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Old 02-13-12, 02:37 PM   #9
MN Renovator
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I've done the math for my gas rate of .75/therm last year (cheaper this year) and electricity of .11/kwh and I couldn't get it to make sense for heating with a heat pump above 40 degrees with a Carrier Greenspeed which is supposed to be the most expensive and most efficient central heat pump out there. At 40 degree daily high outside and a clear sunny day and my house is at 60 degrees before I add heat. Heat pump is a no-go where I am. Even if it was a grand for the heat pump it doesn't make sense here. Our shoulder seasons are short up here in the north so the nightly temperatures between freezing and 50 degrees would make little use of a heat pump if you are far north enough. I considered a mini-split until I got a hold of the Fujitsu service manual(hard as f#@! to get now too) and looked at amp rates and capacity below the AHRI 17 degrees. Thing gets thirsty and will crank full bore at 9.5 amps and as it gets colder capacity drops like a stone. I think the 17 degree 10k rating is very close to max compressor frequency which places it very near the 9.5 amps(2.28kw). The COP is great when the unit isn't running all out but it seems once its at max capacity heating or cooling it sucks power and COP drops. There was an independent study done on Mitsu and Fujitsu inverters and at max load they have worse COP than central a/c and heat pump units. I suppose if I build another house that is well insulated I might use two just to keep them below 70% load except for maybe the absolute worse 2 weeks worth of heating. Non-inverter units are more predictable but if you look at the Carrier Greenspeed balance chart you can see how the 17 and 47 degree points seem to be programmed in the inverter to be the most efficient and the compressor speed varies to keep the capacity nearly the same between those two points. Gives them better HSPF I'm sure but looking below 17 degrees and the capacity drops quick because the compressor was already cranking at its max.
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