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problem with millivolt boiler controls

I'm kinda confused with the whole thermopile and gas valve operation. i'm a younger hvac tech who has very little run in with them. anyways i have a utica 76,000 btu gas boiler. still has the oem L8148G aquastat and has an old honeywell gas valve with a thermopile. Problem is the pilot stays lit but the main gas doesn't. thermostat is calling and the circulator is running so the relay is pulled in from the aquastat. the thermopile is generating about 400-450mv tied in. what now? i jumped the high limit thinking it was that but still nothing. boiler temp of 100 degrees.


  • centralhvactechcentralhvactech Member Posts: 6
    not a parts changer

    im not a parts changer so i dont want to just throw something until it works. I'm thinking the thermopile is fine and the gas valve is bad but still have doubts about aquastat
  • IronmanIronman Member Posts: 5,561
    Gas Valve

    What's the voltage at the gas valve?
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • centralhvactechcentralhvactech Member Posts: 6
    i measured at the aquastat

    it was about 250mv. what i dont understand is why it was different than what was at the thermopile.
  • centralhvactechcentralhvactech Member Posts: 6
    what does the thermopile need to draw

    I thought it was around 500mv give or take
  • Tim McElwainTim McElwain Member Posts: 4,357
    edited December 2010
    HVAC tech

    with the thermopile generator disconnected from the valve and manually holding the pilot knob you should get around 750 millivolts, the minimum for that reading is 540 millivolts. You should drop no more than 10 millivolts across any control. The exception is the thermostat if it is a powerpile Honeywell t'Stat you may get around 85 to 100 drop across the thermostat contacts.

    If you get in touch with me I have a complete manual on Powerpile which you can purchase. E-mail is [email protected]

    For starters just to make sure your source is good pull the pilot and clean the orifice and replace the generator.

    With the thermostat calling and bringing in the relay then the thermostat is 24 volts so forget about a drop across the t'stat.

    Place your meter on the PP terminals on the valve and with the system calling you should read around 250 millivolts.

    Jump out (temporarily) B1 and B2 on the relay does it come on? If yes then there may be a problem with the relay. If no then your problem is related to the generator or gas valve. Jump out the TH terminals on the gas valve does it come on? If yes then check the wires and look for bad splices. If no then the valve is probably bad.
  • centralhvactechcentralhvactech Member Posts: 6

    Sounds like the pile might be weak. It's sounds easy now that you explained it. I'm going to job tonight so ill keep you posted.
  • centralhvactechcentralhvactech Member Posts: 6
    all fixed

    ok after writing a huge paragraph about job and it being erased when i clicked post, this is gonna be a short version. the thermopile was the problem. old one generated about 640mv not tied in, new one about 710mv. hooked up and boiler fired up. thanks for the help.
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