Starting a few months ago, and happening once every couple of weeks (don't you love intermittent issues like this :) ) I will get up in the morning and realize that the heat pump is not doing its job. The system is blowing unheated air (I'm aware that heat pump air is cooler than furnace air) and the room temperature is indeed around four degrees below the set point on the thermostat. In the past I have been able to "fix" it by turning the system to "Off" and letting it sit for 15 minutes or so and then turning the system back on to "Auto" and the heat pump comes back on and brings the temp up to the set point. It will function normally for awhile and like I said, a couple of weeks later it will do it again. The system has propane "aux/emergency" heat which works fine, both when in "Auto" mode and if I force it into "Aux" mode. If we're going to be away from the house for a few days I'll force it into Aux mode to make sure the house stays heated and that works fine (more expensive, but fine). Unfortunately we may not have a chance to get it serviced until after the holidays so I may just leave it on Aux. Annoying. System is York, 4 ton, about eight years old. We have it maintained twice a year. Side note: this happened again last night and when I just now tried my method of turning the system off for awhile and then back on, it didn't work this time. I'll try it again, but am worried.