms will be occupied.
Reply to
Pete C.
In my house..the ex is the one that likes it cold..I like it warm, but not hot.
She considers anything above 50 to be tropical. Who else has a large ceiling fan, and a 20" fan at the foot of the bed, and a 20" fan to her right..all blowing on High nearly all year long?
We have a very large King sized bed..and she puts extra blankets on my side of the bed. Even the dogs cuddle up with me in mid winter..on the left..sheltered by me from the freaking fans.
IMHO, some people here give Jeff far more attention than he deserves, but obviously craves. The most appropriate response, and perhaps the cruelest, IMO, is to simply killfile and ignore him. An alternative, if you must, would be to post the same standard reply to his every post, listing the manifold reasons why he ought to be ignored. Just my $0.02 worth.
Reply to
Gunner Asch
You in Kansas? My sister is single..and working for a Sherriffs department in Kansas.....
Interested? Rather a hotty too as it happens.....
Not really my sister..but has been my "sister" for humm...20 yrs now. I think she is 40..41ish.
If so..Ill turn you on to her face book page..and give you a strong recommendation......
Gunner
IMHO, some people here give Jeff far more attention than he deserves, but obviously craves. The most appropriate response, and perhaps the cruelest, IMO, is to simply killfile and ignore him. An alternative, if you must, would be to post the same standard reply to his every post, listing the manifold reasons why he ought to be ignored. Just my $0.02 worth.
Reply to
Gunner Asch
Furnaces cycle anyway, and often at about the same cyclic rate almost independent of temperature or heat load. To meet higher heat load (higher indoor temp or lower outdoor temp) they just have a higher percentage of "on" time each cycle. Even the venerable Honeywell "round" (T-87), which has been around for over 50 years, did and does a surprisingly good job of this by virtue of it's anticipator -- which almost nobody outside of their engineering org really completely understood.
They'd be wrong about that too. During my 33 years with Honeywell I worked with engineers and scientists worldwide,including those from their homes and buildings divisions.
I was invited by DOE just last week to serve as a technical reviewer for buildings programs. (I'm not going to do it.) I was also on the Technical Advisory Board for DOE's Brookhaven National Labs back in the late 90's.
It is indeed, and it's known in the technical community to be wrong.
Reply to
Don Foreman
Not quite, Jon. You're right that with good insulation and high thermal mass cooldown is very slow even if the heat is shut off completely. Setback can't save much if the temp inside the house doesn't change much.
When the setpoint is again increased, the air temp (which is what the t'stat responds to) can rise to setpoint fairly quickly -- but it will also be cooled more quickly during the off cycles by the cooler thermal masses with which it is in contact. So the furnase runs with higher duty cycle during the warmup period -- but recall that it may have run very little if at all during the cooldown period.
Reply to
Don Foreman
Imagine a bucket with a pinhole leak near the bottom. Let's consider the level of the water in the bucket as analogous to temperature in an insulated enclosure. The diameter of the bucket determines the mass of water needed to reach a given level. It's a valid analogy because temperature is a measure of thermal potential, water height determines pressure pushing water out the pinhole leak. Water leak rate is qty of water per unit time, heat leak rate is qty of heat energy (Joules, BTU, etc) per unit time.
Let's rig a little toilet-valve arrangement to maintain the level except that it will have hysteresis: it will click on when water is below a certain level and refil the bucket until the water rises some incremental amount whereupon it will click off. Refill will stop and the level will gradually go down because of the pinhole leak. How long it takes to go down depends on the size of the leak (insulation) and the diameter of the bucket (thermal mass).
I hope it's apparent that if the leakrate is 1 gallon per minute then the average replenishment rate must also be 1 gallon per minute. If it's less, the level will recede, if it's more the level will rise.
If we now lower the height of the toilet valve, there will be no influx until the bucket leaks down to that level. If we then raise the valve, the valve will stay open until the water has risen to that level. But the average long-term water consumption will still be the leakrate.
Note that this is true regardless of the diameter of the bucket.
It is true that both leak rates depend on potential (depth or temperature difference) so differential equations are required to express them correctly. This does not change the fact that what goes in must equal what goes out long term if the long-term average temperature is constant over several cycles of moving the setpoint up and down.
Reply to
Don Foreman
Let the Record show that "Pete C." on or about Thu, 29 Oct 2009 23:04:40 -0500 did write/type or cause to appear in rec.crafts.metalworking the following:
My problem is that the 'office' is the one room with the worst heat. So I just turn the thermostat down for the house, and use a space heater in the office, and in the bathroom. And dress warm. - pyotr filipivich We will drink no whiskey before its nine. It's eight fifty eight. Close enough!
Reply to
pyotr filipivich
Let the Record show that Gunner Asch on or about Thu, 29 Oct 2009 21:52:22 -0700 did write/type or cause to appear in rec.crafts.metalworking the following:
Ooh, sounds interesting.
Alas, I have not lived in Kansas ... ack. Since before she was born. There were only 48 states! No wait, that was after the start of Camelot ... never mind. There were fifty. It just seems like it was shortly after the glaciers receded, when the central sea dried up...
Alas, that is too far, or not far enough. I'm considering looking into following the Boeing move. - pyotr filipivich We will drink no whiskey before its nine. It's eight fifty eight. Close enough!
Reply to
pyotr filipivich
The Garden of Eden was obviously not in Minnesota. Minnesota Mary thinks 40's is wet sheet weather while Bubba Don about can't sleep unless he's sweating a little.
I can relate to Sam McGee, oh yeah! Don't bury me in cold Fort Snelling, please, take me to a roaring furnace!
Reply to
Don Foreman
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Reply to
Don Foreman

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