Inducer motor cycles on and off and end of heat cycle ???????????????????

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Old 10-24-11, 10:12 AM
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Question Inducer motor cycles on and off and end of heat cycle ???????????????????

Furnace will start normally and heat up the house. Problem is at the end of the heat cycle and the main blowe kicks off, The inducer moter will continue to cycle on and off. I can stop this by removing the tube from the inducer moter and sucking on it. Blowing and sucking on the rubber hose will start or stop the motor. I removed the exhaust pipe and it still did this with no resrtictions. The pressure switch was a .40" and I replaced it with a .40. Still doing it. I dont have a manometer but is this where I should be heading.
Also is the normal cycle of the inducer motor that it starts 1st before the burners and stays on until the main blower kicks off???????
 
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Old 10-24-11, 10:55 AM
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The inducer motor is switched on and should stay on as long as the thermostat is calling for heat. It should switch off and stay off when the thermostat is not calling for heat.

The inducer motor operates entirely independently of the pressure switch.

I'd measure the 24 VAC voltage being applied between the W and C terminals. When 24 VAC is present at W, the thermostat is calling for heat and the inducer motor should stay on. When no voltage is present, it should remain off.

If it's not doing that, very likely you need to replace the circuit board. But check the votages first to be sure of what is happening.
 
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Old 10-24-11, 02:15 PM
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I checked the voltage at the pressure switch between C and NO.The voltage is 0 when the furnace is running. When the furnace isn't calling for heat, it goes from 0 to unreadable after the burners and blower kicks off and it continues to cycle. If I measure the voltage without the pressure switch in the circuit it is around 16 vac? It is a a 2 wire pressure switch. Should this be 24 vac.
So if for some reason the exhaust piping is not working the positive pressure should cause this pressure switch to close shutting down the furnace?
 
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Old 10-24-11, 03:14 PM
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Your method of testing the pressure switch is wrong. What you are doing provides no useful information.

And you have entirely the wrong idea about what the pressure switch is measuring. It measures a negative pressure created by the sucking power of the inducer motor.

You should find a strip of terminals labeled R, W, C G and so on. Connect your voltemeter to that C terminal and use the other test lead to sample voltages.

The pressure switch connections do not control the inducer motor. The pressure switch should close after the inducer motor comes up to speed and creates a negative pressure which should close the pressure switch.

You need to have your voltmeter set to AC voltages and to a level greater than 30 volts.

Coming up with your own theories about how your furnace works is going to do you no good. You don;t have a clue, frankly.
 
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Old 10-24-11, 03:57 PM
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I checked the voltage between W and C and it is around 27 vac when the thermostate is calling for heat, when temp has been reached the voltage goes to 0, but the inducer motor still cycles on and off.
I came to this site to gain some added knowledge a, and support. I do appreciatte your help and believe me I will figure this out. Like everyone else Im trying to save some money before calling for service.
I was hoping somebody would correct my theories on the pressure switch.

Im hoping to have a clue now that Im a member.
 
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Old 10-24-11, 04:00 PM
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Also I hear something like relay or some opening and closing coming from the timing ( control board ) when the inducer fan is cycling on and off
 
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Old 10-24-11, 05:14 PM
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The inducer motor should remain off when the thermostat is not calling for heat and the voltage between W and C is zero.

Apparently the inducer motor is cycling on and off when the voltage is zero.

It shouldn't do that, and the only reason it is doing so is a defect in the circuit board, which needs to be replaced.

Frankly what you describe is very odd --- I've never observed that kind of defect. But odd defects of a wide variety of kinds happen often enough.

Frankly, I can't think of anything else that needs to be checked. You have a bad board in my opinion.
 
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Old 10-24-11, 06:30 PM
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Thumbs up

Thanks for the help, the board goes for $170.00, It is a Kenmore furnace and is 14-15 years old. Is it worth replacing it or is it time to start shopping for a new furnace.
And what would that cost - rough estimate . I'm figuring around $2000.00

Thanks again

Doug
 
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Old 10-24-11, 09:59 PM
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I would replace the part.
 
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Old 10-25-11, 07:57 AM
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Okay thanks, I will let you know how it goes, and I do appreciate the help.
 
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Old 11-04-11, 10:10 PM
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Seattle Pioneer

I recieved the new control board and installed it today. My inducer isn't cycling on and off after heat cycle anymore - PROBLEM SOLVED. Thanks for all the help Seattle Pioneer
 
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