I have an oil furnace hot air system with a 5 ton a/c plus heat pump. The changeover from heat pump to furnace is set at 15 degrees outside temperature. I live close to the airport here in New Hampshire and outside temp is from the airport weather station. The cost variation over heating seasons for electricity and heating oil is dramatic. I would like to r control the changeover temperature from heat pump to furnace using the cost of oil and electricity.I have some general ideas using measurement of electical current to the heat pump, Te furnace uses a specified nozzle size to provide a fixed gal/hr. The heat pump provides about 60% of the Btu/hr of the furnace. It does run about 60% of the time that the furnace runs at a high outside temp. This changes as the outside temp drops. If I had the heat pump current vs outside temp, I could get my answer. Anyone measuring this? I 've seen the Sense Energy monitor…I only want to measure one 220 v ckt with 60 amp breaker over time to get KW/hr.
If you google the model number, you can probably find the performance curves for the heat pump online. It will give you the performance curves as well as the COP or the kW which you can convert to COP. With that information, you can calculate the ideal changeover point. Depending on your model, you may have a time/temp defrost board or a demand defrost board. The time/temp will be less efficient in that they initiate a defrost after 30, 60, or 90 minutes of runtime. Therefore when the temp is under 40 outside, you have to assume that the COP of the heat pump will be a tiny bit lower than the spec due to defrost cycles that are not accounted for in the manufacture performance data. This is especially true with time/temp defrost boards.
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