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I just purchased an Ecobee3 and I'd like to minimize the aux heat usage by setting the "Minimum Outdoor Temperature" setting to the lowest possible value. Ecobee support says that anything below 35 will risk damage to the Heat Pump, but I suspect they're just worried about liability.

I have a Goodman GSH130361AD and I've skimmed through the manual (http://www.hvacpartsshop.com/manuals/gshseriesmanual.pdf), but I can't find anything about min outdoor temperatures or efficiency drop off's. How can I tell what min outdoor temp is appropriate for my heat pump?

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From personal experience, most R22 heat pumps like the Goodman GSH130361AD do not produce much heat below an outdoor air temperature (OAT) of 35F and the system coefficient of performance drops such that running much below that OAT is not very helpful.

The main concern with low temperature heat pump operation is excessive compressor discharge temperature. As the OAT drops relative to the indoor air temperature, the compression ratio increases causing higher heat of compression and reduced coolant flow through the compressor. Discharge temperature should usually not exceed 200F and in no circumstances be higher than 215F.

If you are really dedicated to finding the safe low temperature operating limit for your system, strap a wireless thermometer on the larger refrigerant line where it exits the cabinet of the GSH130361AD and monitor the temperature from inside your home on cold days. When you see the discharge temperature climb to 190F, you will know you have reached the OAT low temperature cut-off.

EDIT: Since I suggested measuring the discharge temperature outside the cabinet instead of 6" away from the compressor, I lowered the temperatures to 215F and 190F to give a margin of safety (vs. the original 225F and 200F temperatures I mentioned).

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