McDave wrote:High speed, Med speed and low speed fans will vary the cfm moving across the evaporator and the rate that the air is exchanged, Yes? So.. variable speed. 8000btu max = hi speed 13,500 at med speed or low speed =?
If you have a window unit that has Auto Fan feature such as the Crosley I use in the bedroom you will get this:
"Note: Fan starts out at high speed and adjusts to slower speeds as the room temperature dictates. For example, if the room doesn't get too warm, it will stay at slowest speed. If the room temp rises quickly, such as a door being opened, it will automatically go to the highest speed. The fan speeds will re-adjust back to the slowest speed as the room returns to the original set temp."
Also in the Energy Saver Mode:
"Note: In this mode the fan will continue to run for 1 min. after the compressor shuts off. The fan then cycles on for 20 sec. at 10 min. intervals until the room temp. is above the set temp., at which time the compressor turns back on."
Can a 13.5k unit do less work? yes. Can a 8k do less work? yes. Can your rooftop have longer run times to chase away damp cave syndrome? Yes. Medium or low speeds.
McDave
Very good points. I think the lower fan speeds helps to a point with humidity for sure but there are many variables in play as you well know. In high humidity, however, and all else being equal, I have seen the Evap Coils Freeze up due to lower fan speeds... I have not studied it much other than just thinking about it, but in addition to the longer dwell time at cool the air at the evaporator, I think the head pressure changes due to the heat load reduction also cause by the decreased airflow and then evaporator drops further below dew point (good), but risk of freeze up as some ice begins to build and then airflow decreases too much and then compressor will shutdown due to protective circuitry. Probably less risk at the medium speeds.. As I understand it, it is basically the same issue you'd have if the Air filter is clogged up and etc. given the compressor speed is not changing also. It is all a trade off. I think the more complex home systems (or any more complex design) get away with this much better because they actually vary the compressor speed as well or automatically vary fan speeds as you indicate. At the end of the day... whatever works best for your needs and situation. No one size fits all, I suppose.