Leave AC on or off all day? (small apartment)

[Jeff7]

<thermodynamics and heat transfer stuff>

http://michaelbluejay.com/electricity/cooling.html

This.
Because science. That's why.
Arguing against that is like arguing against the value of insulation.

But if the AC is on when you're gone, then you've turned your house into a heat magnet. [By] keeping it artificially cool, there's no limit to the amount of heat it can absorb.

I like this way of putting it. ^


Or put another way, it turns your house into a thermal sink, up to the maximum capacity of the air conditioner, and you're paying for every kWh.
If the house isn't being cooled:
- As it approaches equilibrium with the outside temperature, the rate at which it heats up will decline. Energy will hurry across a resistive barrier (insulation, or a wall) in a big hurry if there's a big difference in temperature. As the difference gets smaller, the energy doesn't cross the barrier as quickly. So minimizing that difference in temperature will slow the energy transfer. That's why it saves energy to run your house cooler in the winter: If you've got it toasty inside, that energy is trying hard to get outside. If you've got it frigid, only 10° above the outdoor temperature, the energy inside isn't going to be trying as hard to get out.
- Once the house reaches equilibrium with the outside, the energy has no motivation to move.
The only other factor then is sunlight, which will warm the house. Fun part: As the inside of the house starts to heat up above the outside air temperature, the excess energy inside will now try to make its way out. So you're now dumping excess energy out of the house for free.




I do set the timer on my AC though, so that it turns on a few hours before I get home. It's got enough BTUs to cope with the summer heat, but since it doesn't have any kind of 100,000 BTU surge capacity , and because this apartment seems to have tissue paper for insulation, it takes awhile to get the place back down to a reasonable temperature.
The AC may be a bit undersized for the space it has to cool, but my choice of window unit is limited by:
- 120V lines are available. 240V would require new wiring.
- A large 240V unit probably wouldn't fit in the window.
- My spine would crush itself into something resembling calcium diamonds.

 

[jagec]

Is that less wasteful than just letting the AC maintain during the cooler part of the day (morning to 3) and not having to struggle to drop those degrees later on when it's hotter outside?

Yes. Yes it is.

Not only does less total heat need to be removed if you turn the AC off entirely*, but heat pumps (which includes normal AC units, as they are still "heat pumps" just not reversible heat pumps) are actually less efficient as the gradient between the hot and cold temperature increases. So your AC unit will need to pump MORE heat LESS efficiently if you leave it on all day.




*You've got a bucket with a small hole in it, and a faucet. The faucet can fill the bucket faster than the water can leak out of it, but the more water in the bucket, the faster it will leak.

If you want to have a full bucket at 6 PM, is it better to

a)Fill the bucket at 8AM, and turn the faucet on in short bursts every half hour all day long in order to replace water lost through the hole, or

b)Wait until 5 PM (or whatever) and turn the faucet on, letting the bucket get completely full by 6?

Which case will result in more "on/off" cycles for the faucet?
Which case will result in more wasted water?

 

[SlitheryDee]

It is totally true that leaving your a/c off is the most cost effective solution, but increasing the temperature on the thermostat during work hours to maintain a livable temperature during the day for pets will only amount to a very small increase in your electric bill. I've learned that by experience.

If you have no reason besides a misguided notion of efficiency to leave your a/c on, by all means turn it off. Also don't think that you are breaking the bank by keeping your dog moderately comfortable.

 

[ShawnD1]

If you have no reason besides a misguided notion of efficiency to leave your a/c on, by all means turn it off. Also don't think that you are breaking the bank by keeping your dog moderately comfortable.

It probably is breaking the bank. No joke, it's running AC all the time costs about $100/mo here.
The best solution is hang out in the basement. Even without AC, the basement is usually a lot colder than the rest of the house by a huge margin.

 

[SlitheryDee]

It probably is breaking the bank. No joke, it's running AC all the time costs about $100/mo here.
The best solution is hang out in the basement. Even without AC, the basement is usually a lot colder than the rest of the house by a huge margin.

I've set my programmable thermostat to 85 degrees during work hours and 68 degrees when I'm home for the last 2 years. The bills have averaged about $10 more per month compared to when I turned the a/c completely off during work hours. There is definitely a difference, but it ain't $100 per month. My house insulation is good enough that I doubt the ac cycles more than a couple of times per day at that temperature.

I wouldn't leave it on at all if I didn't have a cat. I assume that there are heat levels that even she can't tolerate, despite her love of basking in sunlit windows. All I'm saying is that it didn't make an enormous difference to set the thermostat a bit higher when I'm away vs. cutting the ac off entirely.

 

[Jeff7]

n/m, misread something

 

[Red Squirrel]

It uses less energy to go to a certain temp, then to keep it there. This goes for anything, heat too.

Though one thing with AC is that as the compressor and condenser coil is running it heats up to a point where it may not be running as efficiently. So if it cycles on/off (not too often, but say, once an hour) then it will run more efficiently. At least that's my theory, I could be wrong.

I have a 1 ton AC unit for my office that's less than 100sqf so I only turn it on when I get home. Nothing like walking into a 28C room and hitting a wall, turning that unit on, GTFO of there, wait half an hour go in and it's like 23. :biggrin: Though if I'm working out or gone for a walk I'll turn it on ahead of time to precool the room for when I'm done.

Thankfully my house does not get that warm unless we get a huge heat wave. It usually sits at 22-25 which is a little above comfort but not to the point of wanting to die. I try to open all the windows at night to let it drop.

 

[nageov3t]

in my own experience, window units (as you'd typically find in apartments in the Northeast outside of newer buildings) don't really do a great job at maintaining a temperature.

mine has an option to, but in practice, it runs until it reaches that temp, turns off, and then kicks back on again when it gets a few degrees warming... so it's just constantly turning on and off.

 

[SlitheryDee]

in my own experience, window units (as you'd typically find in apartments in the Northeast outside of newer buildings) don't really do a great job at maintaining a temperature.

mine has an option to, but in practice, it runs until it reaches that temp, turns off, and then kicks back on again when it gets a few degrees warming... so it's just constantly turning on and off.

That's what central air does too. The only way you can reduce cycle time is to either cool well past the thermostat setting so that it takes a while to get warm enough to cycle again, or to improve insulation in the area being cooled. Turning on and off rapidly just means its keeping the temperature in a very narrow range or that the insulation sucks. Either way it's doing it's job adequately.

 

[Red Squirrel]

in my own experience, window units (as you'd typically find in apartments in the Northeast outside of newer buildings) don't really do a great job at maintaining a temperature.

mine has an option to, but in practice, it runs until it reaches that temp, turns off, and then kicks back on again when it gets a few degrees warming... so it's just constantly turning on and off.

Most heating/cooling appliances work like that. The most efficient scenario is to size it EXACTLY according to the heat loss/gain rate. But it's best to oversize slightly for expansion/temp changes. To prevent short cycling my furnace in winter my thermostat actually lets the temp drop to say, 13, and then it kicks on until it hits 15. When I'm home I may not want such a large temp swing so I may set it to 18-19. So when the temp gets to 17.999 it kicks in until the temp is above 19, so say, 19.001. I'm not sure why this can't be setup in consumer thermostats though, I rigged this up myself with a temp/relay controller and a small server. When I get central air I'll do the same for AC mode.

 

[EagleKeeper]

Spend the $20-50 to get a programmable thermostat.

Set the temp on high 15 minutes before you normally leave and set it to the 70+ that you want 15-30 minutes before you get home.

 

[Red Squirrel]

Yeah programmable thermostats are the best.

You can also go a step beyond and make your own like I did, but I only did that because I work shifts so my days and hours are all over the place so I kept always having to play with the program, now it's set once a month and forget. THough in summer it just runs in "off" mode and I only run the blower once in a while. No AC. Something I want to invest in eventually though.

 

[SP33Demon]

Yeah sorry, aaint turning the AC off so my 95f heat sink will be waiting for when I get home.

This. Also, setting it higher will lower your bill since it will kick on less. This is fcking common sense people, along with the post that said turning it off will lower your bill. Common.sense.

 

[QuantumPion]

I leave mine on 73 because when it's 100 degrees outside it has to run continuously all day long just to keep up regardless of what I set it to.

 

[Svnla]

I always turn off everything while I am not at my place (small 1 bedroom, all electric, and less than 800 sq. ft.).

 

[0roo0roo]

turn it off, i think mythbusters did this?