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Too cold to test an air conditioner...


Haubeil
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I've read related forums and comment here on TIJ stating the exterior temperature was too cold to test an air conditioner unit. ...why?

What exterior ambient temperature is the cut-off for acceptable testing? ...is there a default comment to use?

I always heard the differential for air conditioner & heat pump units was ~30 degress. ...hence a heat pump's heat funciton is not efficient in the colder climate. Although, I've seen some heat pumps running while the exterior temperature was ~5 degrees. Is the ~30 differential a myth? ...because at 5 degrees, that would only generate 35 degree heat!

Thanks in advance,

Haubeil

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Jeff Mr.,

Your question is quite complex because inspectors look at new, old, integrated, stand alone, etc units.

We use 60 degrees during the past 24hour period as our default. "The outside temp was below 60degrees during the preceding 24hrs or at the time of inspection. It was too cold to safely and effectively inspect the system." We just let it go at that. In our region a non-functional A/C is one of the most accurately disclosed defects by sellers. Never had a call back for A/C in the last 20yrs.

If you are unsure about the mechanics of an A/C system, do a little reading or search the archives here.

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The fear in testing an air conditioner in temperture below 60 degrees is that the coolant which is suppose to be in a gasoues state may have changed to a fluid state. Since a compressor is meant to compress a gas tryng to push a fluid through it can blow the compressor head.

Most units have case heaters to solve this but some do not so we default to the lowest common denomenator and write them up as not inspected.

One last thing, even if one would test the unit, the cooling of 65 degree air at 35% humidity is not going to tell one a whole lot about the its operation.

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Bryant A/C says not to use their product below 55 degrees unless specially equiped (i.e. crakcase heater) Other manufacturers use different cut off temperatures, but 60 degrees is a pretty safe outdoor temp. As was said above, A/C units which are not designed for cold weather operation can be SEVERLY damaged when liquid is drawn into the compressor, since liquids do not easily compress (think hydrallic brakes).

The differential you speak of is the temperature differential INSIDE the house not the outside.

Typical cooling differential is 15 degrees, 75 degree return air less 15 degrees equals 60 degree supply temperature coming out of the air vents.

Heat pump mode yeilds more differential at design temperatures 70 degree return air PLUS 20 degrees equals 90 degrees supply coming out of the vents.

Cold outside temperatures lower the amount of available heat to pump inside. Just like pumping water up a hill, the higher the hill, the more energy it takes. Also, there is the reduction of the amount of time the unit can run since the unit will spend ever increasing amounts of time and energy in the defrost cycle when the outdoor coils fall below freezing.

Most heat pumps will produce more heat than they consume in energy down into the low twenties, but only about half the amount available at say 40 degrees, which is precisly the time the house needs the most heat. Systems installed where temperatures in the teens or below will usually have an outdoor thermostat to shut down the outdoor unit when it gets really cold.

There is usually no need to switch to Emergency heat since a properly installed system will automatically bring on the supplemental heat as needed when the T-stat falls behind 2-6 degrees below the t-stat setting.

Jim

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Originally posted by carle3

One last thing, even if one would test the unit, the cooling of 65 degree air at 35% humidity is not going to tell one a whole lot about the its operation.

I also question whether the results would be meaningful at all. I would often be sucking cool air across a heat exchanger and ductwork that were in heating use until I got there.

Brian G.

Not Interested in Buying Compressors [:-weepn]

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