New Coleman/York furnace blower erratic

Discussion in 'HVAC' started by Quattro, Nov 25, 2012.

  1. Nov 25, 2012 #1

    Quattro

    Quattro

    Quattro

    Massive Tool Belt

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    Hey all, it's been a while, but I'm back with a new question. Thanksgiving morning I installed (with the help of my retired-HVAC-instructor father-in-law) a new gas furnace in my house. Everything went fine, except for an errant screw through the A/C A-coil. Oops, that'll need a recharge! :eek:

    Anyway, the furnace lights properly, and the blower comes on to high speed shortly after. The problem is, once it's on, it will try to turn off and on again several times in the heat cycle. It never fully turns off, just "hiccups" as if someone cut the power quickly. So, you'll hear the fan spin up, then it sort of "dips", then comes back up, then maybe 10-15 seconds later, it does it again. Seemingly random...but it might do it 5-10 times in one heat cycle. It's very annoying, and I know it's not "normal". I've tried changing the jumper setting for the fan speed, and for the blower delay...no change.

    Does this sound like a control board malfunction? Could it be my programmable thermostat? There's plenty of heat, the only issue is the erratic fan behavior.

    Model TM9X060B12MP11, 95.5% 60K info here: http://www.micahvac.com/produse/123-heating/124-furnaces/120-lx_series_tm9x060_b12mp11.html

    Any ideas before I make the call to the vendor on Monday? If there's something I could try this weekend yet, I'd love to give it a whirl.

    Thanks!
     
    Last edited: Nov 25, 2012
  2. Nov 25, 2012 #2

    Wuzzat?

    Wuzzat?

    Wuzzat?

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    Measure the voltage across the closed blower motor relay contacts while the blower is running: it should be less than 0.03 vac.

    I had a case just like this and the old relay was right at the 0.1 vac mark while the new relay read virtually zero volts.
     
    Last edited: Nov 25, 2012
  3. Nov 25, 2012 #3

    Quattro

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    Thanks for the tip! However, I'm not sure I know where that relay is, or which contacts to measure. I do know how to use my DMM, though!
     
  4. Nov 25, 2012 #4

    Wuzzat?

    Wuzzat?

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    The relay will be maybe two cubic inches and can be found on the schematic, but going back and forth between the schematic and what you see in the furnace can be pretty challenging.
    In my old furnace the blower relay is on the control board. Hopefully your relay is socketed.
    You measure across the normally open contacts.
     
  5. Nov 25, 2012 #5

    Quattro

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    How do you test across the contacts while the relay is socketed on the board? How likely is it that a brand new furnace needs a new relay?

    Edit: I don't see any relays on the board. At least not "conventional" relays...

    I'll upload pics in the morning.
     
    Last edited: Nov 25, 2012
  6. Nov 25, 2012 #6

    Wuzzat?

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    It's one possibility.
    Normal failures may take years but early failures due to factory-caused defects are called "infant mortality".
     
  7. Nov 25, 2012 #7

    Quattro

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    Here are some photos! Could the thermostat have anything to do with this? The old furnace would do something similar, but not nearly as often.

    image-3525475163.jpg

    image-3938968577.jpg

    image-3568404346.jpg
     
  8. Nov 25, 2012 #8

    Wuzzat?

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    Wuzzat?

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    I'd monitor the current into the motor leads on the far right at whatever lead is supplying current for the speed selected. There will be voltage on all the leads so this may confuse you because the motor acts like a transformer with many taps.

    MOT = circulating motor = blower motor?

    But, the motor seems to run on line voltage so the lead that has the closest to line voltage when measured with respect to the common terminal is probably the one supplying the motor.

    If the voltage changes with the symptom then the motor is being told to act this way and so the problem is upstream.

    Since you can reliably reproduce this symptom you have a good chance of finding the problem. Intermittent problems are much worse to troubleshoot.

    It could even be the line voltage supplied to the furnace.

    I don't see a relay either but there are some indicated in the left hand ladder diagram. They must be somewhere.
    http://en.wikipedia.org/wiki/Ladder_logic
     
    Last edited: Nov 25, 2012
  9. Nov 30, 2012 #9

    Quattro

    Quattro

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    Supplier thinks its the motor. Sending me a replacement. Hopefully I can get it shoehorned in!
     

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