Turn thermostat down?

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.

That is another one of those, " It Depends". In this case on how long you will be gone. ...lew...

Turn it down. A lower temp in the house means a slower loss of heat to the outside. End of story.

"Catching up" is just getting the house hotter so it can loose heat faster again.

Pete Stanaitis

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Storm> Please forgive me while I troll for a moment.....

You start saving energy as soon at the house temperature stabilizes at the lower temperature. Except for very short times, when you let the temperature drop and then immediately ramp it up again, you always save energy by lowering your house temperature.

Here's what the DOE says about it:

"A common misconception associated with thermostats is that a furnace works harder than normal to warm the space back to a comfortable temperature after the thermostat has been set back, resulting in little or no savings. This misconception has been dispelled by years of research and numerous studies. The fuel required to reheat a building to a comfortable temperature is roughly equal to the fuel saved as the building drops to the lower temperature. You save fuel between the time that the temperature stabilizes at the lower level and the next time heat is needed. So, the longer your house remains at the lower temperature, the more energy you save."

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.

This is patently untrue.

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.

True. Yet I still hear this type of "reasoning" all the time. Should be a simple concept even for the technically challenged, for example, people who argued here that you can compress air and allow it to expand (while doing no useful work) with no loss of energy.

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.

That would be almost possible if compressing and expanding was done very quickly, before compressed air cools.

It's not as simple as a lab experiment in a vacuum jar, though, unless you put a big rock in the jar to represent thermal mass.

But it has to be done *awfully* quickly. That's why there's a minimum cylinder size for diesel engines -- something like 300 cc. Below 3,000 rpm or so, the compressed air cools too quickly to ignite the fuel. And heat transfer gets worse as compression goes up.

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.

But that only tells you that a lower-efficiency temperature-recovery system is...lower in efficiency. If you have that particular pair of heating systems, you have one situation. If you have a more-typical single heating system, you have quite another.

In the case you've described, you aren't dealing just with the thermodynamics of the situation. You're also adding the complexity of multiple heat sources that operate under different circumstances.

What you have there *is* a misconception in that it does not account for multi stage / mixed technology heating systems which are not that uncommon. A good example is a high efficiency heat pump with backup heat strips. Depending on the controls, such a heat pump may engage the backup heaters when it is unable to produce an acceptable rate of temp rise with just the heat pump, and this switches the effective efficiency from 300%+ to 100%, making it more costly to bring the temp back up to normal than it would have been to maintain it at normal. This situation was documented on a high efficiency model home.

That isn't a "pair of heating systems", nearly all heat pumps include backup heat strips for times when the heat pump is not able to produce enough heat such as very cold weather / high demand.

That complexity exists everywhere and that was my point - you have to do the actual analysis of the home in question to get the correct answer - you can't rely on blanket statements / myths.

An additional complication is occupancy, since for folks who are retired or work from home, or a stay at home spouse, you loose half or more of your theoretical savings period with the occupants not being away during the day.

Ignoramus10802 wrote: i

No, there is quite an element of truth! You have to compare the thermal mass of the house and the heat loss. If the house had enormous thermal mass, like lots of stone floors and massive stone fireplaces (some people build houses intentionally to have very high thermal mass) then although the furnace could warm the air quickly to make you comfortable, it would still run for hours to warm up all that mass. If you have a lot of thermal mass and low heat loss (good insulation) then turning the thermostat down for a couple hors gives no benefit. If you have low thermal mass and lots of heat loss, then turning it down for even a couple hours will give significant benefit.

Jon

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

Yeah, they are uncommon, in residences. Heat pumps of all types, cumulatively, are used in about 8% of the residential homes in the US.

I'm aware of the principles at work. They just aren't common.