I have a small window AC unit that I occasionally use to rapidly cool my apartment. As I was placing it in my window today I was thinking of how I could maximize it's efficiency so that it either cools my room quicker (or uses less electricity to cool my room due to less work being required).


Say my room is at 80 degrees, and I'd like to lower the temperature to 75 degrees. The outside air temperature is currently 70 degrees.


Ignoring the option of just using a fan to pull cold air in, I'd like to focus on the physics of the AC unit. What is the "best" way to cool my room? The AC has 2 air flow paths.

  1. The cold side: air is blown across the evaporator coil and into the room.
  2. The hot side: air is blown across the condenser coil and vented to the surroundings.

I can control whether the hot side sucks air from inside my room or if it sucks air from the exterior.


I assume that the AC would be most efficient when the coldest air is blown across the coil. So initially I thought it would make sense for it to only suck air from the exterior. However, if I suck air from the interior to cool the hot side side, the negative pressure would actually cause cold air from outside to be sucked into the room to replace the air that is vented. Which would mean that my AC is now acting both as an AC unit and as a regular fan.

Therefore I would assert that sucking air from the interior to cool the hot side would be more efficient than using exterior air only to cool the hot side.

Is this a reasonable assumption? Does using hotter air cause the condenser to work harder and therefore negating any benefits? Seems like an argument could be made for both cases depending on the temperature difference, which is why I am posing this question with the same 5 degree temperature difference on both sides.


closed as off-topic by Kyle Kanos, John Rennie, stafusa, glS, AccidentalFourierTransform Sep 22 '18 at 21:22

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the air conditioner will always work more efficiently when the sink into which it is dumping the heat it collects is at a low rather than a high temperature, and when the air coming in to be cooled is at a low rather than a high temperature.

This means the most efficient way to run it in your example is to use outside air to blow over the hot coil (the one on the outside half of the unit), which now-heated air then gets discharged into the outdoors, and outside air to pull in through the cold coil (the one on the inside half of the unit) which cooled air will then be discharged into the living space.

  • $\begingroup$ Thanks for satisfying my curiosity! That does make the most sense. The unit sucks air into the cold side and blows it out on the same side, so I can't fully optimize it, but close enough for me. $\endgroup$ – Xander Luciano Sep 17 '18 at 9:03

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