Gas Furnace Blower motor does not always cycle

Discussion in 'HVAC Heating & Cooling' started by Dave Savastone, Oct 22, 2015.

  1. Dave Savastone

    Dave Savastone New Member

    Joined:
    Oct 22, 2015
    Location:
    Weirton, West Virginia
    Good Morning!
    We moved into a house a few months ago that have a 20 year old gas furnace with AC and an air cleaner, etc. The unit functions correctly but I noticed lately that this happens during heating:

    1. Furnace fires and heats up correctly.
    2. Furnace then goes off without blower motor engaging
    3. However, I have checked thermostat and the house is staying at designated setting
    4. If I raise the temp on the thermostat, the furnace ignites and then shuts off, then a few seconds later the blower motor starts to cycle warm air. This continues to work.

    Just wondering if this was normal for most gas furnaces. If the thermostat is not calling for additional heat, will the blower just not run?
     
  2. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    Classically, the furnace blower motor is controlled by a thermostat that turns on when the air is warm enough. The few controls I have seen have a setting on that thermostat that allows you to change the temperature at which the thermostat turns on. Maybe you can improve things a bit if you slide the control to a setpoint that is a bit cooler.

    I have very limited experience. If your furnace is a fancy one with a microprocessor, my experience may not apply.
     
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  4. Dave Savastone

    Dave Savastone New Member

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    Oct 22, 2015
    Location:
    Weirton, West Virginia
    Thanks. I have investigated similar threads on the internet and they focus on maybe a bad sensor on the blower motor? I have a repairman coming in the next couple of days to check things. Just seems odd since I thought the blower motor should engage each time the heat exchanger is heated...
     
  5. Dana

    Dana In the trades

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    The more simple-minded hot air furnaces use bimetal disc type thermostats to activate a relay/contactor that turns on the power to the blower. The thermostats are pretty reliable, but they do fail. The relay or contactor is a more likely to fail- dirty or worn contacts that arc every time the blower turns on or off is pretty common after 20 years of service, and 100,000 cycles. But the symptoms don't necessarily point in this direction.

    It's possible that the thermostat wire is broken somewhere, or loose/corroded/ intermittent at some point along it's path. If the initial call for heat fires the burners, but then disappears due to the intermittency, the heat generated on the intial burn can be enough to eventually trip the blower thermostat, causing it to run tepid air for a few minutes.

    Many heating/cooling thermostats will also have a "blower on" switch, which should run the blower, which tests at least part of the system wiring. But pulling the thermostat itself and checking all the wiring at the T-stat for corrosion or loose connections would be a good first step.
     
  6. DonL

    DonL Jack of all trades Master of one

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    It that a learning thermostat by chance ?

    They can do that, In anticipation, They can turn gas off before the the fan bimetal disc senses enough heat to turn the fan on. Normally this will happen when the furnace is in a cold location.

    The fan control is normally mounted on the top of the furnace, and has a Hi/Low temperature setting.
    If you lower the temperature to much, It can blow cooler air, instead of warm air.

    Good Luck.
     
  7. Dana

    Dana In the trades

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    Location:
    01609
    Some thermostat & furnace manufacturer names & model numbers would take some of the guesswork out of it, eh?

    This is sort of like guessing about the symptoms exhibited by a car when all you know about it is that it's is 20 years old, has a motor, uses a keyed ignition, and might be blue.
     
  8. DonL

    DonL Jack of all trades Master of one

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    I agree.

    20 years old covers a few makes and models. :eek:

    It is hard to see from here. :D
     
  9. Dave Savastone

    Dave Savastone New Member

    Joined:
    Oct 22, 2015
    Location:
    Weirton, West Virginia
    I noticed this morning that the blower fan would work if I set it to ON on the thermostat, so, I am wondering if it is some type of sensor on the motor itself that is not working when the switch is set to AUTO?
     
  10. Reach4

    Reach4 Well-Known Member

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    Location:
    IL
    The sensor is not on the motor itself.
    Still getting the repairman?
    If you post a photo of the furnace with the cover removed, somebody might be able to point out your sensor/thermostat.
     
  11. DonL

    DonL Jack of all trades Master of one

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    Do you have a model number ?

    This is a typical fan control that mounts on the side of the furnace.

    [​IMG]
    [​IMG]

    [​IMG]

    You do not have to open the furnace, You just remove the cover, With Power turned Off, to adjust it.

    A person that knows what they are doing, Will know exactly what to do. It may need to be cleaned.

    Good Luck.
     
    Last edited: Oct 23, 2015
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Retired Systems engineer for defense industry.
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    If the fan does not manually come on (and it worked before - it may not have that ability even though your thermostat does), then it's probably the control relay (or if it's smarter, a circuit board) that's bad. Probably time to replace the relay. If you feel comfortable about it, you could measure the coil of the fan relay to see if it is getting power. If it is, you should be able to verify if power is getting through the relay. No power, won't run. Most older furnaces used relays for this. The wiring diagram should show how things are connected, if you don't have the manual, there's usually a diagram inside the cover.
     
  13. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    If the fan does not manually come on (and it worked before - it may not have that ability even though your thermostat does), then it's probably the control relay (or if it's smarter, a circuit board) that's bad. Probably time to replace the relay. If you feel comfortable about it, you could measure the coil of the fan relay to see if it is getting power. If it is, you should be able to verify if power is getting through the relay. No power, won't run. Most older furnaces used relays for this. The wiring diagram should show how things are connected, if you don't have the manual, there's usually a diagram inside the cover.
     
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