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

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.

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.

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).

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.

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.

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.

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.

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.