Power Usage

[NevilleT]

I am only a user of Air Con not an engineer. My question is about power consumption. I have a Misubishi ducted system and was wondering if there is much difference in power usage between high and low fan speed. If I change speed is it just the fan changing speed or does the compressor work harder?

[Hitech]

The more refrigerant that is boiled off in the evaporator the more work that is done to re-condense it. Lower airflow on the evaporator results less boiling. If the system has a orifice/cap expansion valve the Evaporator could freeze up. With a TEV the system will just operate at a lower capacity (with better de-humidification).

[Tesla]

Hi NevilleT
yes you will consume more power with the fan on high. But the fan speed depends on how far you need to push the air across the conditioned space. So if it is a big span you will need the hi fan speed. Other wise you will have an unacceptable temperature gradient in the space. It really depends on the matching size of your AC compared to the load - I have an undersized window ratler in my bedroom to sleep on hot nights but must use the hi speed so that I don't get 20degC on one side of the bed and 26degC on the other. And of course on hot days over 35 I think we all need hi speed on most AC units in Ausi

[NevilleT]

Thanks guys. That clarifies the situation. We are in winter now so in warming the house, there is a balance between a higher temp at low speed with wide gradient across the room, and lower temp at high speed for a narrower gradient. Makes sense.

[jjakucyk]

I've noticed an interesting situation with my own window A/C unit that seems to counter to what Hitech said. It's a 120v 10,000btu R-22 capillary tube unit that I have plugged into a Killawatt power meter. Outside temperature is 78ºF, inside is 74º and here's the power and exiting air temps I get at different fan speeds. I put the power drawn by the fan motor alone in parenthesis.

Low (113W): 890W 38º
Med (133W): 885W 40º
High (151W): 870W 42º

I've also noticed at warmer outside temperatures, like around 85º and with sun hitting the unit, that it draws closer to 1,050W at medium fan speed, with outlet temperatures closer to 50º. The more cooling the better, since it's undersized for my apartment, but the numbers puzzle me. When I turn up the fan speed the power draw does jump as you'd expect, but after a few minutes it gradually drops down to the numbers above.

So what are the factors at play here? I'm not an engineer either, but I've been curious about this stuff for a long time (the liquid<->suction line heat exchanger thread answered questions that have been bugging me since I was a teenager). Is the higher fan speed (and by extension the lower outside temp) causing lower head pressure in the condenser that reduces the load on the compressor motor? I'd expect that to be somewhat canceled out by higher superheat in the suction line though, reducing volumetric efficiency of the compressor. I can even demonstrate that by covering the evaporator inlet with a plastic bag, where the power draw drops by 40-50 watts. That leads me to believe that whatever is going on has to do with the high pressure side. Is the general undersizing of window A/C evaporators a factor? Whatever it is, the numbers indicate that a higher fan speed not only provides more cooling (though I don't know to what extent), but also uses less electricity to boot. So what's the theory on this?

Thanks for all your insight!

[Tesla]

Hi jjakucyk
Welcome to the forum. Very interesting point. You and I have window rattlers which have only one fan motor with two shafts. One for the condenser and one for the evap. So when you crank it up to hi both evap (centrifugal) and condenser (prop) fan speed is increased. Hence the condensing pressure is lowered by increased air flo. The compressor is the biggest load and current drops when the condensing pressure drops. In saying all that there are limits dependent on outdoor and indoor conditions eg, if you had an electric element with a fan speed too hi the air temp coming off would not be hot enough to effectively heat the space. This is why on commercial systems a heating cycle would use about 50% of the airflow as for cooling as on a VAV in an office block.