Correct high elevation de-rating for heat pump AND head?
johngfc
| Posted in Mechanicals on
I understand that building at 7,000 I need to derate heat pump output by about 20%, but recently an HVAC person said I ALSO needed this 20% derating for the head, for a total of 40% derating. I.e., if the compressor unit was rated at 40,000 BTU, I’d only get 25,600 BTU ( .8 *(.8 * 40,000)). Is this correct, and if so, why? Heat exchange at the head would also be affected by air density, but is there a constraint that would prevent using a higher capacity head (i.e., more air flow) that would thereby gain use of the full output of the compressor?
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Replies
Each unit de-rates according to altitude.
Assuming the 20% de-rate, your 24000BTU outdoor unit would deliver 19200BTU and your 24000BTU indoor head would be able to deliver 19200BTU. If you combine the two, you end up with a 19200BTU system, not 40% down.
Derating comes from air density difference, which means the same coil inside and outside can transfer less BTU from the same blower CFM. The refringent circuit is sealed, so it is not effected by altitude at all.
Excellent - that was my understanding. Thanks.