My burner will ignite and burn for about 20 seconds. Fan will come on and burner goes out. Tan will run for a long time and the start cycleing and burner will not relight. I have to unplug the whole unit for about 30 minutes and replug and does the same thing again. Even in the air conditioning mode, the fan cycles all the time. When I turn the switch off on the thermostat, the fan still continues to run. Is this a problem with some sort of limit switch?