In my apartment, I have two window air conditioning units. Both have different settings for the fan speed: one has two speed options and the other has three. Both have the design that the room circulation fan and the condensor coil cooling fan run on the same shaft, so the speeds of the two fans are identical.

My main question is, which fan speed setting should I use if my goal is to make the rooms these A/C units are in as cool as possible? I had originally guessed I would want to use the lowest speed setting, since that's when the air comes out the coolest and also (since I live in a humid area) less air passes the coils per minute and thus less of the cooling power of the unit is being invested in dehumidification but rather spent directly on cooling the air.

However, as I thought about it more, I realized my picture of the situation was oversimplified. For example, running the fans faster moves more air through the unit per minute and thus the temperature of the air coming out is cooler, but there's more air coming out at the not-as-cool temperature so it probably balances out (by energy conservation, it should balance out). Also, running the fans faster means the outside fan is better able to cool the coolant, possibly allowing the unit to run at a cooler temperature overall. And finally, maybe investing more cooling power in dehumidifying the room will help reach a cooler temperature overall since eventually the room will be at a lower humidity and the A/C unit will be investing less cooling power pulling the water out of the air.

Poking around online, I found this statement from an energy.gov website:

Set the fan speed on high, except on very humid days. When humidity is high, set the fan speed on low for more comfort. The low speed on humid days will cool your home more effectively and remove more moisture from the air because of slower air movement through the cooling equipment.

So, according to the US government, running the fan speed on high is the best course of action except on very humid days (but no explanation why). They also say that running on a low fan speed on humid days will both cool your home more effectively and remove more moisture.

So, my questions: If I want to get my rooms as cool as possible, what is the best fast speed setting to use on my air conditioners and why? Does this answer change depending on how humid the day is, and if so, how? Is the advice from energy.gov sound?

(I have a strong physics background, so I'm fine with a detailed physical or mathematical explanation.)

  • fast moving air serves to dry wet coils before they start dripping... – dandavis Jul 5 '17 at 21:33
  • @dandavis, but if the coils are cold they will also be condensing water from the fast moving air. It's an equilibrium between the two processes, evaporation and condensation. – NeutronStar Jul 5 '17 at 21:42
  • true, but enough hot air could warm the coils above the dewpoint, at which point they won't draw out humidity. For pure lowest temps, fans should all be as high as possible. it's worth differentiating between room circulation and conditioning cycle circulation. faster blowing air cools the floor, walls, chairs more, just like a CPU fan revving up when the computer is busy (hot) to remove more surface heat. On a humid day, an external box or pedestal fan can help move the room air to the cycle, while the lower internal fan speed keeps the coils cold enough to drip profusely. – dandavis Jul 5 '17 at 21:56
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    put simply: the room air should be moving as fast as possible no matter what, and the internal fan should be moving fast enough to warm the coils to just under the dewpoint, to get the lowest compressor duty cycle. – dandavis Jul 5 '17 at 22:00

In very simple terms you are trying to "Condition the Air" not just cool it. There is a term "relative humidity" which affects the way you feel at certain temperature. Weathermen refer to it as a heat index. The higher the humidity the more uncomfortable you feel at a lower temperature. So if you are running a fan at low speed it gives the compressor and cooling coil time to dehumidify the air rather than just cool it. On high temperature low humidity days running the fan at a higher speed will allow the air to cool down faster and reduces the air conditioner's ability to dehumidify.

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  • If the air was "cool enough" (itself a vague term and dependent on humidity, as you point out), then the humidity wouldn't matter nearly so much for comfort, which is why I'm focused on the temperature. I ask this question partly because what I have been doing so far leaves both the temperature and humidity a bit too high for comfort, so I'm trying to improve things. And unfortunately due to size constraints (my apartment air conditioning situation is weird), I can't just get a bigger A/C unit to solve my problem. – NeutronStar Jul 5 '17 at 21:45
  • And please note that in any reply to my comment above, I am interested in the answer to my general question in my main post, not just how to address my specific situation (for instance, plugging in a dehumidifier would help my A/C unit, which is limited in size, not have to work so hard to pull water from the air and thus be able to cool the air more and lead to more comfort all around, but this isn't answering my original question). – NeutronStar Jul 5 '17 at 21:48

An AC unit can only shed so much heat. The question doesn't take into account thermal mass of the walls/roof which is another input. A limit on cooling is how well is the unit shedding heat in addition to cooling the air. Slow moving air will cool better, just as air passing through to heat will transfer more heat at a slower speed. RV's have cut the plastic fins off their AC covers to aid units in shedding heat. Outgoing airflow increased by a factor of 7 by replacing the plastic fins with diamond plate mesh. Slower is better as the unit is more efficient, the air is colder which also helps it counter the heat migrating through the walls and roof better.

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A 5000 btu ac unit runs at 5000 btu compressor whether on low fan speed or on hi. The longer the air stays on a cold surface, the more moisture is removed...lo fan speed. the faster the air moves...the better circulation, its 5000 btu's either way.

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  • But also, the faster the air moves, the more air that less moisture is being removed from, so there may still be more moisture being removed. I guess that's the crux of my question. – NeutronStar Aug 5 '19 at 14:58

From an energy efficiency point of view for cooling I'd say running a higher fan speed should effectively "soak" or "pull" more of the cold out of each compressor cycle. The more cold pulled out of the Freon before it's sent back to the outside (to exhaust heat and rechill) should make it more efficient. My guess of course. The fan would use slightly more energy than when running on low however the actual AC compressor uses vastly more energy than the fan. The quality of "pulling" cold out of the compressor makes up part of the unit's efficiency. I'd say a 5K BTU offers just that but at an ideal indoor/outdoor temperatures. Agree slower air will remove more humidity in vast majority of cases but could be argued in special cases where outdoor air is much, much colder than indoor air (due to the dew point).

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