"Mr_Bill" writes:

I would have thought that the load that the compressor presents
to the compressor motor is more or less constant. If so,
then the power factor, whatever it might be, is also constant,
and is factored into the SEER.

Ok. But my point was that you were dividing watts by volts and
expecting to get amps as a result. That only works for resistive loads,
where current and voltage are in phase, and VA is equal to watts.
But for inductive (or capacitive) loads, there's a phase shift between
current and voltage, and watts and VA are not equal.

I agree that the SEER rating is based on some set of operating
conditions, and that in the real world the efficiencies might
go down (for example, if it is really hot then the compressor
radiator becomes less efficient). I would think, however, that
the BTU would go down, and the electrical power consumption
would remain constant. But, obviously this is more complex
than it seems at first glance.

Yeah, it's got to be more complex than that. For example, suppose your
house is quite hot, or the evaporator is small for the house. Then the
expansion valve will let all of the liquid refrigerant available into
the evaporator, and there will be lots of gas returning to the
compressor. This will raise low side pressure, the compressor will take
more mass of gas into the cyclinder for each stroke. This requires more
shaft horsepower from the motor, which will draw more current.

On the other hand, if your house is already cool, the expansion valve
will throttle the refrigerant, the compressor has less gas to pump, and
motor current will go down. You're getting fewer BTU/hr of cooling, but
using less electricity doing it. Overall efficiency could be higher or
lower.

Or suppose outside temperatures are unusually hot. The condensor runs
hot, the compressor discharge pressure goes up, and again it takes more
shaft power from the motor to run the compressor. More power to provide
the same number of BTU/hr cooling when condenser air is hotter than
normal.

Dave