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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?

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  • I would NEVER heat at 90. I always have the temp set to maintain 65 for heat. I really like your suggestion for the two thermostats. That would work fine if the problem is indeed the thermostat - but I guess not if it is something re a part on the furnace that is not communicating correctly or stuck (like a relay) . Or if there was an intermittent short in the wire. Of course hooking up a new thermostat would solve the wiring problem I guess.
    – mymomshouse
    Jan 31, 2018 at 22:03
  • It's hard to say if it's the thermostat or the furnace. If it happens again try disconnecting the thermostat. If the heat stays on it's something on the furnace if it goes off the thermostat is bad. I had a similar problem with a mechanical mercury thermostat. It turned out mice had chewed away the insulation of the thermostat wires and they were touching. The thermostat is basically a temperature controlled switch, touch the wires furnace goes on, separate them turns it off.
    – Mysterfxit
    Feb 26, 2021 at 6:10

4 Answers 4

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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.

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A similar thing happened to me last winter. One evening I changed the temperature setting from 70 F to 71 just before retiring to sleep. Later that night I awoke to find the temperature at 94 F and continuously heating. had to pull the furnace breaker and remove thermostat. The following day, I checked out unit and found that the thermostat will not allow you to set it anywhere near that high. Evidently, if you inadvertently hit a multi-button combination on it, it will lock into a continuous-run mode.

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    What make and model of thermostat did you experience this with?
    – ThreePhaseEel
    Feb 26, 2021 at 2:41
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Not the same brand of thermostat, but I have had two electronic (also however not WIFI or "smart") cause the same problem. Set at 70 and heating runs wild up into the 90s, but only very infrequently.

In my case I believe these internally had relays which would occasionally get "stuck" in the on position. (Audible click when switching on or off). Yours may not be the exact same fault, but I certainly think that your thermostat may simply be defective.

The manufacturer (Leviton) was honorable enough to replace them once we contacted them, so that is one route to follow up on.

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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.

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    Thanks! It turned out to be (hopefully) the AC Ready kit that was the issue. It was just replaced (as well as new batteries in the thermostat for good measure) and hopefully all will end up being fine. Even though it seemed to start and stop today fine with the programmed settings, we got to observe that when turning the thermostat up above the programmed temp the furnace did not respond and with tests it turned out that the AC Ready kit was the issue. I'm very thankful for your assurance regarding house fires. I'll be back to this forum if anything else transpires.
    – mymomshouse
    Feb 1, 2018 at 1:43
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    Glad you got it sorted. Please provide an answer (and accept it) or delete your question. Unresolved posts are not ideal.
    – isherwood
    Feb 1, 2018 at 4:23

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