Thermostat set at 65 - Furnace went to 90 - possible causes?

Question

Oil furnace. Honeywell programmable thermostat (not WiFi-battery operated). Heat set as always at 65 degrees F with auto fan. No changes to settings whatsoever. Had been working fine all winter.

Yesterday, Cold day <20 degrees F and extremely windy. Not home all day (12 hours) - when returned, house was 90 degrees and furnace was blasting. Thermostat was still set at 65 - but temp on thermostat read 90. Turned thermostat off - furnace stayed on. Turned breaker to get furnace to shut down. Off all night.

This morning - house temp 63F - turned furnace off at switch, then turned breaker back on, then turned furnace switch to on. Turned thermostat back on - still set the same at 65F with auto fan.

Has worked fine so far all day (??). Even when turning thermostat off and back on or changing temps - seems to be communicating fine. Furnace goes off with thermostat and back on and stays at whatever temp is set. What could have happened?

Afraid to leave house again unless I shut the heat completely off. Used up almost 1/2 tank of heating oil with what happened yesterday. Could whatever the cause have been start a fire? I know that my chimney needs a repair at the top and is also missing a cap. I also have been finding mice in the kitchen that I've been dealing with - but no sign of them in the area of the furnace that I know of or in the wall where the thermostat is. What are the different options that could have caused this so that I can hopefully prevent it from happening again?

Answer 1

there should be several other layers of protection from a fire, the issue is more economic. It might be a one-off event, like the thing's smarts freezing up. Maybe check the batteries.

If in any doubt, you can put the old thermostat, set at a max cut-off temp like 80, in series with the new one to prevent a "crashed" smart thermostat from running the heat all day. The heat would only come on if BOTH thermostats agreed it should. Since the older one will be on for anything under 80, it normally does nothing and stays "on".

If you have the opposite problem, it doesn't come on sometimes, then you put the backup thermostat in parallel with the smart one, so that either one will trigger. Then you set the analog thermostat for a fail-safe like 50 to prevent frozen pipes.

You can put your backup thermostat out of the way, or next to the furnace; it need not be in the comfort zone to be effective.

Answer 2

The simple answer is to replace the thermostat. They're not that expensive, and this sort of thing is almost unheard of. There are many more likely causes of a house fire.

If it happens again it's probably a bad control board on the furnace.