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?