Gas furnace shows system "on" but doesn't run?

Discussion in 'HVAC Heating & Cooling' started by Confoozled, Feb 16, 2021.

  1. Confoozled

    Confoozled New Member

    Joined:
    Apr 6, 2020
    Location:
    Atlanta area
    Hi all - my upstairs gas furnace will click to system "on" once the temp goes below set point, but it doesn't actually kick on. It will sit there as the temperature continues dropping showing "on" at the thermostat.

    No fault codes in the LED window - light is steady on.

    Some background - thermostat is a 6 month old Honeywell 7-day which replaced the original 20 year old Honeywell which was having trouble detecting battery.

    I did some routine checking of the thermostat. For example with fan set to run, the blower runs fine. I also flipped it over to AC for a few minutes and the blower and compressor both run.

    About 3 weeks ago I replaced the igniter because I had the typical symptoms of having a small crack - one time it'd run fine, the next time it'd kick to heat "on" and blower would run 20 seconds then shut down, indicating the igniter wasn't pulling enough amps to open the gas flow. Then next time it'd run fine again, but then the time after that do the short blower run and shutdown again. So I replaced the igniter. Both the old igniter (about 12 years old) and new one were Emerson 767a-370.

    After I replaced the igniter it ran fine for about 2.5 weeks, but now, like I mentioned above, it says heat system is "on" but just doesn't want to run at all.

    Thoughts on this? I don't think it's the thermostat given it works on fan and AC mode, but I've been fooled before.

    And I'd think it's not the new igniter since it worked fine for a while, and is not now giving the "sometimes works sometimes doesn't" behavior I'd seen before. Instead indicates system on but doesn't do anything. But I've been fooled there before, too.

    Many thanks!
     
  2. fitter30

    fitter30 Well-Known Member

    Joined:
    Feb 2, 2020
    Occupation:
    Retired service tech
    Location:
    Peace valley missouri
    Have a meter? Thermostat calling for heat. There is a push button switch the at the blower door pushes in. Model of furnace and burner control.
    R to w 0 vac
    W to c 24 vac
     
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  4. Confoozled

    Confoozled New Member

    Joined:
    Apr 6, 2020
    Location:
    Atlanta area
    Hi Fitter30 - thanks much for your reply. Unfortunately I'm on the road most of the day so can only give a bit of information for now. The door switch must certainly be pushed in/close, because as mentioned, I can run the blower motor on fan mode and also on AC by toggling the thermostat. I don't think the blower could come on when I did those things if the door switch wasn't pressed in.

    I can check for model number when I return, but I do know there's no brand information on the unit itself as I've looked before. Just a tag from the contractor's installer. And I can check for the burner model number as well. I don't have a multimeter but will see if I can borrow one.
     
  5. fitter30

    fitter30 Well-Known Member

    Joined:
    Feb 2, 2020
    Occupation:
    Retired service tech
    Location:
    Peace valley missouri
    Take a.pic of the tag
     
  6. plumber69

    plumber69 In the Trades

    Joined:
    Jun 20, 2014
    Location:
    Prince Rupert, British Columbia
    You sure its the blower running when call for heat or the inducer. You should be getting some sort of error code. If not that makes me think thermostat. You could jumper r and w on the furnace to eliminate thermostat and thermostat wire.
    See if it fires up for a couple seconds and shuts off
     
  7. Confoozled

    Confoozled New Member

    Joined:
    Apr 6, 2020
    Location:
    Atlanta area
    Thanks both for your continued comments. The TL;DR version of the story below is it seems to be running after I polished the flame sensor.

    To respond to some of your comments - I can hear the difference between the draft inducer (barely can hear) and the blower because the return duct is right above the thermostat, so it's really clear (loud) when the blower kicks in.

    But when I got home last night it was indeed now flashing an error code as you mentioned Plumber69. 1 and 4, which the panel said is burner related. This model states it has a 3 hour lockout once that trips. But it only trips if it short cycles 3 times in a row. Since I didn't know how long ago the lockout began, I powered the the unit down, ate dinner and fiddled around a couple of hours.

    I went up and pulled the flame sensor and polished it a bit with a fine grit per the panel suggestion, put it back in, turned the thermostat back to heat and powered it up and it ran an entire cycle. And it did not short cycle the rest of the time we were awake, and as near as I can tell did not do so throughout the night (none of us woke up freezing). So possibly this has solved my current problem.

    But still, I did find the mfgr. information and burner model as Fitter30 suggested and wanted to post those for future reference. Like if in a couple of days, it kicks out on me again. The sticker was wedged kind of behind the burner unit so I apologize for the quality of the photos (hard to get the phone close enough without getting blurry, and I had to break it into two photos). It's a Payne model PG8UAA036065, serial number 4598A05942 (yes, built November 1998 - I realize I've gotten my money out of this furnace; the evaporator is a bit younger, from about 2010). The burner unit has numbers "36E 24 209" and "EF32CW200A" (note "W" might be a "U") and some symbols that probably indicate a manufacturer's logo, but I'm too ignorant here to know what they might mean.

    Thanks again to both of you for taking the time to respond!

    20210216_173846.jpg 20210216_174141.jpg

    20210216_174250.jpg
     
  8. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    Confoozled, some of those interlock switches let you pull out the plunger to do troubleshooting.

    Is there maybe a schematic inside the blower compartment somewhere?
     
  9. fitter30

    fitter30 Well-Known Member

    Joined:
    Feb 2, 2020
    Occupation:
    Retired service tech
    Location:
    Peace valley missouri
    Payne furnace is made by carrier along with several others. Recommend keeping a spare flame sensor and igniter. That 3 hours wait time for the flame failure thats for auto reset for another 4 tries. Try cycling the power.
     
  10. Confoozled

    Confoozled New Member

    Joined:
    Apr 6, 2020
    Location:
    Atlanta area
    Thanks both Reach4 and fitter30 for your additional follow-ups. For the blower compartment, if I needed power while I had it open I just taped the switch closed. Not terribly professional but it worked.

    Thanks for the info on Payne=Carrier. I do now have two "used but works" for the igniters but I like the idea of getting a spare flame sensor as well. Also good to know I can probably wipe the fault by toggling power if I have to deal with it again. I thought about it but was worried it might just start a new 3 hour cycle and it was already darned cold up there in the attic, and the temp dropping outside. So I was scared to give it a try.
     
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