I have a 1994 Heil gas furnace in a house that I've owned for only a few months. At certain points in the day, unfortunately usually in the middle of the night, the furnace motor starts, runs for about 5 seconds, and then shuts down. Another 5 or so seconds pass, and the cycle repeats. I've let this cycle repeat up to 60 times to see if anything else happens, and as far as I know, it will continue until I do something.
Here's the kicker: the air temp (as per the thermostat) is always above the thermostat set point, e.g., the air is 67F and the thermostat is set to 64F. So thermostat is not asking for heat. As soon as I manually raise the thermostat set point to the air temp, the furnace immediately fires up and runs normally.
I've had two different HVAC companies inspect the furnace. Both of them shrugged and said there wasn't much they could do unless the problem was happening in front of them, which isn't helpful for something that mostly happens after midnight.
I could replace the thermostat, but my understanding of thermostats is that they're either telling the furnace to make heat or not doing that, so it doesn't seem likely to be a problem with the thermostat.
Any ideas? Besides spending $4k on a new furnace, of course.