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  1. #1
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    Goodman gas furnace will not fire with blower set to "ON"

    I have a new problem in one of my units, and I would certainly appreciate some thoughts. The homeowners set thermostat fan setting to "ON" and noted that the furnace would no longer start. I confirmed that myself this morning. Resetting to normal behavior requires turning heat/cool setting "OFF", blower set to "AUTO", and heat turned back on -- then cycles normally.

    Thermostat swap with proven good unit does not resolve the problem.

    The failure behavior seems backwards for a typical fan limit switch failure, but that is the only thing I can think to try next. Any thoughts appreciated. Thanks in advance!

    - jjb

  2. #2
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    I guess the first thing I would try is disconnecting all the wires at the LV terminals on the furnace board , and jumpering there, and see how it acts.

    Sent from my SM-G930V using Tapatalk

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  4. #3
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    Thread Starter
    Thanks -- I'll do that. What would be the expected behavior if I jumper G, and then energize W? Would you expect that the fan stays on during the entire furnace activation cycle, or should the board temporarily suspend the fan as the furnace fires up?

  5. #4
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    Bridgeton, MO, 'burb of St. Louis, Mo
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    Quote Originally Posted by JJBConsulting View Post
    Thanks -- I'll do that. What would be the expected behavior if I jumper G, and then energize W? Would you expect that the fan stays on during the entire furnace activation cycle, or should the board temporarily suspend the fan as the furnace fires up?
    You need to find where the colors changed. It sounds like the installer spliced wires somewhere and crossed colors.

    Example, we generally use yellow for Y. Sometimes installers use blue for Y.

    At the furnace, check voltage at R to G and R to W and G to C and W to C with tstat set for desired operation.

    The genius of the United States Constitution - when it frustrates the government, you know it is working.

    RIP Mr. Gizmo; end of watch 10/27/16

    It was a good situation, until everything went wrong!
    Rub some dirt on it and walk it off...
    We will always know it should have worked!

    “Don't believe signature quotes.” - George Washington

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  7. #5
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    Is this a GMP model?
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  8. #6
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    Jan 2018
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    Thread Starter
    I got back in to see the unit. The plot thickens... I forgot that we replaced this one years ago. Actually a Trane XV95 type unit.

    The furnace board throws a 3-flash error when calling for heat in "FAN ON" mode. So it is a furnace pressure switch failure that is preventing the furnace from firing. The same error does not occur when in AUTO mode. Switch does not close when in ON blower mode. I can create continuity in the switch by sucking on the hose however, and the switch closes normally in AUTO mode.

    Anyone understand this?

  9. #7
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    Dec 2014
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    Possible cracked htex or similar type situation.

    Sent from my SM-G930V using Tapatalk

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  11. #8
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    Sep 2012
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    thomaston, ct
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    Cracked HX


    Sent from my iPhone using Tapatalk

  12. #9
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    Jul 2009
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    I have a newer Concord furnace that does something similar. Only a year or two old in my house. If I leave the Blower on and it calls for heat the furnace will show a pressure switch or sometimes roll out code (But no roll out tripped). Doesn't do it all the time either. Normally when the Blower is on when there is a call for heat it will shut the blower down cycle heat on and when the boards Fan timer is done will bring the blower back on. I recently wired up a second thermostat upstairs to keep the blower on till the upstairs is within a degree or two of the downstairs stat then shut the blower off. I have a open lofted style house. So far it hasn't done it with the upstairs stat like it did when I left the Blower on with the Downstairs Thermostat.
    Last edited by Greend88; 01-04-2018 at 05:48 PM.

  13. #10
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    Thread Starter
    Thank you for your replies. You all had me pretty frightened with the cracked heat exchanger idea -- that makes sense though. If the blower fan was pushing air into the HEX, then the induction fan may not be able to produce enough negative pressure to pull the pressure switch.

    However, direct inspection of the HEX shows no cracks big enough to cause that, and I detect no positive pressure at the pressure switch port on the heat exchanger with the fan running and the furnace off.

    Here's the strange thing -- with the blower fan running, the furnace board throws the pressure switch failure BEFORE the induction fan even tries to turn. With the fan off, everything works just fine.

    I agree with Greend88 -- as I remember, this unit would wind the blower fan down prior to induction and furnace firing, and then start the fan up again after the furnace fires. I assume this is a function within the furnace board and not the thermostat. Now, it is not turning the fan off and I wonder if there is some reason why it can't suspend the fan during this cycle and that is what is causing the error.

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  15. #11
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    Sounds like a board issue. Greend88 should check for a service bulliten on that newer furnace.

    Sent from my SM-G930V using Tapatalk

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  17. #12
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    Quote Originally Posted by JJBConsulting View Post
    I got back in to see the unit. The plot thickens... I forgot that we replaced this one years ago. Actually a Trane XV95 type unit.

    The furnace board throws a 3-flash error when calling for heat in "FAN ON" mode. So it is a furnace pressure switch failure that is preventing the furnace from firing. The same error does not occur when in AUTO mode. Switch does not close when in ON blower mode. I can create continuity in the switch by sucking on the hose however, and the switch closes normally in AUTO mode.

    Anyone understand this?
    Could be a Bad Board, or in the worse case scenario, the "New" Stat that you swapped out, is also bad.

    Icy78 gave you Good Advice.

    Ck the Board first & if it's Good, then the low voltage control wiring, as well as this "Swapped out Stat".

  18. #13
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    Billington Heights, NY
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    Quote Originally Posted by JJBConsulting View Post
    However, direct inspection of the HEX shows no cracks big enough to cause that,
    What?! If you have cracks you have a failure. Multiple cracks can be cumulative. Plus, if it's cracked you'll also likely have other faults you cant see (like opened seams).
    Experience - knowing when to get the hell out of the way and plug your ears. "Don't be a sissy. Turn it on!"

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    Jtrammel - "I’m going to sell hvac systems derp derp derp"
    BBeerme - "every time he opens his mouth, he reminds me of a cow without the fart bag."

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