Suspect bad inducer motor in gas furnace - need help

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Old 12-08-12, 10:58 AM
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Suspect bad inducer motor in gas furnace - need help

I have a 373LAV (Horizonatal) Bryant Model Gas Furance (about twelve
yrs old). The furnace works intermittenly, but usually doesn't work. I
am getting a 31 code, meaning "Pressure, Draft Safeguard, or aux limit
switch did not close or reopened."


What I noticed is that when the thermostat calls for heat, the inducer
motor starts to spin, but trips prior to reaching full speed, and
thereby stopping the furnace startup sequence. Sometimes (rarely) the
inducer motor will reach full speed, which allows the startup sequence
to continue to flames. But usually, the inducer motor trips prior to
reaching full speed.


I have replaced the pressure switch and draft safeguard limit switch.
I do no supect the furnace vent exhaust duct piping to be plugged,
because when the unit trips, the limit switch does not need to be
reset and it also trips with this limit switch jumpered. In addition,
the flue stack has a well designed cap on the roof that would make it
extremely diffcult for anything to get in.


I have also run the self component test on the unit and the igniter
and blower seem to function fine, but again the inducer motor does not
run to full speed and trips pretty quickly.


I suspect the inducer motor is going bad...though I am not sure how to
check it. I don't want to replace the inducer motor unless I will be
doing so with some favorable confidence that it is indeed bad. Any
thoughts on how I can check the inducer motor or does the inducer
motor have a start capacitor that might be bad as well?


Appreciate any help please.
 
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Old 12-08-12, 12:20 PM
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Is the inducer getting 120VAC when it quits?

Have you removed the vent pipe and inspected the inducer for debris that would prevent the inducer from spinning? (mud divers nest)
 
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Old 12-08-12, 02:11 PM
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As Houston implies, the circuit board is probably not turning on the inducer motor reliably.

Use a meter to see if you have 24 VAC being switched on reliably by the thermostat to the W terminal. If you do, the inducer motor should have power switched on reliably to the inducer motor whenever you have 24 VAC to the W terminal.

Check to see if the circuit board is failing to keep the 120 VAC power on to the inducer motor, which is the likely problem. If so, replace the circuit board.

If you do have 120 VAC power to the inducer motor and the inducer motor is starting or staying on, you probably have a bad inducer motor.
 
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Old 12-09-12, 05:49 AM
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Thank you both for the feedback, very helpful.

As suggested, I tested the AC voltage on the W terminal during an attempted startup of the furnace and it kept constant at 26VAC, even after the inducer motor failed to work.

I also tested the AC voltage on one of the wires going to the inducer motor during an attempted startup and noticed the voltage was at 0 volts initially and about a minute later the volts jumped to 31VAC at the same time the motor began to spin, then less than a second later the volts went to 0 VAC and the motor stopped.

As additional information: during my testing, on two occasions, the inducer motor worked well (was getting a steady 122VAC) and allowed the startup sequence to proceed to flames.

Is this a conclusive test proving I have a bad circuit board? Or is there something else I can test to confirm, prior to replacing the board?

Thanks again for your advice.
 
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Old 12-09-12, 07:06 AM
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Trying to Help

There is a draft safety switch also. It should be located where the inducer blower is connected. This switch is for a down draft. sometimes a pic can help. Can you take a pic of the inducer fan assembly inorder to see how it is configured with the vent system.
 
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Old 12-09-12, 07:58 PM
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This heat exchanger is known for failure. Please have it inspected by a professional.
 
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