Derating a HP in higher elevations?

[12stones]

Just wanted to see if anyone here has heard of de-rating a heat pump in higher elevations such as around 6000 ft. I can't think of any reason other than that the air is a little thinner hence less energy per cubic foot of air. I wanted to see if anyone here could help with this question.

[cem-bsee]

ask a local dealer

[hvacker]

Yes you need to derate the capacity to 4% per 1000 ft of elevation. Also a units input like natural gas has to be derated the same eather with orfices or gas pressure.

The thinner air has less transfer effect and equipment is tested and rated at sea level.

Even air measuring test equipment has to be factored for altitude.

[cem-bsee]

so, how does one know the elevation of the lab where the unit was rated?
else, how does one know where 1000ft factor kicks in?

here, with my elevation at 540ft, should a unit be derated ~2%?

[hvacker]

Equipment like instruments are rated at sea level. Instruments are also rated at Standard Air. Temperature 59degF, Barometric pressure 29.921in Hg (Ashrae) At 540' Alt your Standard Pressure would be about 29.38" Hg

And yes equipment at 540' would be derated about 2%

[mark beiser]

Yes you need to derate the capacity to 4% per 1000 ft of elevation. Also a units input like natural gas has to be derated the same eather with orfices or gas pressure.

How about if the indoor and outdoor fan motors are both variable speed ECM motors?
They maintain a constant mass flow, so you should still have about the same number of pounds of air through the coils that the equipment was rated with.

Even air measuring test equipment has to be factored for altitude.

Thats why vane anemometers are so great, the readings don't have to be corrected for air density.