INDUCED DRAFT BLOWER RUNS CONTINUOUSLY

Discussion in 'HVAC Heating & Cooling' started by Thomas G, Jun 11, 2019.

  1. Thomas G

    Thomas G New Member

    Joined:
    Jun 11, 2019
    Location:
    Akron OH
    I have an Armstrong furnace installed when I built the house in 2000. Just recently I started to have problems. First issue was the induced draft blower would start, burners would come on but main blower would only run occassionally. Had the blower motor replaced by a tech. Main blower runs fine now but can't get heat. The Induced draft blower (IDB from now on) runs continuously and never seems to clear the sequence to let the burners fire. AC runs fine. Tech thought it would be the relays on the control board but wanted too much to replace it. I replaced it with the factory authorized upgrade and the problem still exists. I have checked the IDB flue discharge and plenty of air is coming out so there is no blockage. I've tried disconnecting the hoses to the diaphram on the furnace front, tried pinching them and tried disconnecting the wires from the switch on the diaphram (pressure switch?). IDB still runs. Looking for what to try next to narrow this down so the next replacement part is the right one.
     
  2. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    Replacing a 20 year old furnace one part at a time is expensive and frustrating. I feel your pain, but it's hard to offer specific advice without so much as a model number.

    Given it's age and the fact that it's most likely to be ridiculously oversized it's probably time to start thinking about replacing the whole unit with something right-sized for the load. Since you probably have multiple heating season's worth of fuel use data (exact dates and amounts) it's possible to use the old furnace to measure the heat load to get a sense of how big a replacement furnace should (or more specifically shouldn't) be for maximum comfort. (Bigger is the opposite of better when it comes to hot air furnaces.) In my area it's far most common to see furnaces 3x or more oversized for the 99% load, and relatively rare to find one right-sized (= 1.2-1.4x oversize factor for the 99% load).

    At least think about it. Most furnaces will make it to 25 years or more before the reliability, maintenance, and repair issues point toward outright replacement, but many hit that point before age 20. Yours may be one of the latter. Replacement time is an opportunity moment to right size and gain an improvement in comfort, not just reliability or efficiency. Even if you're not ready to make that step this year, running the fuel-use load measurements way ahead of time is worth it, since that day is eventually coming.
     
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  4. Thomas G

    Thomas G New Member

    Joined:
    Jun 11, 2019
    Location:
    Akron OH

    Thanks for your note Dana. I have all the model info, as follows:

    - Furnace - Armstrong GUK125D20-3A
    - Serial No. - 8400E16109
    - Main Blower Motor - A.O. Smith 3/4 hp, Model F48J38A45 ( replaced with something "universal" by the repairman)
    - Control Board - Honeywell ST9120C2028, PN 40403-001, replaced with PN 40403-003 (current upgrade)

    Had the repairman take a look when he did the blower and heat exchanger is in good condition. Considering furnace replacment is probably in the 5 grand range I think I'm still ahead of the curve on this one. That said, these things don't last forever so I'll be taking a look at the data as you suggest so I'm prepared when the time comes.
    Back to the issue of the IDB running constantly, based on the wiring diagram on the furnace panel and that came with the control board I put that in correctly. The exception could possibly be the thermostat wires; for those I did not look at the diagram but installed them to the same terminals as they were originally. Tomorrow I'm going to connect the 2 wires that go to the diaphram and see if completing that circuit stops the IDB. Any other trouble shooting suggestions would be welcome.

    Regards
    Tom
     
  5. Stuff

    Stuff Active Member

    Joined:
    Mar 7, 2013
    Location:
    Pennsylvania
    To identify what is going on:
    1. Power off system
    2. Disconnect all thermostat wires from the control board.
    3. Power on system
    Do indoor blower and inducer come on? If so indicates a roll-out or limit switch is tripped.

    Most of the time the diaphragm/pressure switch is only looked at after a heat call from the thermostat.
     
    cacher_chick likes this.
  6. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    I couldn't find an online manual for the GUK125D20, but I suspect the "125" is the input BTU (125,000 BTU/hr), which means it's likely to have at least 100,000 BTU/hr of output.

    For reasonably tight 2x6 /R19 type construction with clear glass (not low-E) double panes that would be right sized for a ~4500-5000' house at +5F (Akron's 99% outside design temp), enough oversize capacity to cover even Polar Vortex disturbance event cold snaps. Is that your house? 100,000 BTU/hr is enough furnace output to heat my 2x4 framed sub-code 2400' house + 1600' of insulated basement with clear glass storms over the 1923 vintage wood sashed single panes down to about -100F outside, a temperature not seen in my neighborhood (or Akron's) since before the last Ice Age.

    For a typical 2000-3000' new-ish house in Akron it's likely to be on the order of 3-4x oversized. (Remember, ASHRAE draws the line at 1.4x oversizing for best comfort & efficinecy in residential heating.) If it's married to a central air conditioner using the same air handler the AC is probably similarly oversized. A pretty-good 30-40,000 BTU/hr condensing 2-stage can be had for under $1500, and installation isn't rocket science- often DIY-able, but it will be more complicated and expensive if the AC has to be replaced too. If that's the case it's a good idea to get a handle on both loads. Sometimes/often the best solution is a modulating heat pump to handle both heating & cooling, even if gas is currently somewhat cheaper from an operating cost point of view. If gas is a LOT cheaper a right-sized hydro-air solution with a cooling coil and hydronic heating coil in the air handler operating off a condensing water heater can work out well.

    The AC oversizing factor can be inferred by measuring it's duty cycle on afternoons when the temperatures are near the 1% outside design temp, which in Akron is 85F. As with heating systems, oversizing the AC by more than ~1.5x is going to reduce rather than enhance comfort. To the untrained it might seem like it's "struggling to keep up" with very long on-cycles, but that will do a lot more for reducing the humidity than a 3x oversized AC in

    So, go ahead and patch the furnace up- control components are relatively cheap, but start measuring your loads, think about what makes sense for a replacement.
     
  7. Thomas G

    Thomas G New Member

    Joined:
    Jun 11, 2019
    Location:
    Akron OH
    Hi Stuff,

    Thanks for your suggestion. Sorry it took so long but I messed up my back and getting that low can only be done early in the day and I've had other demands on my back that prevented me from getting to this sooner.

    I disconnected everything as you suggested. Induced Draft Blower comes on but main blower does not. I also jumpered the diaphram switch with thermostat connected and disconnected with no impact; IDB still comes on right away. The only things I see left are the ignition control that attaches to the gas valve and a sensor of some sort on the side of the igniter chamber. I have no idea how to check the ignition control and I looked at the sensor on the side of the ignition chamber and didn't see anything, but don't know if I would. I'm starting to think the brand new board I purchased is bad but I have to think the likelihood of that is remote. Ready for the next step.

    Regards
    Tom
     
  8. Thomas G

    Thomas G New Member

    Joined:
    Jun 11, 2019
    Location:
    Akron OH
    Dana,

    My home is 2x4 construction, 33oo sq ft plus about 1200 sq ft basement. Based on that I'd say my furnace is probably about right based on your note. AC is about 3 ton as I recall, maybe 3.5 so that should be about right too. Will keep your notes in my furnace file for future use though.

    Regards
    Tom
     
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