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  1. #1
    Join Date
    Feb 2013
    Location
    Champaign, IL
    Posts
    45

    This call has me confused!

    One of our guys performed pm on a two system house, got a call a day later that the second floor system wasn't working/keeping up. He worked on it for a while, I was told to go see if I could figure it out. Here are the details:

    Call for heat, inducer kicks on, pulls 1.97" wc, ps needs 1.35 to close. Ps closes, ignitor heats up. Gv energized, opens for less than a second and closes. Not open long enough for the flame to flow acrossed all four burners, literally less than a second and the gv is de-energized by the board.

    Call for heat does not drop out.
    Pressure switch does not drop out.
    Inducer does not drop out, continues to pull 1.87 to 2" wc throughout the cycle.
    No limits are opening to cause interruption.

    Burners are clear all the way acrossed, flame sensor is clean (not on long enough for flame sensor to come into play at this point)

    Here is the kicker, twice while trouble shooting the furnace lit and stayed lit. Once I shut it down and attempted to refire, problem reoccurred.

    Have a new board installed at this time, same issue.

    Furnace is a 94 Trane 90% upflow.

    Any ideas/thoughts are greatly appreciated!

  2. #2
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    Does it use the old radiant flame sensor system, or does it have a normal flame sensor rod?
    If more government is the answer, then it's a really stupid question.

  3. #3
    Join Date
    Dec 2004
    Location
    Caledonia WI
    Posts
    882
    Some times things happen faster than your meter can respond. Did you jump out pressure switch or thermostat? Also check wiring and gas pressure when valve is powered. At least it happens enough for you to see it.
    Nothing is magic, it works or doesn't work for a reason.

  4. #4
    Join Date
    Dec 2012
    Location
    Spokane, Washington
    Posts
    124
    Check your ground. A simple test is to make a "test" 12g ground wire from the board to a "good" known ground. It doesn't take long, is cheap and if it doesn't work you can at least cross it off your list. Good luck
    A smooth sea has never made a worthy mariner
    English proverb

  5. #5
    Join Date
    Oct 2011
    Location
    Las Vegas, NV
    Posts
    526
    The few times I've had the gas valve being mysteriously de-energized it turned out to be the pressure switch dropping out faster than my meter would respond. For a split second.

  6. #6
    Join Date
    Apr 2010
    Posts
    93
    What piece of equipment are you working on?

    Did you try to jumper the pressure switch? Do you continue to have 24v signal to the valve?

    I had similar occurrence with an older Rheem 80%. Valve would open for a split second then close, occasionally staying open.

    I'm calling a failing solenoid in your gas valve.

    In past experience mentioned, I would measure ~20-22 volts when the valve would open for a split second, sometimes not open and measure ~15v, sometimes 24v and would hold and valve would stay open, but rarely and very intermittently.

    Removed leads from solenoid and test, would measure 24v and hold consistently.

    Replaced gas valve and batta bing.

    And btw although those funky voltage readings did obviously tell me of a problem, they may jot be exactly entirely accurate, as like someone said above, can happen so fast your meter barely detects fluctuations.

  7. #7
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    19,531
    The ground jumper should generally be used between the burner closest to the flame sensor, and to the board ground. Aside from boards in modern equipment that sense the presences of the equipment grounding conductor that comes from the service panel, the ground between the burners and the board are all that the board cares about for flame sensing.

    Yes, you can jump out the safeties one at a time, but I'd consider if another gas appliance is running when this one tries to fire, causing low inlet pressure. Or, a defective gas valve.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

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  8. #8
    Join Date
    Jul 2005
    Posts
    5,576
    You have a poor connection somewhere, track it down. Could be literally anywhere in the active circuits, both low and high voltage, including the premise wiring, breakers, etc.,. It's a process of elimination. Test jumpers should help. If you're not losing 24 volts to the valve, then it's the valve.

  9. #9
    Join Date
    Sep 2002
    Location
    Virginia
    Posts
    3,327
    snug every wire nut , and every screw with a wire

    spade connectors , take off , gently pinch tighter with pliers , put spades back on

    Now keep your meter on the gas valve and fire it up ... see if you lose power to valve when it drops out

  10. #10
    Join Date
    Mar 2001
    Posts
    1,529
    This is one of the times that an old analog meter will be better at checking the valve input than a digital.
    One way to outthink people is to make them think you think. They'll think you're not really thinking what you're trying to get them to think you think...........

  11. #11
    Join Date
    Feb 2013
    Location
    Champaign, IL
    Posts
    45
    In response to all of the above advice:

    Normal flame sensor rod.

    I have jumped out the stat, but not the pressure switch - I will try that.
    I understand that these things can happen faster than a meter may be able to respond, yet did not think to jumper out pressure switch. I did monitor inducer pressure, did not go low enough to affect ps, yet could be that my meter response time is too slow.

    I am losing 24 volts to gas valve.

    I will make sure that no other gas appliances are fired. Incoming gas pressure: 6.5. Unable to measure manifold gas pressure, drops out too quickly.

    I will check all connections, ensure good ground, and jumper out limits one by one when I go back. Thanks for the advice so far!

  12. #12
    Join Date
    Jan 2007
    Location
    Mn the state where absolutey nothing is allowed
    Posts
    1,381
    do you get a fault code?

    im thinking cold solder joint on the board or possibly in the gas valve internals.

    sometimes the best tool for this is you... start wiggling wires. hw smartvalves have been known for poor internal connections a grabbing the harness and moving it around works on thoses.. can happen on the IFC at the point where the gas valve wires connect.

    good luck
    my boss thinks its possible to repeal the laws of physics

  13. #13
    Join Date
    Dec 2011
    Location
    Twin Cities Minnesota
    Posts
    277
    I made up a "Test" 24 volt transformer. plug into 120volt.,two jumper wires with clips! works great to test gas valves etc.

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