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.

  • Can you post a wiring diagram for the furnace? Also, what do you have for a thermostat, and how is it wired? – ThreePhaseEel Dec 12 '16 at 23:25
  • Some thermostats have a "fan" setting. It's not something going on with that, is it? – Joe Phillips Dec 13 '16 at 0:23
  • The fan setting is "Off" or "Auto". I have it set to "Auto". As far as I know the fan setting on the thermostat is for the blower, not the furnace motor. – Bob Dively Dec 13 '16 at 0:41
  • The thermostat is a Honeywell 97-5110. I imagine that it's probably the same age as the furnance—1994. I don't have a wiring diagram for the furnace. – Bob Dively Dec 13 '16 at 0:52
  • It tries to turn itself on and off every five seconds even when it's not calling for heat? Or are you saying it works during the day when you call for it, but at night it gets wonky and then you have to reset it? I'll assume the latter and suggest you get a new pressure switch for the exhaust blower. My guess is that the rubber gets sticky when temps drop at night. – Mazura Dec 13 '16 at 4:22

After more than a month and service visits from multiple technicians from multiple companies, I finally got a technician who solved my problem. He listened to me describe what was happening and said "That's the thermostat. Let me see if I've got an extra one on the truck." 10 minutes later he drove off without charging me and the furnace has worked fine ever since. I feel like an idiot for not just installing a new thermostat in the first place, but at least I don't have to get up three times a night.

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