# Turn thermostat down?

Please forgive me while I troll for a moment.....
Is it energy saving to turn the thermostat down, when leaving the house? I mean, the furnace has to run to catch
up when I get home. I have a way of looking at the matter. I'll explain my point of view after the argument is underway.
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Christopher A. Young
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Imagine for a minute that you have to leave house for a month.
Would it be energy efficient to turn thermostat down? Of course, as less heat will be produced for a whole month, with only a few minutes to catch up.
The same applies to only one day.
i
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Ignoramus10802 wrote:

It's far more complicated than that. Factors such as insulation / heat loss, type of heating, multi-stage heating, electric backup heat on heat pumps, etc. all come into play in determining the away duration and temp reduction where savings begin, and in some cases (typically high efficiency homes) it can require a multi day absence to see any savings.
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This is patently untrue.
i
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Correct - whatever the net effect of insulation is, there is a net negative heat flux from the house to the outside. The flux is proportional to the temperature difference (the exact equation will depend on the radiation, convection and conduction components - radiation alone is governed by the Stephan-Boltzman equation). The larger the difference the greater the flux. Averaged over any period of time, any time spent with the thermostat set lower will yeild a lower internal temperature, hence less heat flux. Whether that is enough to show up in your bill is another question, but from a energy savings point of view, it is incontestible.
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The confounding issue, though, is the thermal mass of the house. That's why the DOE explanation says that the savings occur when the temperature inside the house has stabilized at the lower temperature.
When you shut off the furnace, the thermal mass of the inside of the house is what's giving up heat to the outside. That's stored energy that came from the furnace heat. When you raise the temperature, you have to restore that heat to the thermal mass. So with the furnace off and the temperature inside of the house dropping, you're losing stored heat. When you turn the thermostat back on, you have to restore that lost heat, which will also heat up the atmosphere inside of the house (which is a very small portion of the total inside thermal mass).
That's what I read from their description, anyway, and it comports with things I've read about it from other sources. There is no (theoretical) net gain or loss when the thermal mass is put through the cycle of cooling down and heating up. The savings occur when the temperature is reduced and stabilized.
This all assumes that a house is decently insulated and that the thermal mass of the house is substantial. Of course, the thermal differential between the inside and outside temperatures are always at work, suggesting that there is less heat loss with each degree of reduction of inside temperature, as you say. But the DOE's reference to actual testing agrees with the fact that, as soon as you turn the thermostat down, you begin losing *stored* heat, and when you turn it back up, 100% of that lost heat must be restored, regardless of actual thermal losses through the walls and ceiling.
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Ed Huntress

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Ed Huntress wrote:

Yes, if the house has only dropped a couple degrees when you get back, then there's little benefit. if the house cools rapidly to the lower temperature and stays there for, say, 7 hours before you return, then you get a benefit. Of course, if your house cools off very quickly, then you might do best to invest in insulation.
We had a big ice storm a couple years ago, and found we could be moderately comfortable for about 8 hours before firing up the generator to bring the furnace online. I think that means our insulation is doing pretty well.
Jon
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Either that, or you live in a cave.
You could judge how much of it is insulation effectiveness and how much is high thermal mass if you had a few thermometers, a clock, a calculator, and more time on your hands than anyone should have. d8-)
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Ed Huntress

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On Thu, 29 Oct 2009 12:46:13 -0400, "Ed Huntress"

Savings begins the instant inside temperature is reduced, whether or not it is stable at a lower setpoint. This is true regardless of how well the house is insulated, what the thermal mass might be, who the president is, which party controls congress or whether DOE likes it or not. Rate of heat loss at any (and every) instant depends upon temperature gradient from inside to outside.
Whether or not the savings on the energy bill is noticable is another question depending on how much the setback was for how long.
I wonder how much the DOE spent on a study to address what would be an easy test question in thermodynamics 101.
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The point is that what you're talking about is insignificant in most homes. That's what the DOE statement is all about: cycling of the heat mass overwhelms the effect of the insulation, until the temperature is reduced and stabilized for a while. Otherwise, all you're doing is cycling the heat retention of the thermal mass, with relatively much less actual savings as the temperature drops in the house.

Probably enough to know that you learned your thermodynamics with a calculator and a vacuum jar, rather than a house. <g>
I've seen those studies for years, Don, starting with a book I read in the '70s, titled something like _Low-cost, Energy-efficient Shelter_. What the DOE says is widely known.
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Ed Huntress

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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
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wrote:

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.
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wrote:

No, it's dead on, and based on a lot of actual experiments.

But he explained the wrong problem. He explained a theoretical thermodynamics problem. The question is whether the savings would be worth it if you only shut off the furnace for a relatively short time. And the answer is, unless you live in a lightly-built and underinsulated trailer, probably not.

Again, you're talking about a vacuum-bottle experiment. The question is whether the savings are worth coming back to a 50 deg house.

Approximately 8% of homes.

All of this is very nice for armchair philosophizing, Dan, but you have to know the actual *values* involved in the practical problem to know whether they're significant issues, in terms of your monthly heating bill. DOE has done the work, and I've shown what their actual experiments show. The savings are quite small in a typical house unless you leave the furnace off, or the temperature set low, for a long enough period for the STABILIZED temperature to be maintained for a significant portion of the total cycle.
Thermal mass works against you, by extending the ramping-down and -up portions of the cycle. Likewise, insulation.
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Ed Huntress

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"Is it energy saving to turn the thermostat down, when leaving the house?"
It was not whether the savings were worthwhile. Or whether a 50 degree house is comfortable. Incidentally the problem can not be accurately described by lump constants. The electrical analogy is a transmission line, not a capacitor.
Dan
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wrote:

Right. So the question was not whether it was worth doing, but whether one could calculate some reduction in total joules of energy by working it out with a calculator.
Save your pedantry, Dan. It's not illuminating. It's just annoying.
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Ed Huntress

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Well everyone else was answering the question that Stormy asked. You are the only one that decided to ignore the question and answer what you wanted to talk about. Which explains why you could not understand why so many intelligent people disagreed with you. And to be pedantic, the question was not about calculating the amount of reduction. There are way too many variables to do that.
Dan
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wrote:

Pfffht. Go correct a kid's homework or something.
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Ed Huntress

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On Fri, 30 Oct 2009 14:49:38 -0700 (PDT), " snipped-for-privacy@krl.org"

An accurate simulation is far more complex than any simple models using leaky buckets, capacitors or transmission lines. Building such a simulation that actually did show good correllation with observed (measured) behavior in a range of buildings and climates took some very competent senior engineers a couple of man years and a whole bunch of programming.
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Right. The transmission line analogy is a significant step closer than a capacitor and works well enough when one is working with something simple as a homogeneous mass surrounded by insulation. A house is not homogeneous and then there are other things as solar gain.
My current house has significant solar heat gain. On a sunny day the outside temperature can be about 40 degrees and the house will be comfortable with the furnace turned off. I take the window screens off in the Fall to maximize the solar heat gain.
Dan
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On Fri, 30 Oct 2009 12:31:30 -0400, "Ed Huntress"

[snip]
Really?
I'm not going to contradict you, since I've never seen any data, and heat pumps are probably not very effective in areas where winters stay pretty cold, but here in the sort-of-south (SC Appalachians), heat pumps are installed in nearly every house that is worth more than about 80 grand. My old 10 SEER unit works fine down to about 38F. If 8% is correct, then I'd say that a lot of people in the more temperate regions are missing out on a fair amount of savings, especially now that NG is about the same cost as electricity per BTU, oil is higher and LPG is outrageous.

I don't know what intervals the OP was thinking of (he didn't say), but I drop my setpoint for the 10 hours I'm gone to work, and for the 8 hours I'm asleep (well, not as much sleep nowadays, but, you know). I've done no studies to measure my savings, but even 10% would be great. If a large proportion of the population believes that they can save half of their energy bill by occasionally dropping their setpoint, well, that says a lot about our citizenry.
Joe