Quote:
Originally Posted by Hv23t
...I also just ordered a digital temperature switch that I'm going to set to bypass the heat pump altogether when the air temp gets below 45 deg f. And then it will use the 4000 watt 240 volt element, that is running on 120 volts so is only 1000 watts...
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Hv23t,
I think that having a temperature controlled cut-out is a good idea.
However, there is a curve that results when you plot COP vs. air temp, and COP drops as air temp drops. When the COP approaches 1, it is definitely advantageous to switch to resistance heat.
Here is a plot I found on the web just to show you the shape of the curve. The temperatures here are in C not F, and the place on the graph where COP = 1 will most likely be very different from yours, but the idea is very similar to yours.
I guess my point is that 45 degrees seems to me to be an arbitrarily high point to set your cut-out, and that even though the COP is declining, it's still a lot better than resistance heat.
But exactly what your curve looks like will depend on more factors than I can possibly guess. You can get a rough idea by monitoring the system as ambient temp changes.
By the way, there is a
very timely conversation going on right now on the 'Manifesto' thread that concerns R-290 efficiency. You should check it out, as it pertains exactly what you are doing.
Best,
-AC