hi, no matter what setting of fan control (cir/auto/ on AND no matter what temperature I set heat on....heat works for 10 minutes and blows cold air for 1/2 hour! I have to go shut off, turn on again to get heat. I cant understand how any furnace or programming would allow for heat for 10 minutes then blow cold air to loose all the heat you just received. Making me absolutely crazy! This is my 2nd thermostat! both are Robertshaw thermostats. 4000 and 5000 series.