I turned on the through-the-window air conditioner in the bedroom to cool it down earlier and wondered which setting, high or low, would be more energy efficient in pre-cooling a room. It seems that setting it on high would cool the room quicker then cycle on and off to keep it at the desired temperature. However, the high setting does use more energy when it's on, I believe, so maybe putting it on low and allowing a slow, gradual cool-down would be better. Or maybe it doesn't make any difference. Does anyone have any ideas about this?
The high-low settings are only for the fan, not the compressor so any difference is very minimal. The compressor is either on or off and takes the same power no matter what the fan setting is. To cool the room down, you have to remove a certain amount of heat and will have a pretty fixed cost per thousand Btu's removed.
Run the fan to what seems best for the room and noise level you can tolerate. High circulates more air so the more distant parts of the room may feel a little cooler faster, low is usually best for sleeping as long as the cool air is reaching you.
It would seem that for pre-cooling, a high fan speed would be better. The hot air in the far reaches of the room would reach the return intake sooner. Then slow it down after it's comfy. Not disagreeing, just wondering.
It would make the room feel cooler all over faster for the reason you state. Depends on how long you pre-cool and how hot it is starting out. The system can only take away so much heat at a time so that is the overall answer for total comfort.
The compressor is the big energy draw. High fan does take more energy per hour, or per minute. But, the AC spends less time with the compressor on. The high fan setting is most efficient.
Also, a fan blowing towards the AC helps cool the room, faster. If you put a fan blowing away from the AC, it dosn't seem to help any where near as much.
I'm not so sure that it's true that the compressor stays on for a shorter time with the unit on high, at least from first order, direct effects. As Ed pointed out, the compressor only has one speed, at least in the units I've seen so far. Which means it pretty much has to run the same amount of time in either case, to move X amount of heat and get the temp down to the desired level. If you have the fan on low, the compressor will start to cycle at some point, so while it will take longer to reach the target temperature, the compressor isn't necessarily running all the time.
If you factor in the second order effects, meaning that on low it will take longer to get the room to the desired temp and that during that longer time, more heat will be entering the room, whcih then also needs to be removed, then I can see where running it on low will take more compressor run time and energy.
I would think that from an energy usage standpoint, the whole thing is probably so close it doesn't make much difference. Hence, I'd go with the high speed, so it gets cool faster.
With the high fan setting, the cold air is "thrown" farther into the room. Low fan, the AC sits in a small puddle of cold, near the unit. Which is why I also reccomend the fan pointing toward the AC, so that the AC can do its work, cooling the room.
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