gas furnace fan reversing by itself

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Old 10-23-11, 06:25 PM
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gas furnace fan reversing by itself

Iíve noticed that my York gas furnace does not heat up the house properly. At first, I thought that the air flow velocity was not high enough and I still donít know if it is. Nothing has been changed since the last winter and the AC worked fine this summer.
I opened the cabinet and inspected the operation. The sequence is normal: my digital thermostat calls for heat-the induced draft blower comes ON-the pilot light comes ON-the burners light up-the fan limit switch starts rotating-it reaches the ON position-the main blower turns ON-the limit switch keeps rotating and gas burning. But at this point something strange happens: it very quickly reaches the Limit position and shuts the gas OFF. The main blower keeps blowing, but at some moment it all of a sudden stops and by itself reverses rotation. And it may do reversing back and forth a few times in the row, until it finally stops completely.
I donít believe it should EVER reverse, but I may be wrong. The fact that the limit switch very quickly reaches the Limit position is also not good and I could try replacing (it is about 25 years old. The settings are: 90-140-190.)
Someone suggested that it may be a bad blower motor capacitor. I visually checked it out: it has white rundown trace on its side. I replaced both the motor and the capacitor in 2008.
Before I do more troubleshooting, I need to know if motor reversing is a problem and what I should be looking for.
 
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Old 10-23-11, 06:45 PM
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I've seen reversal happen twice but both times is was with a condenser fan motor on an A/C. That is not to say it can't happen with a 115 volt furnace motor. The easiest way to check is to pull the blower assembly & fire up the furnace.
The fact you are reaching limit very quickly throws up a red flag. Given the age of the furnace, it is entirely possible the heat exchanger has failed. Check the fan rotation first but if it is correct, I suggest calling a local professional to examine the heat exchanger.
 
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Old 10-24-11, 05:01 AM
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Grady,
Since posting, I went ahead and switched the blower wiring for heating from low to medium speed (connected blue wire instead for red). By eye I could not tell if the speed increased, but seems like the random reversing stopped and the limit switch does not reach 190 degree as frequently. (I will continue monitoring it for a few days.) Could it be that it helped properly cooling the chamber and it is not overheating as bad now?
My other idea, besides replacing the capacitor, is also replacing the limit switch, since it may be tired after all those 20+ years.
And I did not get the suggestion of "pull the blower assembly & fire up the furnace. " What would I check by doing that and how? Are you suggesting pulling out the blower (but not disconnecting its wires) and starting the furnace with it sitting and spinning outside? That is to just check rotation? Or seeing how the limit switch operates without air flow? Am I getting your idea wrong?
If I wanted to examine the heat exchanger myself, what would I have to do and look for? I've been inside my furnace several times, replacing various parts and I am not afraid of doing more work.
Thanks.
 
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Old 10-24-11, 07:19 PM
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The pulling the fan was to check rotation but it's probably not a good idea to do so. Far easier would be to start the furnace, allow the blower to come on, then take a telescoping inspection mirror & watch the blower as it slows down.

Have you by any chance changed the type of filter you use? Some of the pleated filters create a lot of air restriction.

Properly inspecting a heat exchanger is not a DIY job. A pro should have combustion analysis equipment &/or fiber optic instruments he can use to determine if the heat exchanger is cracked without having to take the furnace almost completely apart.
 
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