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Lennox furnace stuck in heat mode

pinkstonms
pinkstonms Member Posts: 9
in Gas Heating

Furnace kept recycling never getting a flame, after checking power through the system I diagnosed the gas valve was bad it was getting 24-28v but valve never opened. I replace the gas valve now the system cycles and the flames ignite. Both red and green lights are flashing fast indicating normal operation and heating is functional. BUT now I can't get it to turn heat off. Every time I turn the system on it acts as though there is a call for heat from the thermostat and the flames ignite even though I have the thermostat in the OFF mode.

I have reset the thermostat, I even swapped it with another one in the house that was functioning properly.

I am sort of lost at this point could the board be bad? How would I check to see if the board is malfunctioning?

Comments

  • ratio
    ratio Member Posts: 3,923

    What happens if you take the (should be white) wire off of the "W" terminal on the furnace control board? If it stops then, the stat wire is probably shorted somewhere.

    bjohnhy
  • pinkstonms
    pinkstonms Member Posts: 9

    I will try that and get back to you, Thank you

  • pinkstonms
    pinkstonms Member Posts: 9
    edited February 23

    OK pulled the white wire in terminal W, powered on furnace does nothing. I set thermostat to heat

  • ratio
    ratio Member Posts: 3,923

    Ok, control board seems to be working properly so put the wire back onto the W terminal. Does the thermostat plug on to a subbase that stays wired to the thermostat wires? If so, unplug it; in not, take the white wire (or whatever color was attached to the W terminal in the furnace) off & see what happens. Also take a close look at the wires & how they are stuck into the connectors on the thermostat/subbase. Sometimes they'll be stripped back a little too far and will make contact—the furnace can't tell the difference between that and a heat call.

  • pinkstonms
    pinkstonms Member Posts: 9

    Yes it is a Nest thermostat

    IMG_2939.jpg
  • pinkstonms
    pinkstonms Member Posts: 9

    I pulled the white "W" wire off the terminal on the control board

  • pecmsg
    pecmsg Member Posts: 5,746
    https://forum.heatinghelp.com/discussion/comment/1844485#Comment_1844485

    🤭

  • ratio
    ratio Member Posts: 3,923

    Ok, with the Next unplugged (& the white wire landed back on the furnace board), does it still heat?

    How 'bout with the white wire removed from the Nest subbase?

  • pinkstonms
    pinkstonms Member Posts: 9

    OK Pulled off the Nest Thermostat and pulled white wire going to W1, the system shut down.

    Replaced White wire to W1 but left thermostat off base system comes on and stuck in heat mode, flames don't go out.

  • ratio
    ratio Member Posts: 3,923

    When you swapped the thermostat, did you swap the base as well?

    Right now, we tried:

    1. no wire to heat terminal (W) in furnace = heat off
    2. wire on heat terminal but no subbase at the other end of the wire = heat off
    3. wire and subbase = heat on

    Nests are known to be finicky from time to time, but your application seems to be vanilla, it should be working—but there are a few more things to look at before we condemn the subbase. In particular, what terminals are the 5 wires landed on in the furnace? Red → 'R', White → 'W', Yaller → 'Y', Green → 'G', and Blue → 'C' or 'Com', right?

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 17,281

    Shut the furnace off at the power switch

    Connect the thermostat wires at the furnace for normal operation.

    Take the Nest off the wall and disconnect all the wires from the sub base.

    Leave the thermostat wires at the thermostat separated. Turn on the furnace power switch

    Touch the R wire and the W1 wire together the furnace should run. Disconnect them the furnace should turn off.

  • pinkstonms
    pinkstonms Member Posts: 9

    I only swapped the head of the thermostat not the base.

    correct

    1. no wire on W furnace terminal FURNACE OFF
    2. no wire on W thermostat terminal FURNACE OFF
    3. wire on furnace W and Thermostat W head off thermostat off FURNACE ON

    I have attached photo of control board terminals

    IMG_2941.jpg
  • ratio
    ratio Member Posts: 3,923

    Ok. It appears that the subbase is failed. Try swapping in the subbase from the other thermostat too (you don't need to screw it back onto the wall for testing) & see if it works right.

    Nest tech support might be able to help you further, but at this point I'd condemn the Nest subbase & replace it along with the stat itself, but maybe hang on to the old stat for a spare since it seems that part isn't defective.

    You can try @EBEBRATT-Eds suggestion above (Red & White wires together = heat on, Red & White wires apart = heat off), but I expect it will work right.

    HVACNUTEBEBRATT-Ed
  • pinkstonms
    pinkstonms Member Posts: 9
    edited February 23

    Thanks Ratio yeah pretty sure ebebratt-eds is right but I will test, not sure what that will tell me though. The furnace will come on just cant get thermostat to tell heat to shut off.

    I will try swapping base from other unit and see if that works.

    i'll get back to you, Thank you really appreciate your help.

  • JUGHNE
    JUGHNE Member Posts: 11,357

    I believe I am blessed to never have even seen a Nest.

    Does the Rc and Rh need jumped together or is the Nest an all knowing device?

    LRCCBJPC7060
  • ratio
    ratio Member Posts: 3,923

    Nest Knows.

    Goodness, is that their next sales jingle???

  • pinkstonms
    pinkstonms Member Posts: 9

    OK light at the end of the dark tunnel. ebebratt was right red and white connected furnace on, diconnected furnace off. More importantly I swapped the thermostat and base from other side of the house and furnace is functioning as it should heat comes on demand and off when switched to off.

    Ordering a new Nest thermostat

    Thank you Ratio, God bless you! HVAC service call out of the budget for this retired guy.

    bjohnhyPC7060
  • ratio
    ratio Member Posts: 3,923

    Glad you got it figured out!

  • pecmsg
    pecmsg Member Posts: 5,746
    https://forum.heatinghelp.com/discussion/comment/1844562#Comment_1844562

    the NEST IS the problem

    LRCCBJ
  • HVACNUT
    HVACNUT Member Posts: 6,607

    That model Nest is famous for shorted sub bases. But at least they surprise you. Sometimes you get heat with your AC or AC with your heat.

  • LRCCBJ
    LRCCBJ Member Posts: 904

    The Nest is a good example from my comments on the other thread. An expensive device that is fairly low in quality compared to others…………….that the company has marketed very successfully to people who don't know any better. They believe the product is superior to an alternative simply via the marketing.

    Very similar to the company selling gutter covers…………….!!!

    Learn from THEM.

  • bjohnhy
    bjohnhy Member Posts: 106
    https://forum.heatinghelp.com/discussion/comment/1844546#Comment_1844546

    A jumper is not needed

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 17,281

    @JUGHNE

    "The Nest is all knowing"

    I fixed one at my nephews house. He installed a Nest himself with no "C" wire and it was hooked to T & T on a cad cell control. It worked for a year and the battery died.

    Why it didn't cook the transformer in the cad cell I have no idea.

    I added a sep trans and relay and hooked up the C wire

    I had to take the Nest off the wall to pull more wire but the R wire was hooked to Rc (he has no cooling) and i couldn't pull the wire off the sub base it wouldn't release so I cut it.

    When I reconnected I put W & C and put the R on Rh and it worked it also worked when he had it on Rc so it must be "all knowing"

    JUGHNE

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