I set the thermostat at 70 degrees,the house is currently 63 degrees... the heater comes on, I see the flame and hear the blower,but it will only work for 2 minutes at a time.I cahnged the battries in theHoneywell electric thermostat but there was no change. Is there a way to troubleshoot(other than the things I just mentioned) before I call a repairman?