Heater Running But Thermostat Went Blank -- Help Please!

Discussion in 'HVAC Heating & Cooling' started by e2139adfa, Jan 7, 2021.

  1. e2139adfa

    e2139adfa New Member

    Joined:
    Jan 7, 2021
    Location:
    California
    My honeywell wifi thermostat went blank while my heater was running yesterday and now I can't shut off the heater unless I unplug the York furnace. Here's what I've checked so far but still can't figure out what's wrong and the cause of my blank thermostat. Would appreciate if anyone can give their expertise and suggestions, thank you in advance.

    1. Changed battery on thermostat (it's powered by c-wire but changed battery just in case).
    2. Used multimeter to get a low voltage VAC reading from the wall where my thermostat is installed - no voltage detected.
    3. Checked circuit breakers. Not tripped. Checked voltage at breakers, confirmed breakers are working fine.
    4. Checked fuse inside furnace - fuse is fine, not blown.
    5. Checked furnace transformer. 120v coming in fine and there are two low voltage leads going out. One had 24v reading (#1 in picture) and the other had 0v reading (#2 in picture). Did not do a resistance test.
    6. York furnace light blinking green at intervals of four.
    7. Checked low voltage wiring leads on furnace board (wires that go to the thermostat, outlined in red in picture) -- no voltage on any of the leads. (heater was turned on and running at this time)

    Tempted to just replace the transformer but concerned it might go out again if something is wrong with the system. Also tempted to just put in a new fuse (even though it looks fine) or resort to replacing the thermostat but it's only 4 years old and if I'm not getting power at the thermostat wall, highly doubt the thermostat is the issue.

    Appreciate any/all help, thank you so much!


    IMG_0001.jpg
     
  2. fitter30

    fitter30 Well-Known Member

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    Occupation:
    Retired service tech
    Location:
    Peace valley missouri
    Terminal strip that you box if it has marking R, C,G, W, Y
    Meter
    R-C should have 24vac
    R-W 0 vac thermostat calling for heat
    R-W 24 vac thermostat not calling for heat
    Disconnect W at terminal strip with power off, turn furnace back on heat should not run
    If that works reconnect W
    Thermostat
    R-C 24vac stat has failed
    Disconnect W at thermostat heat should shut off
     
    Last edited: Jan 7, 2021
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  4. Reach4

    Reach4 Well-Known Member

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    Location:
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    There is one transformer in that picture, and the 1 and 2 labels label output terminals.

    To measure the transformer output, you want to measure between 1 and 2, which should be the same as measuring R-C as fitter30 said.
     
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  5. e2139adfa

    e2139adfa New Member

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    Location:
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    Thank you. I measured the 1 and 2 out from the transformer and got a 25 vac reading. However tried measuring R-C at the terminal strip like @fitter30 suggested and got only a 1.2 vac, likewise for the reading of R-W.
     
  6. Reach4

    Reach4 Well-Known Member

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    Location:
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    I think transformer #1 would make it to R, except for something blocking the path.
     
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  7. fitter30

    fitter30 Well-Known Member

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    Where is 1&2 landed? Take a pic of the wiring diagram and the model number.
     
  8. e2139adfa

    e2139adfa New Member

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    Hope these pictures help. 1 and 2 are the gray wires along the bottom of the second picture that land on the board.
     

    Attached Files:

  9. e2139adfa

    e2139adfa New Member

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    could you explain what you mean? Thanks
     
  10. fitter30

    fitter30 Well-Known Member

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    9 pin plug on board unplug power off . With wire side facing you check continuity 3-8 top left to middle right both red wires
    On the top of diagram there are 4 safety switches a limit ,2 roll out and rds (don't know what it is). If theres a open follow the red wires check each safety.
    On diagram four flashes limit open. Doesn't make sense that the heat runs with limit open. If that is true pull board has to have burn marks on the back side.
     
    Last edited: Jan 10, 2021
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    The thermostat acts like a switch. WHen it is calling for heat, it applies the transformer output (R) to the furnace via the W wire. That 24vac goes through (usually) a relay where the other side is connected to C, or the other side of the transformer to create a closed circuit for the voltage/current to operate the relay. The switch in the thermostat may not be perfect, so when you measure some voltage across it, you're measuring what amounts to the resistance of the switch relative to the coil in the relay. If the switch was 'perfect', the voltage drop across it would be nearly zero. That 'switch' may be a triac, and that will have some voltage drop across it. That's essentially an electronic, solid-state switch.

    As said, if you remove the W wire from either the furnace or at the thermostat, if the furnace doesn't turn off, the furnace is at fault. If the furnace does turn off when you remove the wire, the thermostat is bad.

    Using test tools can mislead you to where the problem is without a bit more understanding of how to use it, and what it means. If you do have a triac versus a switch in the thermostat, you couldn't get a reasonable resistance measurement across the two lead, and, you can't measure resistance with the power on.
     
  12. DavidDeBord

    DavidDeBord Member

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  13. e2139adfa

    e2139adfa New Member

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    Location:
    California
    I checked the red wires on the 9 plug and one of them have a 25 vac reading and the other red wire gave 1.3 vac.

    I don’t understand the other directions you gave. Can you explain? Thank you
     
  14. fitter30

    fitter30 Well-Known Member

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    Location:
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    If there is 25 volts between the red wires there is a limit open there should be 0 not move than 2 volts. A closed switch will have 0 volts across contacts full voltage if open.
     
  15. e2139adfa

    e2139adfa New Member

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    Jan 7, 2021
    Location:
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    Ended up being a stuck limit switch. Called a HVAC guy out and he tapped it and wallah!
     
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  16. Dana

    Dana In the trades

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    In my local dialect of 'merican that's spelled "voilà ", but of course YMMV. ;)
     
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