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airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 8383
Experience:  Furnace, A/C and heat pump specialist.
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Furnace wont turn off at thermostat

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Came downstairs to find the Nest thermostat had a power error code - ran through all steps and still nothing. Just replaced with original thermostat and furnace still won't shut off unless we flip breaker. Any ideas? It's a little cold to have the furnace off for an entire night.


We've checked wires - they are in proper place and no visible shorts at the thermostat.


Had HVAC repairmen out about 5 days ago to replace draft inducer motor and housing. Seemed to be working fine until today as far as we could tell.

Hello. My name is XXXXX XXXXX I'll do my best to help. Troubleshooting often requires meters and test equipment. Let's get started and see what we can do.
Ok let me understand this. With power to furnace and thermostat off, furnace continues to run?
Customer: replied 3 years ago.

That's correct - the only way to get the furnace to turn off is at the breaker. Neither thermostat were able to control the furnace.

Lets do this to determine if issue is in the furnace control board or external problem. Go to furnace and remove all of the thermostat wires from the furnace .Then turn breaker back on. If furnace does not run, we need to trace down wiring to see if we can find a short. If furnace still runs with thermostat wires disconnected at the board, you are going to have a defective control board.probably took a power surge or got wet.
Customer: replied 3 years ago.

Pulled all wires, flipped breaker, furnace did not turn on.

Good deal. so we know board and furnace are probably ok.
Do you by chance have an ohm meter?
Customer: replied 3 years ago.

CORRECTION - didn't see safety switch.


Pulled all wires and furnace DID turn ON. lets reconnect all of those wires to the furnace and go take wires loose at thermostat then power back up. If furnace runs , we have a short somewhere in the wiring between thermostat and furnace if not problem is in thermostat (strange it would be in both thermostats), but let's make sure.
Customer: replied 3 years ago.

All wires pulled at thermostat and furnace turns on when breaker reset.

Ok, we are going to have to trace those wires.I would start by looking where they enter the furnace to make sure sheathing has not rubbed against metal and nicked the wires. If all looks good there, we will have to follow them until we find the short. Could be a squirrel or mouse chewed the sheathing in attic or basement somewhere.
Customer: replied 3 years ago.

I don't know if we have the energy or skill to accomplish that. We just tried to reinstall the Nest thermostat and it doesn't seem to be getting power. Without multimeter not sure how we would verify but I'm guessing we've lost power at the thermostat?

Somewhere between the furnace and the thermostat in the control wiring, the white and red wire are touching together. That is completing the circuit to the furnace when furnace is powered up.Can you physically get to the control wire to follow it from thermostat to furnace to see where the wire is shorted? Most likely a knick or a chew by an animal
Customer: replied 3 years ago.

The thermostat and furnace are on different levels. We can trace the wires until they go into a brown harness and up into the ceiling and that's about it until they pop back out on the wall upstairs where the thermostat is.

Hang on. I just got the update where you made the correction. You are saying with the wires disconnected from the thermostat at the furnace itself and held in door switch the furnace DOES come on? If so you have a bad control board in the furnace.In otherwords wires are not connected to furnace but furnace runs?

Customer: replied 3 years ago.

Gotcha - ok.

Yes if the furnace comes on by itself, with no thermostat wires connected, you have a bad board in the furnace. Sorry I did not get that correction you posted earlier.
Customer: replied 3 years ago.

No problem, I didn't see a good way to edit/correct the note. Is this a job for a professional or possible as a DIY project? Either way, do you have a ballpark for what it will cost to replace? Thanks!

You can do it yourself. Just remember to turn off power and replace one wire at a time so we don't get anything crossed up.These boards run between 100 and 300 bucks wholesale. If you can give me the model on the furnace I can tell you exactly. You can do it yourself for half what we as hvac companies will charge.(Model will be in burner compartment on right or left inside panel).
Customer: replied 3 years ago.

Heil on outside of furnace

Inside: Inter-city products Corp USA

Model/Mfg No NTC5100BJC1

You can purchase at the link below for 129.99 with free shipping. A contractor will charge about 400 bucks with installation.
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Customer: replied 3 years ago.

Thanks for your help!