What is the crossover point where it is more efficient to use electric heater coils (EH setting on my thermostat) as opposed to allowing the heat pump to operate normally, using heat exchange with the outdoor air?
there is one sure way to find out your break even point, go to the "crown boiler" website and do a heatloss for your house. there is a lot of information to put in and the results will tell you how many btus of heat you need at a certail outdoor temperature. do not trust manufacturer info! use manufacturer info to find how many btus your unit can produce at 40 degrees. i found that most heatpumps fall into 35 to 45 degree range. as the temp outside drops the amount of heat you get from the heatpump drops too. once you get below those temps you will have the compressor, possibly defrost heat elements and auxiliary heat operating - EH. if you do the heatloss on your house you need to reduce the temp difference to 35 or 40 degrees, because the temp difference for your regular heatloss will be higher than that depending on where you live. you use these numbers because you need to find how many btus to maintain say 70 degrees indoors. in northeast ohio where i live it is 65 to 70 degrees difference for heating. if you have any other questions let me know.
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It really has nothing to do with the heat loss from your house. If your heat pump cam provide more heat than the electricity it consumes it is cutting your bills. Of course you may want to not use it to that limit because wear is a factor as well but even if it can't provide all the heat your house needs it will displace some of your more expensive resistance heating. I would take a clue from the cutoff points the manufacturer uses. Some units incorporate resistance heating to take over when needed. I'm not sure if they would only run one type to cut down their current requirements.
Why not try to use test equipment to measure the right thing. There are for rent out there.
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