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thermostat not turning on gas wall heater

LandW
LandW Member Posts: 4
Hello!
my thermostat seems to have stopped getting my gas wall heater to turn on.
It is a relatively old heater and a relatively new thermostat. Someone who came to look at it said they were incompatible but it had been working fine for years.
The pilot light stays on and all the connections seem tight.
any help much appreciated!






Comments

  • Jamie Hall
    Jamie Hall Member Posts: 20,421
    What if anything happens if you jump the two terminals at the heater to which the thermostat connects? If that doesn't work, then it's a problem at the heater. If it does, then try jumping the same two wires at the thermostat. If it DOESN'T turn on there's a problem in the wires. If it turns on, then the problem is in the thermostat...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    MikeAmannrick in Alaska
  • neilc
    neilc Member Posts: 2,085
    land that red wire better at the valve, from here it could be shorting to the valve case
    known to beat dead horses
    MikeAmannHomerJSmithrick in Alaska
  • neilc
    neilc Member Posts: 2,085
    check them all for clean and tight
    known to beat dead horses
  • pedmec
    pedmec Member Posts: 712
    If you can jump the thermostat connections at the gas valve and it works and then jump at the thermostat and it doesn't work doesn't necessarily mean its a bad wire. It could be a dirty or bad thermocouple. Its only generating a very small voltage. To put it in perspective it is .001 of a volt. So to open that gas valve the powerpile is designed to generate .750 volt or 750 millivolt. Another term for voltage to understand is its called ELECTRICAL PRESSURE. As the voltage travels thru the wire it acts much like water flowing thru a pipe. Its going to lose its electrical pressure as its traveling thru the electrical loop. When the millivoltage finally returns to the valve it needs a minimum of 350 millivolts to open the main valve. 350 might not apply to all millivolt gas valves but there is a minimum required. So the thermocouple needs to be clean to generate the maximum amount of millivolts for its journey. Remove and use scotch brite pad to clean. Then test. if you have a multimeter that can read millivolts you can measure the millivolts
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 4,918
    edited October 2022
    Someone who came to look
    needs to learn to read. the MV on this device means Millivolt.
    This is the perfect thermostat for your heater.


    To test the gas valve and pilot generator place a jumper wire where indicated on the following picture
    Finally if you find the burner is working, then you need to follow good wiring practice and make the wire connections under the screw terminals in a proper fashion
    Edward Young Retired HVAC Contractor & HYDRONICIAN Services first oil burner at age 16 P/T trainer for EH-CC.org
  • HomerJSmith
    HomerJSmith Member Posts: 1,920
    It's either the thermostat (not likely), the wiring (probably not likely), the gas valve (maybe), or the thermopile (most likely). The pilot assembly need cleaning, too, when replacing the powerpile. If you never replaced the powerpile in a bunch of years do it.
  • LandW
    LandW Member Posts: 4
    Thanks everyone!
    when I jump the wires at the heater it turns right on!
    I don't have a millivolt tester, is it safe to say the problem is the thermopile and I should just replace?
    /there seem to be different brands that all look the same, will any 750mv thermopile fit?

    thank you!
  • Jamie Hall
    Jamie Hall Member Posts: 20,421
    Good start! But before you condemn the thermopile, try jumping the wires at the thermostat. Granted, it could be a weak thermopile, but you still haven't eliminated a possible problem in the wires, nor in the thermostat.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • LandW
    LandW Member Posts: 4
    sorry, I misread your first instructions.
    I put the old thermopile back in. When I jump the wires at the heater it turns on, and when I jump them at the thermostat it turns on as well. So that means the thermostat must be the problem? Should I just replace it with the same model?
  • MikeAmann
    MikeAmann Member Posts: 681
    Don't condemn the thermostat just yet. It could be just dirty connections. I see that the front and back mate together with this connector. Clean those pins with a pencil eraser and wipe on a little dielectric grease (if you have some) and try it again.


  • neilc
    neilc Member Posts: 2,085
    woah, woah, woah,
    check that first picture above, the tan wire, is bare at that bend to the right, just above the "BD#9" hashed over with red,
    cut that bare bend back and reland it,
    it's probably shorting to the stat
    known to beat dead horses
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 4,918
    LandW said:

    Thanks everyone!
    when I jump the wires at the heater it turns right on!
    I don't have a millivolt tester, is it safe to say the problem is the thermopile and I should just replace?
    /there seem to be different brands that all look the same, will any 750mv thermopile fit?

    thank you!

    The Thermopile is good if "the heater turns right on"

    There is a series of wires and switches that start at the Red Wire on the Gas valve and end at the White Wire on the gas valve. As long as the burner is able to operate when the jumper wire is placed between the Red and the White wire, then the burner will operate when the wires from the Red wire screw to the White wire screw are all properly connected and all switches and thermostats are closed circuits, the burner should operate.
    If there is a problem, you need to trace the wires to each switch and check which switch is not making a closed circuit. (Closed circuit means: the switch is turned on or thermostat is calling for heat.)



    Edward Young Retired HVAC Contractor & HYDRONICIAN Services first oil burner at age 16 P/T trainer for EH-CC.org
    MikeAmann
  • LandW
    LandW Member Posts: 4
    neilc said:

    woah, woah, woah,
    check that first picture above, the tan wire, is bare at that bend to the right, just above the "BD#9" hashed over with red,
    cut that bare bend back and reland it,
    it's probably shorting to the stat

    got excited but did this and still nothing...

    LandW said:


    If there is a problem, you need to trace the wires to each switch and check which switch is not making a closed circuit. (Closed circuit means: the switch is turned on or thermostat is calling for heat.)

    sorry that's a little over my head.. where are the switches I would trace the wires to? what are the "limits" in your diagram?
    when I close the circuit at the heater it works, when I close it at the thermostat it works, where else could the circuit be broken?



  • Jamie Hall
    Jamie Hall Member Posts: 20,421
    Think. if you close the circuit at the heater and it works, and then if you close it (I presume you mean cross the two wires) at the thermostat it works... there's only one thing left, isn't there? The thermostat. It's either miswired (it happens) or not working.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • mattmia2
    mattmia2 Member Posts: 7,148
    edited November 2022
    Do you know those batteries are good? Might be enough to light up the lcd but not to pull in the relay.
    MikeAmann