That sounds like a convenient thing to believe, similar to family telling me to just leave the heater on all day on a base level heat to supposedly save energy because it doesn't have to do a lot of heating when I come home from work. But, obviously, a lower temperature differential between inside and outside is going to lose less heat/cold, so unless the heater/AC is more efficient at lower power levels (they're not) it cannot possibly make sense to leave the heater/AC on when you're not making use of it to save energy. Sure it'll have to work harder when you get home, but that's better than it trying to maintain that temperature all the time. I guess people want to believe this though because you'll have a higher thermal comfort if you don't come home to a hot/cold house.

I tried to find the source for this claim too in the past. The nearest I can find is that it implies the HVAC system is sized correctly: if the system is optimally sized, it should need to run most of the time to stay at the desired temperature. If it changes the temperature too fast, then it may end up with wild temperature swings that overshoot the desired comfort level and waste energy that way. In certain situations, with good insulation and building design, this waste can apparently exceed the savings from a few percent lower degrees for part of the day. If I understand correctly, this is a bigger problem for heat pumps that take a long time to spool up/down so simple bang-bang hysteresis control has poor performance in this scenario.

Edit: as another poster mentioned, for heat pumps (like AC, but frequently not furnaces), it is also much more efficient to run them when it is cool outside (the morning), and then maintain that temperature through the day as a cold reservoir, than to try to expel the heat when it is hot outside. Which is probably a better explanation for the effect.

I find that a bit hard to believe.

The rate of heat flow across an insulator is proportional to the temperature difference across the insulator, if I recall correctly.

Suppose you want the inside of you house to be at temperature Tin when occupied and the temperature outside is Tout.

The rate of heat flow across the boundary between inside and outside will be k|Tin-Tout|.

Imagine two identical houses.

In house #1 we keep it at a constant Tin. In house #2 we keep it at Tin while occupied but when empty we let it move toward Tout, turning on heating or cooling when it is going to be occupied again to get it back to Tin when the people return.

For each house, if we plot |Tin-Tout| over time, the total energy used over a given interval will be proportional to the area under that |Tin-Tout| curve.

For house #1, that curve is simply a horizontal straight line.

For house #2, that curve matches the curve of #1 while the house is occupied, but during the unoccupied times falls toward zero until it is time to get it back to Tin in anticipation of people returning home. It then rises back up to Tin.

The area under house #2's curve will be less than the area under house #1's curve.

Thus, in terms of energy required to produce the desired temperature curve, house #2 uses less than house #1.

There are inefficiencies in producing and applying that energy, though, and so getting a given amount of energy applied to heating or cooling the house from your heating or cooling system will take more energy, and that extra energy might depend on Tin or Tout, so it is possible that this might be a big enough effect to counter the savings from letting the unoccupied temperature move toward Tout.

Isn't it harder to heat 32 degree air 1 degree than heat 70 degree air?

This was my 400 level thermo 2 class, we calculated everything, inefficiency, convection, conduction.

> Isn't it harder to heat 32 degree air 1 degree than heat 70 degree air?

From a pure heat capacity viewpoint, heating cooler air is very slightly easier (on the order of a few thousandths at most for realistic temperatures) [0]. Don't think it'll be noticeable for the average heating system, though.

> we calculated everything, inefficiency, convection, conduction.

I'd be curious to how those calculations affect things. If the heating element is the same temperature, I'd think heating cooler air would be faster because heat conduction is faster across a larger temperature differential and you'd get "faster" convective mixing for air with bigger temperature differences. Have to admit I'm not sure what inefficiencies might come into play, and I haven't taken classes at that level so I wouldn't be surprised if I missed something.

[0]: https://www.engineeringtoolbox.com/air-specific-heat-capacit...

I could not find the problem I was referencing. I'm either wrong, or the problem wasn't in the book and was in the professors notes. I'm leaning on the later, but unless I reply with something else, assume I'm wrong.

Sorry to hear that you couldn't find the problem. Was hoping that I'd get to learn something new

Much of the energy drain when heating/cooling a home is the fan to blow the air. These days, keeping the fan running is quite efficient. Turning it off, then putting in a large draw to get it moving again is expensive.

So at least part of the efficiency comes from not restarting the fan repeatedly.

My furnace has an 80,000 BTU burner (equivalent to 23,000 watts), and a 1/2 horsepower blower, which would use around 600W.

So the blower uses less than 3% of the energy used by the burner. Some of the waste heat from the blower motor probably helps heat the home, so in reality it's probably less than that.

While startup current is significant (for a very short time), starting the fan up in the morning and again in the evening after work doesn't add that much to the total power drain, or even cycling it on/off a few times an hour to maintain temperature. Starting wattage on that motor is around 1800W, but only lasts for a few seconds.

This is wrong on several levels. The energy expended by a fan is proportional to the mass flow. The energy required to accelerate the rotor is negligible compared to the ongoing energy required to keep it turning and doing work. Also, I'm pretty sure that the energy use of the fan is itself negligible compared to the heat pump.

Hm - everything people are saying makes sense. I was repeating what I've been told by literally every HVAC guy I've had work on multiple houses over the last 10+ years. If the professionals are all wrong, I suppose it could just be one of those tropes that persists in an industry?

If the fan is off for at least a minute, the energy to start it should be much smaller than the energy to keep it going. If that's not true, then fix the fan design.

What fan are you talking about ?

This seems like it would heavily depend on how well insulated the house is, and for how long it will be unoccupied.

Based on my experience with most houses’ insulation, especially bigger ones, I cannot imagine it being economical to cool it for 8+ hours when no one is there (for work and school), rather than letting it warm and then starting the AC 30min before people start coming back home.

The short answer is that you should keep the house relatively cool, but maybe above your comfort level while you're not there, and well insulated. The details of when it's best to run the AC for maximum efficiency pales in comparison.

The AC will be more efficient in the morning, for one, than during the heat of the day. And if you let the entire house get truly hot inside, it will take a lot longer than 30 minutes to bring the temperature down (it's not just air that has to get cool, it's all the solid objects as well). And when it's fairly hot outside, it may not actually be possible to get it cooled down if you wait until the heat of the day. E.g. my AC will keep my house at 70F when the temperature outside is over 110F, but if I let it drift up to 80F inside and then try to bring it down to 70F in the mid-afternoon, it won't get any lower than maybe 75F until the ambient temperature outside drops into the 90s.

The same effect works in reverse though, maintaining temperature through the heat of the day is wasteful if the temperature drops before you get home. If you’re dealing with significant solar gain then running a fan before turning on the AC becomes even more efficient.

Only if it's already cooler outside than inside the house. On a day where it's only 90 out during the day, it's not dropping to 75 until 8-9pm at the earliest.

Also can't use a fan on the west coast in the later parts of the summer when the air is full of wildfire smoke.

An example of significant solar gain would be a car sitting outside in sunlight. A surprising amount of modern architecture would work just fine as a greenhouse. Assuming a 9 hour workday it’s going to get well above ambient before you get home.

I could not find the example problem I was referencing in my textbook, consider me either incorrect or ambiguous until I do. I'm going to look through my notes next. I believe it was a problem from my professor.

I remember this problem being absurd, like bigger than a mansion.

Gosh I wish I could find it. I've referenced this problem a few times, I need to find it. I might have the book, if I can find it I'll update this post.

These jackets are powered for hours from a rechargeable battery about as big as a fist. I don't know of any house-sized AC unit that could run from such a battery for more than a few seconds. So that doesn't make intuitive sense.