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Thread: Furnace/ AC troubleshooting - Furnace blower fan won't stay on

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    Default Furnace/ AC troubleshooting - Furnace blower fan won't stay on

    Out at a family members cabin in Invermere. AC wasnt working well when the last family member was here, but of course they didn't do anything about it.

    Background info: This is a Napoleon 9600 furnace, converted to propane. the External AC is a big IQdrive Fridgidaire. Thermostat is honeywell I think.

    Now, when the thermostat calls for cooling, the large exterior AC unit starts up and runs, but the furnace blower fan inside that would normally circulate this cooled air tries to start, but after about half a second it shuts off. Then it does the same thing every 5 minutes or so, and never runs at all. Eventually the Exterior AC unit turns itself off, without the thermostat ever sensing the house air cooling off (because it hasn't).

    First step in troubleshooting was to bypass the fan limit switch, but that didn't change anything.

    I don't suspect the thermostat, as the furnace seems to be attempting to cool when called for.
    I don't suspect any issue with the AC unit, as it's starting and stopping normally, and the coil in the furnace is getting cold.

    Something is preventing furnace fan from starting and running.

    Since it's a long weekend, and this is BC, there's no way anyone will come take a look until Tuesday, or more probably several days after that. If there's any troubleshooting I can do before then, I'd like to try.

    @Darell_n or anyone else?
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    That last family member is a jerk.

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    What happens when you just turn the furnace fan on with the thermostat? If it doesn't start up regardless of mode, the start capacitor or the entire fan motor may be shot.

    If the furnace board has more than one speed output for the fan motor, try switching the leads to a different speed.

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    Quote Originally Posted by BerserkerCatSplat View Post
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    What happens when you just turn the furnace fan on with the thermostat? If it doesn't start up regardless of mode, the start capacitor or the entire fan motor may be shot.

    If the furnace board has more than one speed output for the fan motor, try switching the leads to a different speed.
    Yeah, the fan doesn't run in "cool" or fan-only modes. I can try a different fan speed. This board is jumpers, not different taps. It's an ECM motor (I think)
    Last edited by ExtraSlow; 08-01-2020 at 11:40 PM.
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    Can you reach the fan and manually turn it?
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    Quote Originally Posted by adam c View Post
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    Can you reach the fan and manually turn it?
    Will check. Did notice that the "CFM" light blinks on when the fan is attempting to start up, and then goes off again when it stops trying.

    EDIT: Fan spins freely and smoothly by hand. No grinding or other noises.
    Last edited by ExtraSlow; 08-02-2020 at 12:11 AM.
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    Quote Originally Posted by ExtraSlow View Post
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    Yeah, the fan doesn't run in "cool" or fan-only modes. I can try a different fan speed. This board is jumpers, not different taps. It's an ECM motor (I think)
    The other fan speed jumper positions made no change.

    I'm assuming it can't be good for this thing to be trying to start the fam every ouple of minutes all day, so I've shut it all the way off for the night. Hope having the windows open overnight can cool this place off a bit or we won't be sleeping much. Sigh, just came from a cabin without AC and was really looking forward to having a restful sleep. That's already fucked.
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    I'm definitely not an electronics engineer but if I was to take an educated guess, the control board for the fan isn't getting a signal that the fan is spinning so stops and restarts it every few minutes.

    Similar to my furnace sometimes in winter, there's a sensor that I need to clean otherwise it doesn't get a signal that the furnace is firing and ends up going into fail safe mode
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    As a blockage engineer, my first thought would be to check the intake and exhaust tracts for obstruction and drug money.

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    Is there a run or start capacitor on the blower motor, if so check it.

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    Quote Originally Posted by adam c View Post
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    I'm definitely not an electronics engineer but if I was to take an educated guess, the control board for the fan isn't getting a signal that the fan is spinning so stops and restarts it every few minutes.

    Similar to my furnace sometimes in winter, there's a sensor that I need to clean otherwise it doesn't get a signal that the furnace is firing and ends up going into fail safe mode
    Could be right, it spins up, but stops spinning within a second or so. Makes me think that it's stopping itself because of some sensor. the CFM light that flickers on for a second might confirm that? Or maybe that's me just wishing.

    So, on a furnace, what sensors need to agree to allow the blower fan to start and remain running?
    - I think there's some kind of "overcurrent protection" for the blower motor. is that part of the control board? Not a physical fuse, because it's happening repeatedly.
    - I think the "limit switch" needs to be closed. Normally used for heating, but I think it's in the loop for the fan in all modes? I don't think it's this, because I bypassed that and it didn't change anything.
    - what else?
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    Quote Originally Posted by BerserkerCatSplat View Post
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    ..., the start capacitor or the entire fan motor may be shot...
    Quote Originally Posted by The_Penguin View Post
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    Is there a run or start capacitor on the blower motor, if so check it.
    Hmm, I'll have a look. I have spent my life knowing nothing about how furnaces work, but I'm learning slowly.
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    Man, all of this technical/sensor shit that can go wrong with a modern furnace just scares the hell out of me if our old beastie with no electronics ever gives out.

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    Should have purchased a Weber. How is it flashing? I still bet some timbits on the blockage/pressure switch failure.
    Do you have something long and skinny you can stick in the intake/exhaust apertures?

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    Last edited by The_Rural_Juror; 08-02-2020 at 09:04 AM.

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    Quote Originally Posted by The_Rural_Juror View Post
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    How is it flashing?
    It's not the red or green fault codes LED's that flashes. it's a different amber one on teh other side of the board that flikers on for a moment only.

    Quote Originally Posted by The_Rural_Juror View Post
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    I still bet some timbits on the blockage/pressure switch failure.
    Do you have something long and skinny you can stick in the intake/exhaust apertures?
    Hmmmm, yeah there's a long "direct vent" type intake and exhaust. I'll check those.



    And to follow up on the capacitor idea, doe ECM motors even have a start/run capacitor? this furnace is ECM, and I can't see the capacitor anywhere. Is that only for PSC motors?
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    Try to jump out R to G on the thermostat input terminal strip. This should run the blower at 50%. This will eliminate the thermostat as a problem, but it sounds like the control module integrated into the X13 motor is bad. Would require a voltmeter to confirm. The X13 blower is the basic replacement of the old PSC motors, but not a full blown variable speed drive.

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    Quote Originally Posted by Darell_n View Post
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    Try to jump out R to G on the thermostat input terminal strip. This should run the blower at 50%. This will eliminate the thermostat as a problem, but it sounds like the control module integrated into the X13 motor is bad. Would require a voltmeter to confirm. The X13 blower is the basic replacement of the old PSC motors, but not a full blown variable speed drive.
    Thanks Darell. And just to confirm, there's no Capacitor on this type of ECM motor, right?
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    Could be shutting off because not enough airflow. Had exact same symptoms on my furnace one winter and it’s because the filter was all clogged up and not letting enough air through (we had Reno’s going on and it had been a while since I checked it).

    Checked the filter yet?? Seems stupid but it fits the description of the problem.

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    Quote Originally Posted by cjblair View Post
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    Could be shutting off because not enough airflow. Had exact same symptoms on my furnace one winter and it’s because the filter was all clogged up and not letting enough air through (we had Reno’s going on and it had been a while since I checked it).

    Checked the filter yet?? Seems stupid but it fits the description of the problem.
    Have been running my diagnosis with filter removed. No change.
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    Looks like all else has failed. APEGGA handbook for troubleshooting methods says to deliver a swift strike, with precision, to each discreet component.

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