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by triceratops
515 days ago
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> In areas with moderate outdoor temperatures. Anywhere that the heatpump is going through defrost cycles, that statement is likely false because the COP is likely 1 or less when that starts happening. Modern heat pumps have a COP close to 2 at 5F/-15C while still delivering 50k+ BTUs. Here's one example: https://ashp.neep.org/#!/product/68628/7/25000/95/7500/0///0 |
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But it has a 150W pan heater, but I don't think that is its primary defrost mode, at least its not going to be at 50k btu at those temperatures unless its also really dry. The pan heater is probably to just assure that the coil melt water doesn't build up in the base pan.
For most of the forced air systemms in the USA, the deforst mode is as I mentioned triggered via outdoor coil temp/runtimes and somewhere below an ambient of ~30F, which will be a coil temp of ~20F. The exact algorithm changes from AC unit to unit, but you get say 20 mins of heating, and then it will flip to 5 mins of AC while not running the oudoor fan, where an electric (or maybe gas) furance will heat the indoor air after it flows over the indoor coil which is cooling the air. So its a double wammy, its taking 5+ mins of operation back at an even higher wattage.
AKA the COP goes negative for 5 minuites... Which will pull the SEER numbers down pretty hard, and that unit actually has pretty poor numbers for being variable speed compressors/etc.(edit: should be SCOP, since SEER(2) is just the ac side, that unit looks to be optimized for heating. Either way, depending on which efficienty spec you pay attention to, the 'problem' tends to be that the SCOP/HSPF/etc numbers are calculated using 'moderate' temp data, so below freezing temps are a minority of the calculation)
I poked around a bit looking for the install/service guide for that unit but didn't find one detailing the defrost algorithm.