Furnace Burner Short Cycle Time

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Old 01-21-11, 05:16 PM
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Furnace Burner Short Cycle Time

I have a TempStar Gas furnace. Recently I replaced a bad inducer motor on the unit. Since then, the burner cycle time has been very short. The burner will be on only for a few seconds, shut off and then relight almost immediately.

The unit powers up normally. The inducer motor starts, the burners ignite and heat for a minute or so then the blower motor starts.

Once the unit is "warmed up", the burner starts its very short cycle time; on for a few seconds at a time. What is causing the short cycle time?

Thanks for your advice!
 
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Old 01-21-11, 05:54 PM
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Did the new motor come with a gasket ? If not you need to use high temp gasket sealer to seal it otherwise maybe the pressure switch is dropping out due to a leak around the new motor.
 
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Old 01-21-11, 06:28 PM
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The old inducer motor had what looks like a fiberglass mat that would act as the gasket. I attempted to reuse the mat on the new motor but most likely it leaks.

It seems like a vacuum leak is the problem! I disconnected the vacuum switch tube from the inducer and provided a forced suction on it while the heater ran. It seems to run normally.

I will try to fix the leak and post the results! Thanks!
 
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Old 02-01-11, 11:58 AM
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Ok, so the gasket is not the problem.

Since everything works ok for the first minute or two before it starts its short cycle, Im wondering if the change in temperature could be affecting the vacuum.

ie - cold air is more dense which equals a higher induced pressure. Then when the temperature increases, the air is thinner equaling a lower pressure, causing the switch to drop out?

Btw, is the inducer supposed to run the whole time and vent hot air out of the system?
 
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Old 02-01-11, 12:46 PM
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It doesn't sound like you've even verified that the pressure switch is what is opening to shut off the burners.

Have you checked to identify which of several things are shutting off the burners?

My #1 suspicion would be that the furnace is overheating and shutting off on the limit switch. Has that been checked?
 
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Old 02-05-11, 03:38 PM
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Actually yes - the limit switch is fine. That's why I'm focusing on trouble-shooting the vacuum/pressure switch (besides, by simply listening, I can tell it is the pressure switch that turns off the gas). The only thing that has changed on the unit was the inducer motor assembly, thus, the obvious place to start looking for the source of the problem (assuming that multiple components didn't fail at exactly the same time).

I have actually seemed to have fixed the problem, though I'm not sure why the logic works.

If the seal between the inducer assembly and the heat exchanger housing is loose (ie, letting the inducer suck in a small amount of outside air) the system works fine. If I have a complete seal around the inducer, the problem arises.

If anyone has any ideas why this works, I would love to hear them... but, the unit seems to be working just fine now.
 
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Old 02-05-11, 05:28 PM
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Leaving the inducer motor loose is not a solution.

The furnace venting system needs to be working properly to avoid safety hazards and insure the correct operation of the furnace.


If you can't fix it, call a repairman who can.
 
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Old 02-06-11, 12:12 AM
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Its not loose; its simply not a 100% seal.

It IS working, venting properly and a very "knowledgeable" repair man wasn't able to give any better answer. Constructive guidance would be appreciated.
 
 

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