How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Airheatman Your Own Question
Airheatman , HVAC Technician
Category: HVAC
Satisfied Customers: 7333
Experience:  Furnace, A/C and heat pump specialist.
Type Your HVAC Question Here...
Airheatman is online now
A new question is answered every 9 seconds

Furnace wont turn off at thermostat

This answer was rated:

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 2 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 2 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 2 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 2 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 2 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 2 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 2 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 2 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 2 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.
Airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 7333
Experience: Furnace, A/C and heat pump specialist.
Airheatman and other HVAC Specialists are ready to help you
Customer: replied 2 years ago.

Thanks for your help!

JustAnswer in the News:

Ask-a-doc Web sites: If you've got a quick question, you can try to get an answer from sites that say they have various specialists on hand to give quick answers... seen a spike since October in legal questions from readers about layoffs, unemployment and severance.
Web sites like
...leave nothing to chance.
Traffic on JustAnswer rose 14 percent...and had nearly 400,000 page views in 30 days...inquiries related to stress, high blood pressure, drinking and heart pain jumped 33 percent.
Tory Johnson, GMA Workplace Contributor, discusses work-from-home jobs, such as JustAnswer in which verified Experts answer people’s questions.
I will tell you that...the things you have to go through to be an Expert are quite rigorous.

What Customers are Saying:

  • You did one super job of explaining to me everything there is to know about this fridge. I'm looking forward to asking you questions in the future. Jimmy Bagley, IA
< Previous | Next >
  • You did one super job of explaining to me everything there is to know about this fridge. I'm looking forward to asking you questions in the future. Jimmy Bagley, IA
  • Wonderful service, prompt, efficient, and accurate. Couldn't have asked for more. I cannot thank you enough for your help. Mary C. Freshfield, Liverpool, UK
  • This expert is wonderful. They truly know what they are talking about, and they actually care about you. They really helped put my nerves at ease. Thank you so much!!!! Alex Los Angeles, CA
  • Thank you for all your help. It is nice to know that this service is here for people like myself, who need answers fast and are not sure who to consult. GP Hesperia, CA
  • I couldn't be more satisfied! This is the site I will always come to when I need a second opinion. Justin Kernersville, NC
  • Just let me say that this encounter has been entirely professional and most helpful. I liked that I could ask additional questions and get answered in a very short turn around. Esther Woodstock, NY
  • Thank you so much for taking your time and knowledge to support my concerns. Not only did you answer my questions, you even took it a step further with replying with more pertinent information I needed to know. Robin Elkton, Maryland

Meet The Experts:

  • Phil


    Mechanical Engineer

    Satisfied Customers:

    Retired HVAC/ Electrical & Boiler contractor. Industrial
< Last | Next >
  • Phil's Avatar


    Mechanical Engineer

    Satisfied Customers:

    Retired HVAC/ Electrical & Boiler contractor. Industrial
  • gotoman for ac's Avatar

    gotoman for ac

    HVAC Technician

    Satisfied Customers:

    51 years of experience install design and repair of all makes and models of AC
  • Rick Mather's Avatar

    Rick Mather

    HVAC Technician

    Satisfied Customers:

    35 years experience, HVAC
  • Matthew's Avatar


    HVAC Technician

    Satisfied Customers:

    14 years experience in the HVAC field of service, installation, Manual J & D calculations
  • Kenton Guidry's Avatar

    Kenton Guidry

    HVAC Technician

    Satisfied Customers:

    12 years install/service experience.
  • Rick Ridley's Avatar

    Rick Ridley

    UK Heating Engineer

    Satisfied Customers:

    Retired But have had 30 years on the tools, Ex CORGI
  • Tom's Avatar


    HVAC Technician

    Satisfied Customers:

    16+yrs HVAC/R service. Residential Commerical Industrial