We just purchased a house with a York 10 SEER heat pump being controlled by a Robertshaw 9620 thermostat. The whole system is about 11 years old. I don't think the thermostat is working properly. This week it has been about 40 - 50 degrees outside. I have the d1 temperature set to 1 degree and the d2 temperature set to 2 degrees. Current temperature setpoint is 68. The thermostat will bring on the heatpump when the temperature falls to 67 degrees (normal operation). Then after 5 - 10 minutes it goes straight to aux heat and stays in aux until the call is satisfied. The temperature on the stat never drops below 67. My understanding is that the heatpump should run 100% of the time (except defrost) until 1) the call has been satisfied or 2) the temperature falls to 66 and the aux heat needs to come on. I have tried setting the d2 temperature to 3 degrees and it behaves the same way. It also leaves the heat pump on while in aux heat. This is a dual fuel electric / gas system, should it be doing that?
Thanks for the help.