I recently replaced my 19 year old heat pump and this is the first winter I have used the new one. It will keep the house at set point but when the outside temp drops to around mid to low 20's the compressor on the unit will cycle and maintain inside temp but the fan will run continuously. I have not noticed aux heat coming on even with outside temp near 12 degree. Should I turn on emergency heat or continue to let the unit run. Service tech has been called and has checked unit 2x.

I would turn the outside unit off and put emergency heat on
Did you buy the same brand heat pump as your old one if not sometimes you cann't mix match units of different brands.Sometimes low voltage wiring is different from brand to brand.

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Originally Posted by builderbob1

I would turn the outside unit off and put emergency heat on
Did you buy the same brand heat pump as your old one if not sometimes you cann't mix match units of different brands.Sometimes low voltage wiring is different from brand to brand.

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The new unit is a Tempstar while the older one a Whirlpool... however after checking the unit out this evening the tech seems to think it may be a thermostat issue. I had installed a Honeywell 7400 it was removed and tech installed 5000 Honeywell. The 7400 was programmable while the latter not so. Still have the problem, now the tech will contact folks to learn more about the thermostats.
Can't tell if they work on timing and temp or just temp. Curious though. Thanks for the input and I will pass this on.

It makes sense that the compressor cycles on and off but the fan stays on when the temp outside drops. Heat pumps don't perform well when the temp drops below 30s so that's why second stage heat is used (called auxiliary or emergency heat) in conjunction with the heat pump. Second stage is usually a high resistance heat element that is installed in the ductwork and as voltage passes through it, it gets red hot and heats the air passing over it (just like a gigantic hairdryer). The problem could be that the thermostat is not making the connection to the secondary heat wire, or if it is maybe the wire isn't connected. It could be frayed or broken somewhere where you can't see it (under the house or in the attic or wall). You (or the tech) can check the voltage from the red wire and the second stage heat wire and it should be around 24v. If that's okay, it could be the relay that turns on the heat element. If its not the relay, it could be the heat element itself.. maybe burned open, or not connected properly to its high voltage source. Also, there are safety high temperature switches that will disengage the heat element if it gets too hot and shut your heat element off, maybe one of those has malfunctioned and won't allow the relay to open. Hopefully your tech knows these things and can check them out for you