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As the title suggests, my furnace will sometimes shut off as if it's reached the target temperature, while it is still far below (ex. set to 70F, stops at 63). If I unplug the thermostat, the furnace will turn back on (even before I plug it back in).

This happens when running on a schedule or on 'permanent hold'. It doesn't happen reliably, sometimes it goes until the target temperature is reached and turns off normally. It may be related to temperature (it might take more resets to go from 60 to 65 than from 65 to 70), though I also just might not notice as much at higher temperatures.

It seems to me that either:

  1. Something is wrong with the thermostat, and unplugging it (switching from wall to battery power) updates the signal appropriately.
  2. Something is wrong with the furnace, and I'm overriding some shutoff condition.

The second one sounds dangerous, though I hope that the furnace is smart enough to not be overridden if it's unsafe.

What's going on here?

UPDATE: The thermostat is a Honeywell model TH6210U2001: thermostat The pins connect to the base plate which connects WYGR wires to the furnace and air conditioner (separate units). The furnace is a Jonson AirEase 80 Plus. furnace

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    What model furnace? What model thermostat?
    – manassehkatz-Moving 2 Codidact
    Feb 11, 2021 at 18:25
  • Is this a heat pump with an air handler? Gas or electric heat all have different types of safety’s that may be shutting the system down. Cycling the call for heat may reset the controller like a power cycle on some models.
    – Ed Beal
    Feb 11, 2021 at 18:53
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    "If I unplug the thermostat" What does that mean? Thermostats are generally hard wired to the heating unit, not "plugged in". More info is needed if you want an informed answer. "switching from wall to battery power" I have never seen a unit that has the option for "Wall" power, curious. What is wall power?
    – Alaska Man
    Feb 11, 2021 at 19:17
  • @AlaskaMan it's mostly a DIY thing, I think. Thermostats that need a return wire to the furnace transformer in order to power themselves can alternately use a local transformer when no return wire is available, in an existing installation. These are marketed as "C Wire Adapters" since they eliminate the need for the (usually) blue wire on the C terminal for thermostat power. That sounds like what OP might be describing.
    – Michael - sqlbot
    Feb 11, 2021 at 21:54
  • If you wait until the furnace shuts off early then IMMEDIATELY yank the thermostat and plug it back in, then it runs again for a while and shuts off a second time before reaching the desired temp, my money would be on a safety in the furnace shutting it down. HOWEVER the Honewell T6 is just a cheap P O S thermostat Honeywell makes better ones that can be controlled remotely from a cell phone, etc. So you have an excuse here to get a better tstat and if the problem keeps happening you know it's the furnace. If it turns out the T6 isn't bad you can sell it on Ebay to some Honeywell purist.
    – Ted Mittelstaedt
    Feb 11, 2021 at 23:27

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A professional HVAC guy would have a spare tstat and swap it for testing if you were bent on keeping the old thermostat. Unfortunately, while for a century furnace thermostats were as reliable as a rock, and nobody would ever suspect them, furnace thermostat makers have discovered if you build it just expensive enough to last 5 years, when it breaks people will just accept it because so much other electronics these days has designed-in short lifespans. Then they will buy another.

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