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Electrical Problem

I've got a customer that owns a small apartment building with radiant heating in Berkeley.  They have 2 original A.O.Smith cast iron boilers that are wired in parallel.  When a thermostat calls for heat, a relay closes sending 24 volts to both boilers via an outdoor sensor (WWSD) and an aquastat; very simple.

The problem is that when you try to connect the green ground wire on boiler #2 from the ignition control module (S8610U) to the grounding clip on the gas valve, there's a spark and then boiler #1 shuts down and boiler #2 won't fire.

I've replaced the ICM to no avail.  And I've by-passed the WWSD and aquastat......nada.

I could leave the ground clip off the gas control to allow it to work properly, but this does not seem right.  I'd like to find out the answer.

Can anyone help?
Often wrong, never in doubt.


  • Steamhead
    Steamhead Member Posts: 15,329
    There's a ground fault somewhere

    at this point, you'll have to isolate each component from ground and test for the fault with a voltmeter. This will take some time but I can't think of anything faster.

    First place I'd try this is that gas valve.

    Once you prove your innate genius to the owner, I'm sure you'll talk about the benefits of a 2-stage ODR setup.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Ground Fault

    Hi Frank:

    How exactly do you do that?
    Often wrong, never in doubt.
  • Steamhead
    Steamhead Member Posts: 15,329
    What I think has happened

    Is that the metal housing of some component has 24 volts on it. When you connect the ground wire, it shorts the 24 volts to ground. If you left it like that long enough the transformer would blow.

    If it is the gas valve, it's possible that the pipe dope at the joints is a good enough insulator that 24 volts won't overcome it.

    With the boilers running, take your voltmeter and measure between the gas valve's aluminum housing and ground. I bet you read 24 volts between these two points. If so, remove the gas valve from the piping and re-test it to be sure. If the gas valve still shows 24 volts between the housing and ground, replace it.

    If you remove the gas valve and there is no voltage between the housing and ground, reverse the 24-volt wires and try again. It may be that the fault is more towards one terminal than the other.

    If you reverse the wires and still get no reading between the housing and ground, there is something else energizing the pipe and housing. Look for a bare wire or something similar.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Wayne_16
    Wayne_16 Member Posts: 130
    Check for any voltage on the netural wire

    First of all place a resetable circuit breaker (3 amp) in your ground wire circuit to protect the transformer.

    Sounds like there is some voltage leaking someplace to ground.  Check each leg of power to each boiler.  Check incoming power for proper polarity, check on how much voltage or amperage is on the netural and ground wires.

    Check tranformer phasing issues, If thats the case, I thank someone on the wall for providing the pdf document.

    Dont overlook there may be some other appliance or wiring in the building creating the grounding problem.  I would not overlook anything.  One idea would be to turn off circuit breakers one at a time to see if you can identify the circuit having the problem.


    Minnsesota Wayne
  • jp_2
    jp_2 Member Posts: 1,935
    edited May 2010
    lost the common

    sounds more like somewhere you have lost the common return.

    take a long wire and connect it to a known ground, then connect that to your meter and start measuring voltage here and there.

    you could also have lost your ground connection too.  or a real poor one causing voltage drop.


    or you have 2 problems, a short to ground by a device and a floating  ground, so when you ground the ground, you see another problem.
  • Timco
    Timco Member Posts: 3,039

    I Have seen many issues like this where the polarity was reversed at the disconnect for the boilers. Makes it impossible to shut off power to boiler without killing the main.

    Just a guy running some pipes.
  • Short

    Thank you, Frank for describing that procedure in layman's terms; something I can deal with.

    And thank you, Wayne for that most excellent .pdf on transformers.  It explained a lot of things and taught me how to phase a transformer.

    I believe that the problem is localized at boiler #2 since boiler #1 works fine until you connect the ground at boiler #2.

    I'll let you know what I find out.  I'm with Frank so far to say it's a short within the gas valve. 

    All the best,

    Often wrong, never in doubt.
This discussion has been closed.