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Thread: Thermocoupler

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    DIY Junior Member flex's Avatar
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    Default Thermocoupler

    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
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    Homeowner Thatguy's Avatar
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    Quote Originally Posted by porter357 View Post
    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
    Gas valve solenoid coil resistance too low?
    Two defective TCs?

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    DIY Junior Member flex's Avatar
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    Quote Originally Posted by Thatguy View Post
    Gas valve solenoid coil resistance too low?
    Two defective TCs?
    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

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    Homeowner Thatguy's Avatar
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    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.

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    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    Quote Originally Posted by porter357 View Post
    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
    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.

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    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    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.

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    Homeowner Thatguy's Avatar
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    Quote Originally Posted by Doherty Plumbing View Post
    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.
    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 by Thatguy; 11-28-2009 at 05:47 PM.

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    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    Quote Originally Posted by Thatguy View Post
    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.
    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.

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    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    Quote Originally Posted by Doherty Plumbing View Post
    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.
    Have you tried this yet?

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    In the Trades Wally Hays's Avatar
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    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.

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    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    Quote Originally Posted by Wally Hays View Post
    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.
    Sure if he wants to pay for one. This is always the best bet.

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