Well, I'm not really sure. It was only 12 degrees today/last night. DH got up and left for work and the thermostat was set at 69 degrees and it was working fine. He left at 5:45. At 6:14, it made a weird noise outside like a leaf caught in the fan blades, then it wouldn't shut off. And the temp in the house steadily declined. I watched it go from 69 at 6:00 to 63 by 7:00, I put it on Emergency heat hoping that would work and it was blowing freezing air. Something about the heat strips not heating up. We just put this in 18 mos. ago, it is very frustrating. But they rigged it to work tonight, and will fix for real tomorrow while DH is here.