Furnace shuts off and restarts back up repeatedly

Discussion in 'HVAC' started by Christian, Jan 11, 2011.

  1. Jan 11, 2011 #1

    Christian

    Christian

    Christian

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    Hi all.

    Our furnace is doing this thing where it'll start up, run for 1 minute, then shut off. Then rinse and repeat. Sometimes it'll get up to the temp set on the stat, sometimes not. This is a forced air gas furnace, new in 2006.

    If I go downstairs and hit the electric kill switch on the furnace to shut the power off, then turn it back on after a minute or so, the problem seems to go away temporarily, but after a few minutes of heat running we're back where we started.

    Thoughts??

    Thanks in advance for the advice guys.
     
  2. Jan 11, 2011 #2

    paul52446m

    paul52446m

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    There are different things that can give you this problem. Is this a 80+ or 90+ furnace. Does it have plastic stacks? What make and model is it?
    If you have a thing that looks like a nail by your burners, this would be a flame rod. They get a coating build up on them. Take some fine sand paper
    and clean this flame rod.
    Are you burners burning good and blue?
    Check all hoses on the pressure switches for cracks and bad ends.
    If this is a 90+ furnace take off all drain and pressure switch hoses and use a small wire to see if the ports are clear.
    Is this a up flow or a down flow furnace
    You have changed your filter so you have good air flow?
    When the burner turn off , does the blower keep going Or does the burner turn off before the blower comes on?. You need to stand in front of the furnace and give me as much info as you can about when it goes out.
    Ok you have things to check and questions ti answer
    Also if its plastic stacks, check them out side to see if they are clear.
    Later Paul
     
  3. Jan 16, 2011 #3

    Christian

    Christian

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    Sorry for the delayed reply...still behaving the same. Would really love some help!

    Is this a 80+ or 90+ furnace. 80+

    Does it have plastic stacks? no

    What make and model is it? Mod#: n8mpn075b12a1

    If you have a thing that looks like a nail by your burners, this would be a flame rod. They get a coating build up on them. Take some fine sand paper
    and clean this flame rod. I also read that this could be dirty and be the problem, but I'm unable to find it. See picture below...maybe you can tell me where to find it?

    Are you burners burning good and blue? whey they're on, yes they look blue and full

    Check all hoses on the pressure switches for cracks and bad ends. where can I find the pressure switches?

    Is this a up flow or a down flow furnace return is at bottom, so flow up?

    You have changed your filter so you have good air flow? yes brand new filter

    When the burner turn off , does the blower keep going Or does the burner turn off before the blower comes on?.

    OK, when it starts up after 'resting' a while, the burners kick on then the blower kicks on and it works good for a while. Once running for maybe 10 mins, the burners go out and there is a bright orange light glowing for a couple of seconds, then that goes out (blower motor sounds like it's still running). Orange light goes out then comes back on and off again another time or 2, then everything shuts off. Just before everything shuts off, there's an audible 'click' noise, and the code light starts blinking as follows:

    The Honeywell valve code light (I think that's what it is...the green light on the front of the valve by the on/off switch?) is blinking 6 times, short break, 1 blink, and repeat. Maybe that will help diagnose the problem? In the picture, the light is clear, located on the front of the unit near the bunch of colored wires.

    Here's what I'm working with:
    [​IMG]
     
    Last edited: Jan 16, 2011
  4. Jan 16, 2011 #4

    kok328

    kok328

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    Should be between burners #2&3 where those two sheet metal screws are holding the ignitor assembly in place.
     
  5. Jan 16, 2011 #5

    Christian

    Christian

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    That's where I thought it was...but I wasn't sure.

    I located it, took it out and cleaned it with light sandpaper, and put it back in. I didn't notice any really significant buildup of crud on there or anything though.

    I'll post tomorrow morning on how it worked out. Thanks guys!
     
    Last edited: Jan 16, 2011
  6. Jan 17, 2011 #6

    Christian

    Christian

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    OK...since cleaning the flame sensor...same as before. No change in behavior. Is this worth outright replacing (they're not too expensive from what I can see).

    I also tried changing the batteries in the stat...no change (figured there wouldn't be but we tried that anyways). Could this be a stat problem? How can I check that without buying and installing a new stat? Is there a way to bypass it and 'force' the furnace into heat mode? If it stays on then, it's not the stat right?

    Thanks for the help guys.
     
  7. Jan 17, 2011 #7

    paul52446m

    paul52446m

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    If you want to jump the stat. , go to your furnace and where the low volt wires tie on, put a jumper wire from r to w and that will bypass the stat and make the furnace run all the time. On one of your door there should be a code for
    you blinking lite. Later Paul
     
  8. Jan 17, 2011 #8

    Christian

    Christian

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    Is there anything else that would help a newb distinguish these wires from the other bundles of wires in the compartment? :rolleyes:
     
  9. Jan 17, 2011 #9

    kok328

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    I would only jump the stat in the event that you want to eliminate the stat as a source of the problem. For all other troubleshooting, simply set the desired setpoint on the stat a few degrees above actual.
     
  10. Jan 17, 2011 #10

    Christian

    Christian

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    Yes that is my plan.

    The house is holding steady at 64, but won't heat further than that...so I thought it could've been the stat...but I successfully jumped it with no luck. The furnace did not stay running.

    Still in search of ideas! One other thing I'm going to try is to check the exhaust piping to make sure it isn't clogged...that could cause it to overheat also from what I understand.
     
    Last edited: Jan 17, 2011
  11. Feb 21, 2011 #11

    suckassmule

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    Christian, does your furnace run (heat for 10min.) in approx. 1 degree intervals, then shut down, then restart, heat approx. 1 degree, shut down, etc., etc?

    If so, it sounds like yours is doing what mine does. I've been poking around looking for answers myself and believe it may be a defective Smart Valve. Seems Honeywell produced a batch of defective valves or built them with defective circuit boards/soldered connections that crack over time and create intermittent and various problems with the flame sensor and/or ignitors within your furnace. I'm not a 100% certain that is what is wrong at this point with mine but I'm leaning heavily towards that being it. I need to pull the cover off the Smart Valve and verify the defective soldering points to confirm and likely will suck it up and purchase a new SV rather than repairing since a few posts have indicated that the re-solder is a temporary fix in some cases and living in Colorado w/ occasional sub-zero weather cannot take a chance of a complete furnace failure.

    Hope that helps. It's all I know at this point.

    Mule
     
  12. Dec 27, 2012 #12

    j7735

    j7735

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    hi my furnace wont stay lit it cuts off and the blower motor never turns on only when in ac mode the pilot lite stays lit
     
    Last edited: Dec 27, 2012
  13. Dec 27, 2012 #13

    tinmantu

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    from afar, it sounds like the blower motor died and it's tripping out on high limit. Could just be a capacitor. Might be worth a service call if you aren't handy with testing electrics.
     
    Last edited: Dec 27, 2012
  14. Jan 13, 2013 #14

    jwm02g

    jwm02g

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    Did you ever figure it out? Mine is a gas furnace, pretty new (probably at least after 2000, maybe more like 05 or later). You can hear two distinctive switches when it comes on, I would guess the heat and the blower. But now, sometimes only the first switch clicks, after a min the second one flips, but then immediately flips back of. It's like the heat comes on, but the blower doesn't blow. Then it blows for literally less than 5 seconds, and goes off. Sometimes it flips back and forth several times in just a few seconds. Diagnostic lights show no errors, and changing the filter doesn't help. Haven't tried cleaning that rod yet because I am not sure I could find it.
     

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