I live in a home with 2 other roommates. One is a neuroscientist from MIT who contends that opening the window "wastes a week's worth of cooling down the apartment". Her perspective is that even if there is a 10 degree temperature difference between indoors and outdoors, then all AC utility is lost.

I agree there is a point where this is true, but not to the extreme of leaving the AC on for a week or two will meet this happy temperature saturation point.

My argument (not successfully explained) is that the apartment (2,000 sq ft) cools to a reasonable temperature quickly when two AC units are on, furthermore it will take a long time to reach maximum cooling. Even if maximum cooling is desired, it's just not efficient past a certain point because cooling loss through the glass windows means the AC unit must compensate for that.

Furthering my annoyance is that the apartment is empty for the entire daytime (9am-7pm) hours, when the outdoor temperature drops, making the two AC solution even more efficient in my mind.

The 2005 model AC doesn't list the BTU, but the following info is on the sticker:

Volts 208
Total Amps Clg 7.4
Total Cmps Htg .82 
RA Fan Amps .8
RA Fan HP .08
DA Fan Amps .83
DA Fan HP .13
Compr LRA 38
Compr RLA 7.1
R-22  35.1 oz
Design Pressure: 350 PSI

Am I correct in saying the two AC solution is more efficient? At which point would it be more efficient to leave the single AC on all week?

  • $\begingroup$ I need some clarification on your question. Are you asking for a choice between two options: 1) leaving a single A/C unit on all the time (although subject to its thermostat), versus 2) opening up the windows during the day, but having a pair of units to quickly cool the apartment down at night? $\endgroup$ Jun 12 '15 at 23:56
  • $\begingroup$ @DanielGriscom - That is correct, that is the choice $\endgroup$ Jun 12 '15 at 23:57
  • $\begingroup$ I actually don't know the physics (I'm curious about it), but my experience in very humid Japan is that turning on the AC results in a comfortable temperature in less than an hour, and turning it off results in discomfort within an hour - which suggests that leaving it on all day is more or less pointless. With heating in winter it seems different, with the time scale being several hours - I've often wondered why. (Note that Japanese apartments tend to have a light weight construction with no double glazing and little insulation.) $\endgroup$
    – Nathaniel
    Jun 13 '15 at 0:34
  • 1
    $\begingroup$ A couple of points 1) the function of air conditioning is not just cooling, but also dehumidification 2) calculating the amount of energy required to cool a volume of air depends on the initial and final relative humidity. $\endgroup$
    – paisanco
    Jun 13 '15 at 1:22
  • 1
    $\begingroup$ Your neuro roommate clearly knows no physics. It's trivial to look at the power rating of an A/C unit and calculate the cost of the kilowatt-hrs it took to run for however long it took to cool the house down. And further, it should be bloody obvious that the interior (assuming shades, etc) won't get much hotter than outside no matter what you do, but if you run the A/C all day you're constantly "shoveling against the tide". $\endgroup$ Jun 13 '15 at 12:06

Think of an air conditioner as a device that pumps heat from inside a room to outside. To bring the temperature down to a given point, an amount of heat needs to be pumped out of the room. If the room is perfectly insulated it will take a fixed amount of energy to pump that amount of heat out, call that e. It will also take a fixed amount of time call it t.

Now assume that the room is not perfectly insulated, but that heat is transferring into the room. We know that the rate at which heat transfers is always faster the greater the temperature difference between inside and outside. Now to bring the temperature down to a given point, the air conditioner needs to pump out the usual amount of heat, plus the amount that transfers in -- so it will take e+et (where et is the incremental energy to pump out the transferred in heat,it is based on the rate of transfer in over time t).

Now that it is down to temperature, heat continues to transfer in.. so it will take eh energy to hold the temperature (eh is the energy to pump out heat as fast as it is transferring in over the time the temperature is held down).. This is now the scenario where you leave the ac on all day.

Now think of the other scenario.. you leave the AC off all day and turn it on when you need it. Again to bring the temp down to a given point it will take e2+et2.

So now lets compare them.. e+et+eh < or > than e2+et2 ?

e and e2 are the same, (if you assume the outside air is constant temp.. the inside will be heated to the same level for both scenarios)

et and et2 -- are equal. based on the same assumption the starting temp is the same, the outside temp is the same, so the amount of transferred in heat during the cool down is the same.

So it looks like under these conditions the outstanding factor is eh, so it is better to leave the AC off all day and turn it on when you come home.

You might question the assumption of constant outside temperature.. but if you look at things... even if the temperature rises during the day and thus leaving scenario 2 with a higher starting temp for the cool down (e2 > e), et and eh are now larger than they were before. (e2 - e) is always < eh (Since there is no heat transfer in once the room temp equalizes with outside temp for scenario 2, yet there is a constant transfer in in scenario 1 because the ac maintains a temperature difference, and remember the greater the temp difference the faster the heat transfer).


Let's assume a few things:

  • You never open the windows
  • You're only dealing with dry air
  • Air conditioner efficiency doesn't depend on the air temperature involved

With these assumptions, you are correct: the energy needed to cool the apartment down after a hot day will be less than the energy needed to keep the apartment cool all day. Think of the heat coming into the apartment through the walls, which will eventually be pumped out by the air conditioner(s): the greater the temperature difference, the greater the heat flow, and the greater the heat that will (now or eventually) have to be pumped out.

Let's start relaxing assumptions. If you open the windows, but only when it's warmer in than out, then the choice gets even clearer. However, you'd need to have someone on hand to open/close the windows at appropriate times. If you just open the windows when you leave in the morning, and close them when you get home, then that argument goes out the window. Now you have to balance how much heat comes into the house via the open windows with how much leaves via the same route.

Finally, if you add moisture, then you're really in trouble. In a moist climate, a lot of A/C energy goes into taking humidity out of the air. So, if you leave the windows open all day, and the house heats up AND soaks up moisture (not only in the air; in the curtains, carpet, and even wood), then the A/C will have to pump all that moisture back out of the house at the end of the day.

And, something I really don't know is whether A/C units have preferred operating temperatures. In the extreme I'm sure they do (e.g. a freezer in your garage will stop cooling when it's too cold, and your goodies will melt), but in the range temperatures you're considering who knows. If they lose efficiency when the indoor air gets above 90, say, then that's really going to argue for leaving them on all the time.

All of this, though, is trumped by comfort, and what is A/C for if not comfort? Maybe not walking into a broiling house at the end of a long day of work is worth leaving the A/C on all day. Maybe being hot is fine when you're awake, as long as it is eventually cool enough to sleep. There's no way to answer this for anyone else; you can only answer it for yourself. (That's the fun of roommates and spouses: they aren't you, and understanding that is part maintaining the relationship.)


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