Thermocoupler

Discussion in 'Boiler Forum' started by flex, Nov 23, 2009.

  1. flex

    flex New Member

    Messages:
    5
    Location:
    ohio
    hi guy's this is the thermocoupler i use one year I had to replace it two times is there something I'm missing here. its on a boiler that heat's the house.

    Honeywell 36 In. Lead Length Universal Thermocoupler - Gas

    Attached Files:

  2. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    Gas valve solenoid coil resistance too low?
    Two defective TCs?
  3. flex

    flex New Member

    Messages:
    5
    Location:
    ohio
    thanks do you have any pictures so i can see what I'm suppose to replace or at lease have an idea when i call a repair guy
    thanks
  4. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    You'll have to find an HVAC forum or ask the manuf. You also may need to run some voltage/current/resistance measurements but the elec. spec's on HVAC parts seems to be Top Secret.
  5. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    Put in a brand new thermocouple and do a drop out test on it to see how strong of signal it's putting out.

    If it puts out a weak signal try another new thermocouple (some are just bad from the factory). If BOTH seem to be putting out weak signals it's quite possible your gas valve is starting to fail.
  6. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
  7. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    Ignore this procedure, or atleast the valid times it gives for how long the drop out should take. Unless you have a Bosch water heater but I reread the entire post and don't see where you said you had one.

    In BC the thermocouple is allowed to keep the gas valve's pilot light on for up to 90 seconds after you blow the pilot out.

    If I do a drop out test on a thermocouple and it can't keep the gas valve on longer then about 30 seconds I change it out. I like to see 50 seconds+ though.
  8. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    While I have your attention, do you have any factory spec's on mV or 24 vac gas valve solenoid min. voltage/min. current requirements? A lot of people want to use a 1.5v AA cell to check a mV valve, and these spec's will probably show it is damaging to the valve.
    Thanks.
    Last edited: Nov 28, 2009
  9. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    Well a mV gas valve running off of a thermocouple uses about 28mV.... I don't know if a 1.5v AA battery would hurt it but I wouldn't think it's good for it.
  10. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    What current does the 28mV valve take to open? That's a spec I can't find.

    What about the current those valves draw that use a 750 mV TC? These TCs have an internal resistance of 3 ohms so pulling 125 mA to run the valve would drop the valve voltage to ~370 mV.
  11. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    Those valves run off a thermopile which puts out 750mV, or 27x that of a thermocouple. Like a fire place....etc.

    What the amperage draw is of the valve I'm not sure. If I can find my old school books I can look it up cause I know there is info like this in there.
  12. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,060
    Location:
    Cave Creek, Arizona
    t'couple

    A thermocouple that gets too hot will fail, or more precisely will stop generating power to maintain the coil in an open position. IF the main burner flame is so large it is encompassing the thermocouple, or if the pilot light is not on the tip of the thermocouple, its operation will be erratic. Overheating and age are about the only two causes of thermocouple failure, and if yours are going bad that quickly age is probably not the cause.
  13. flex

    flex New Member

    Messages:
    5
    Location:
    ohio
    i think its more about the flame i looked at the old one and its burned on the side the the top
  14. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    Have you tried this yet?
  15. nhmaster3015

    nhmaster3015 Master Plumber

    Messages:
    836
    Location:
    The granite state
    Wouldn't the best thing be to call a service tech out to check the gas valve and thermocouple? They have the proper test equiment.
  16. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    Sure if he wants to pay for one. This is always the best bet.

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