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Thread: Tempstar gas valve cycling off and on

  1. #1

    Default Tempstar gas valve cycling off and on

    I have a Tempstar gas furnace (model NUGE105kgo2) that while the furnace is running will have the gas valve cylce off and on a couple times during the heating cycle. It will turn off then immediatly turn back on and the igniter will relite the burners. Does not seem to happen every cycle but most of them. I checked voltage to and from the thermostat at the transformer and when the cycle happens there is a small voltage spike of less than 1 volt at both red and white wires. The time the gas valve is off to when it comes back on is so short it may not give my meter time to see a voltage drop. Anyone have any ideas?

    Mike
    Mike
    Tacoma, WA

  2. #2
    Join Date
    Dec 2003
    Location
    Kent, WA
    Posts
    8,431

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    Are you seeing a voltage spike (increase) or dip (decrease)? It is normal for an inductor (e.g. a heating contactor and perhaps even a gas valve) to produce a voltage spike when it is de-energized.

    So not really sure if the problem is a loss of command for heat (thermostat) or some internal problem shutting down the gas valve. You could just jumper the red and white wires to bypass the thermostat and see if the problem persists. May also want to measure the voltage directly at the gas valve to see if it is being commanded off or if it is just stopping on its own.

    Some better volmeters have a bar graph display along with the digital numbers. This will allow you to see faster transitions which you can't see by number.
    Mark
    Kent, WA

  3. #3

    Default

    Mark,

    I am seeing a slight voltage increase. I figured it could be caused by the gas valve energizing or deenergizing for a moment. My next test was to jumper the thermostat. I will let you know what I find.

    Sure is cold here at least for us in the Puget Sound. We are just not used to these kind of temps.
    Mike
    Tacoma, WA

  4. #4

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    Okay, after jumping the transformer I was still getting cycling out of the gas valve. So I know it is not the thermostat. I think since the problem is internal to the furnace I will just call a repair man and let them figure it out. I am not going to mess around with the guts of a furnace.

    Thanks again Mark. Stay warm!
    Mike
    Tacoma, WA

  5. #5

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    The tech adjusted the blower speed up a step. That seems to have taken care of the problem. It may not have been pulling the heat out fast enough. Why it would start now however is a mystery. I had not changed the speed of the fan. He said if that doesn't work I would probably need a new control module. I was able to get one off ebay for cheap just in case that becomes a problem. On another note, what kind of oil do I use to lube the blower motor? I want to add a few drops as it has been a while since it has been serviced.
    Mike
    Tacoma, WA

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