Minimum Heating Temperature for Heat Pump
I recently had a ducted heat pump put into my house (I’m in the northeast) — this configuration (two copies: one in attic, one in basement):
https://ashp.neep.org/#!/product/34518
The house is old, and probably could use better sealing / insulation, but the question I have is related to ramping down. From the charts, I can see each system should ramp down to 550W, and that’s when it hits the highest COP, at least at moderate temperatures. I had initially set the system to 68 degrees and left it and I noticed that it never ran that low — on warmer days it would shut off more, but it never ran below around 900W (that’s _per system_ — I monitor them separately of course).
Reading one of the manuals, I saw that it said something like “guaranteed heating performance 70-80 degrees” — so I wondered if it wasn’t going to perform as well below 70 degrees. So I tried raising the temp to 70, and it seemed to work (though by the time that happened it had gotten colder so not much opportunity to test) — at least one or two days I saw it running for periods of time around 550W.
Is this expected behavior? Or any other ideas? It seems very odd, especially because, at least for me, even 70 feels a little warm.
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I have been monitoring the electricity consumption of my 2 12k single zone wall mounted units for 3 years. I don't see my units running at their very minimum capacity often. Your proably only going to see 550 watts when its 47 or above and under low load. Once you start going below 47 the minimum electricity consumption of the compressor will start going up. When its 17 out your compressor will almost double its electricity usage and use 870 watts. The compressor can't turn down any lower than that at that temperatures because of oil control. It needs to run at a certain minimum speed to circulate oil through the system. At 17 degrees since heat pumps lose capacity as it gets colder that requires more electricity at 17 than 47.
Mitsubishi might have the data on your system for the minimum capacity and electricity consumption at various outdoor temperatures. Check on their mylink website.