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  1. #1
    Join Date
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    Default Short fan cycle on oil furnace (different from other post)

    Our furnace is 16 years old and we live in an older home.

    For the past two days our furnace starts to heat, the fan starts and then a few seconds later the fan shuts off and the burner continues to run and it continues this short cycle. We have had two different technicians come in. The first one replaced the fan/limit switch and that made it a lot worse. Then someone came back and a made some adjustments to the switch but it still runs a short cycle.

    We added two returns upstairs in the fall. Is there anyway that this has something to do with the short cycle and is it really possible that the air is so cold that it is drawing in that it would shorten the cycle so much? It didn't do this last year and it was just as cold.

    Does anyone have any thoughts on this? (I apologize if I didn't describe this too well, heating systems are not my speciality I will be more than happy to answer clarifying questions if I can.

    Thank you, I really appreciate it.

  2. #2
    Join Date
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    Default Re: Short fan cycle on oil furnace (different from other post)

    The blower kicks on when the high limit is reached in the heat exchanger and shuts off when the low limit is reached. By adding additional returns you are supplying additional cold air cooling the heat exchanger.

    You may be able to balance the system by adding baffles in the returns or, if the motor is multi speed, running the blower on a slower speed. Proper balance is necessary for proper function. You may also be able to change the nozzle for more heat but you'll use more fuel when it is running.
    Jack
    Last edited by JLMCDANIEL; 12-16-2009 at 06:15 PM.
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  3. #3
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    Default Re: Short fan cycle on oil furnace (different from other post)

    It could be a bad stat, loose G wire at the Stat. Are the settings for the Blower on 130 and off 110 at your limit switch? I doubt very much that your two new returns have anything to do with whats going on but, you could cover them with a piece of cardboard to see what happens. Don't upgrade the nozzle without first calling your Furnace Manufacturer and seeing which ones can be used, you don't want to put more BTUs into the furnace than it's been designed for, you could end up with a cracked heat exchanger.
    Goo Luck!!!

  4. #4
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    Default Re: Short fan cycle on oil furnace (different from other post)

    Ditto on the return vents. They wouldn't have anything to do with the blower motor shutting down.
    The " G " wire really only has to do with the manual on for the blower ----- I don't believe it would cause what's happening here ----- though it doesn't hurt to check it anyway.

    Did any of the tech's check the motor itself or the wiring the the motor inside the furnace or the capacitor or the motor start contactor ?

    If there is an Auto / Manual ( Auto / On ) switch on the thermostat ----- try switching it from Auto to Manual to turn on the blower to see it if quits after awhile.
    If it does then there is something wrong with the motor or it's associated wiring.

    2 cents.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  5. #5
    Join Date
    Dec 2009
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    Default Re: Short fan cycle on oil furnace (different from other post)

    Thank you for your '2 cent' responses. It is so frustrating that we are the one trying to figure it out when I would be hoping the technicians would just take the time to check everything.

    Tried the cardboard, no luck. Not sure about the nozzle, I know that we got a new one last year when we had the service done, but didn't pay attention to what it even was until I saw it on the receipt.

    I don't know what the G wire is on a the stat (or even what that is). I do know that the settings on the limit switch are at 180 and 90 (I don't know what they were before but this helped to increase the timing. We also turned the fan down to medium. At the onset of starting the heat, the fans shuts on and off quickly. As the temperature increases, the fan stays on longer. Now I asked both of the technicians if it could be the fan motor, because that was something that my father mentioned. They both said no (and they didn't do anything to test it) I think that there is an auto/manual switch because I remember the tech saying "Automatic". Things to try out (at least my husband can try it)

    I should see if I can find a schematic of the furnace ******. I am welcome to more options if anyone else has some.

  6. #6
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    Default Re: Short fan cycle on oil furnace (different from other post)

    I don't know what the G wire is on a the stat (or even what that is).
    stat = thermostat

    The termostat has wires that are connected to label points . Typically if there are 4 wires attached to the thermostat they would be connected to the points labled .....
    W
    R
    G
    Y

    3 wires would be attached to ........
    W
    R
    G

    The wire attached to the " G " goes to the furnace to control the blower ( fan ) to turn on manually when you move the switch on the thermostat to Manual ( or On ).
    Normally this switch is in the Auto position.

    If you try the test mentioned above and the blower ( fan ) doesn't shut off then it sounds to be an intermittant wire connection either from the limit switch or to / from the motor or the limit switch itself. The limit switch is what turns on and off the blower ( fan ) automatically.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  7. #7
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    Default Re: Short fan cycle on oil furnace (different from other post)

    The only thing I can think of is your limit switch is gone or as Canuck says, the blower motor. Good Luck!!!

  8. #8
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    Default Re: Short fan cycle on oil furnace (different from other post)

    Or the tech may have installed a smaller nozzle when he serviced it.
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  9. #9
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    Default Re: Short fan cycle on oil furnace (different from other post)

    Would that cause the burners to stay lit and shutdown the blower ?
    "" an ounce of perception -- a pound of obscure "
    - Rush

  10. #10
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    Default Re: Short fan cycle on oil furnace (different from other post)

    Quote Originally Posted by canuk View Post
    Would that cause the burners to stay lit and shutdown the blower ?
    Yes, it would reduce the BTU output below that needed to keep up with the designed airflow.
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

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