This is an issue that constantly comes up, and we generally tend to base what we do on how long we are going to be out. But it would seem to me, based on my rudimentary understanding of physics/thermodynamics, that pure energy considerations would dictate that you always turn it off. IOW that it will always consume less energy by being turned off and then turned back on, as opposed to being left on, no matter how long your absence.
This is because the total amount of energy needed is the amount needed to offset the amount of heat that comes in through the insulation. And the total amount that comes in will be in proportion to the difference (or maybe the difference squared?) between the two areas, i.e. the inside and outside of the house. So that the cooler the house is kept, the more heat comes in and the harder the a/c works to offset this. The warmer the house is kept, the less additional heat comes in, and the less energy required to offset it (when it is ultimately turned back on). At the extreme, when the inside temperature matches the outside temperature, no additional heat is coming in, and no additional work is needed to cool it off.
(Of course, this is looking at strictly energy grounds. Some people may want to leave the system on when leaving for a short trip simply because they want to come home to a comfortable house, and don’t mind paying a bit extra for this).
The correct answer is “turn it off to save energy”. Good luck on finding a consensus on this. I even did some sample calcs at some effort a while ago on here to prove that this should work from a heating standpoint, and no one even looked at them or cared - they just kept repeating (the very next week) the same incorrect assumptions and BS.
Since the issue most people are facing with this question is partly efficiency but also partly comfort, it seems to me that any calculations you do need to take the thermal mass of your home into consideration. (I read the other thread but didn’t see this mentioned. If it has been, I’d be grateful for a link.)
That is, if your house has a Relatively Large Thermal Mass, if you allow it to absorb heat while you’re not around, it will return it to you when you come back in the evening, even though the outside temp has dropped. Therefore, if you have a RLTM, it may be worth it to leave the temp set lower while you’re gone.
If, on the other hand, you have a little house on the hillside and it’s all made out of ticky-tacky, like ours–zero insulation and a frosting-layer foundation-- cooling it when it’s empty won’t help you feel appreciably more comfortable when you come home.
What if you happen to have twelve windows facing all three north, south and west (being an important factor, as the hottest sun of the day seems to always be late afternoon), your apartment building has aluminum siding, your floors are all lacquered hardwood, it’s nearly 70% humidity and 90º F, and your apartment is an incredibly long and narrow railroad apartment. Without being a physicist or one with much skill in that area of science, I think that I still would have to disagree on a few levels. Now don’t get me wrong, if all you physicists out there are right, you’re right, but hear me out: what about the laws of convection? The building materials of my apartment building and the weather conditions of NYC are as such that my apartment in no exaggerative way turns into a convection oven when the A/C is turned off for even an hour. And from personal experience, it always appears to take an incredibly long time to re-cool it if we do leave them off when leaving the house. From a strictly energy conserving stand-point, I can see how it utilizes less energy to turn them off, but take these other factors into consideration (especially since I have pets that might die if the heat becomes too unbearable). Another point to consider is the relative amount of BTU’s to square footage in a given living space… wouldn’t the square footage and BTU’s have to factor into the physics as well? Two 5,000 BTU A/C’s placed strategically at opposing ends of the apartment with fans circulating the conditioned air to the center seems to work well for us. It just seems to simple to do the physics without considering first the weather and the conditions of the space being air conditioned. But I’m not a physicist.
emilyforce. the answer to your scenario is a timer: turn on the A/C, say, 1 hr before you return and turn it off 1 hr, before you leave. It works the same with heating.
sailor, absolutely, of course; timers work. If you have one. I just meant to point out that the equations and logic showing (indisputably, AFAIC) that it’s less expensive to heat only while you’re in the house don’t account for the realities of comfort vis-a-vis thermal mass.
I leave mine on, leave the windows closed, but turn up the thermostat.
The major reason is to keep the humitidy down. When I return, it takes just a moment to get the temperature down, its getting the humidity down that takes hours, and that is why you should leave it on, but at a higher temperature, to keep the humidity down.
I started a thread on this regarding heating. The consensus is that when you turn down the heat, and the house reheats, you use the same energy to reheat that you saved in the cooldown process. However, there is a savings in that the heat loss is lower during the time that the house temperature is lowered, because heat loss is proportional to the difference between the temperature of the object and the (cooler) temperature of the ambient air.
But with cooling, another factor comes into play. A house does not heat only from heat transfer from the ambient air.
It heats from radiation from the sun, also. I will posit, but don’t really know, that heat transfer due to radiation is constant and not related to the temperature difference, so I would think that the same answer holds as with the heating scenario.
Just don’t turn off your air conditioning and close up for 3 months in the hot, humid Alabama Summer.
Yesterday was the first day of school and now every school in Alabama is calling me wanting to know how to clean their classrooms that are covered in mold.
It changes nothing. The fact remains that net heat transfer into the apartment/house decreases as the indoor temperature becomes higher. If you leave the A/C on while you are away, the indoor temperature is lower than if you’d turned it off so there is more heat transferred and more work for the A/C.
This thread is about energy conservation only. If you have pets and need to keep the AC on then go ahead, all I’m saying that it costs extra to do so.
If you want a quantitative analysis then yes, you need to consider the square footage and all that. But no matter how you juggle the numbers, you can’t build a house which uses less power to keep the A/C on all day than to turn it off when you are away.
As for preventing mold, a ventilator fan and/or an open window is usually sufficient.
By the way, CookingWithGas is correct about readiative heat input by sunlight is a constant and not dependent on the indoor temperature.