Better to run air conditioner fan on high or low speed?

Question

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.)

Answer 1

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.

Answer 2

I have some data that may help here. I have a GE 10,000 btu AC, single double-shaft motor. I’ve measured the wattage on fan only to be 149 W on high, 140 W on medium and 135 W on low. In cooling mode (compressor running) this changes to 832 W on high, 790 W on medium and 777 W on low. In fan only, there is 14 W difference between high and low, while in cooling mode this jumps to 55 W. The refrigeration system has to be running at reduced capacity to account for this larger difference in low/hi power consumption. This makes sense because at low fan the refrigeration unit is picking up less heat in the room and rejecting less outdoors. So, score a point for the Department of Energy when they suggest running the fan on high for maximum cooling.

Looking at dehumidification, I think they’re right as well, for two reasons. First, as others have said, the room air moves more slowly past the evaporator coil in low, giving the air more time to give up water vapor. There is another thing, too. Air conditioners can only dehumidify to the extent that the room air also needs cooling. Say it’s 74 F both outside and in the room but it feels too damp because it’s raining. If you turn on the AC and crank down the thermostat so it’ll run, the room will soon begin to get too cold for comfort. Even more significantly, cooling to dehumidify starts to become harder as the room temperature drops. This is relative humidity we’re talking about, so 75 percent humidity means the air is holding 75 percent of the water vapor it would have the capacity to hold at that temperature. Air’s capacity to hold moisture increases as the air warms. Condensation, whether on an AC evaporator or on the outside of a glass holding an iced beverage, simply indicates that the air in the vicinity of this cold object has been cooled to a temperature below its dewpoint. That air around the glass or flowing out of the window air conditioner will be at or near 100 percent relative humidity for its new temperature because if you look at the curve of how temperature drops when liquid water condenses out of water, you’ll see that this process gives up so much heat (which the refrigeration system must remove) that the ability to remove sensible heat is then diminished.

So, to reduce relative humidity with an air conditioner, two things have to happen: The air must be cooled to its dewpoint so water vapor in it condenses out, and the air must then mix with warmer air already in the room so its relative humidity drops. This occurs easily in a warm room because the air quickly warms up after it leaves the air conditioner, acquiring a lower relative humidity. But if the room starts to cool to where it’s not too much warmer than the air exiting from the AC, the relative humidity will stop dropping. In cases of a cool room you want to dehumidify, a dehumidifier solves this by first cooling the air to its dewpoint, then warming it by passing it over the condenser of the refrigeration unit. The dehumidifier will in this way act also as a heater that warms the room in an amount equivalent to its power consumption. If you were using only an air conditioner to dehumidify a room the same effect could be achieved by increasing the heat added to the room from other sources, such as by using a small electric heater. Obviously this would be wasteful of energy, so in practice you want to simply run the refrigeration system at as low a capacity as possible if your goal is dehumidification, to minimize loss of room heat. Score point #2 for the DOE.

Answer 3

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).

Answer 4

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.

Answer 5

If you want the area as cool as possible, run both units at high fan. A rule of thumb is 4 air changes per hour under normal humidity and 12,000 BTU (1 ton) per 500 sq.ft. If the unit is oversized, it will cool the space too quickly and cycle off before enough humidity is removed causing the walls to sweat. In very humid environments, such as along the Gulf coast, it is better to slightly undersize the unit so it will run longer removing the humidity. Like 3 1/2 tons for 2000 sq.ft.

Answer 6

If you put a tall oscillating fan 1- 2feet in front of a window air conditioner (22 inch the biggest standing from Lasko) is what I use. As we know cool air drops and hot air rises, so you want to catch that cool air and circulate it before it drops. I run my 8000 btu window air conditioner at 79 temp, fan on medium, floor fan on medium. This cools off my 15x15 room in 10 minutes, after it's cool I turn down the floor fan to low as it blows really hard. My electric bill using this about 6 hours a day for approx 3 months is only 10 to 15 dollars more per month than regular bill in winter. I also have a large ceiling fan in room that runs always. This method works very well. Give it a try, good luck.

Answer 7

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.

Answer 8

Better run fan on higher speed if you want more cooling effects. On a windy day, your body feels a lot cooler as compared to a non-windy day of the SAME temperature. Similarly, a bigger fan you set in your AC will cause the unit to generate stronger wind, enabling the room to cool faster and reach set temperature faster. A setting of 23°C with max fan feels a lot cooler as compared to a setting of 23°C with min fan. But, once your room has reach set temperature, do set your fan speed to smaller to prevent overcooling of your room and you yourself catching a cold.

Answer 9

Everyone here seems to have forgotten the real purpose of an AC in it's original journey. The cold flowing air is simply a benefit of this magnificent dehumidifier that was named AIR CONDITIONING, sounds better that dehumidifier. Anyway I agree with the low fan speed for hight moisture removal, therefore I have set my 220v window unit on cool/auto fan. Remember it's a magnificent dehumidifier whether it's a 5k unit or a battleship.

Answer 10

Long story (and lots of thermodynamics) short, fan speed makes NO DIFFERENCE in how many BTUs of heat energy the unit sucks out of the room. The thing that sucks out the heat energy is the compressor. In small window units (not like your car where the compressor runs at different speeds off the fan belt)window unit compressors do a constant amount of work at steady state (regardless of fan speed). It is that work of the compressor that accounts for the cooling. A fan at high speed may give you more air, but it is warmer than the air at low speed. Because the compressor is the limiting factor the variables associated with fan speed (temperature vs volume of air) will EXACTLY balance out to produce the EXACT same BTU effect on lowering the room's temperature. Yes,fan speed may impact how much humidity the unit removes (low speed more of a dehumidifying effect) but that does not effect the room's temperature. Bottom line the air conditioner does NOT have a different BTU rating for low vs high fan ... and it is the BTUs that change the temperature of the room. Hope this helps.