SEER and power consumption

Posted by Mr_Bill on April 10, 2007, 11:59 pm

Dave -

Thanks for the reply.

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.

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.

Anyway, thanks for the reply.

Posted by mm on April 11, 2007, 12:41 am

>Dave -
>Thanks for the reply.
>I would have thought that the load that the compressor presents
>to the compressor motor is more or less constant.

I would think that the outside temperature is to the desired inside
temperature, or the closer the inside temperature is to the desired
temperature, the less the refrigerant would be warmed in the
evaporator. So the less effort it would take to compress it at the
next stage.

This is sort of the corollary to, You don't get something for nothing.

You also don't usually have to expend effort for nothing. If you get
so much cooling for so much effort, you can usually get half as much
cooing for somewhat more than half as much effort.

> If so,
>then the power factor, whatever it might be, is also constant,
>and is factored into the SEER.

>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.

I dson't think the electric would stay constant. Same reason as
above.

> But, obviously this is more complex
>than it seems at first glance.
>Anyway, thanks for the reply.

Posted by Dave Martindale on April 11, 2007, 12:43 am

>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.

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

Posted by on April 11, 2007, 8:18 am

"The spec sheet says to use branch circuit protection of 30A minimum
and 35A maximum. The spec also says to use wire of at least 22A
ampacity. This all seems reasonable so far.
"

Am I the only one that sees a problem with the above?

Posted by Mr_Bill on April 11, 2007, 2:04 pm

On Apr 11, 5:18 am, trad...@optonline.net wrote:

> "The spec sheet says to use branch circuit protection of 30A minimum
> and 35A maximum. The spec also says to use wire of at least 22A
> ampacity. This all seems reasonable so far.
> "
> Am I the only one that sees a problem with the above?

Actually, this thought occurred to me right off the bat.

I can assure you that the factory spec says 22A ampacity,
30A min breaker, 35A max breaker.

In any case, I am using 50A ampacity (8 ga) wire, and a 30A breaker.

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SEER and power consumption - Page 2