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Old 10-16-2012, 07:22 AM   #16
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fan limit switch?


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Originally Posted by jbrown0902 View Post
I have a older early 80's carrier furnace model number 349FAD036075 if that helps. I turned the heat on for the first time this year a couple of days ago lit the pilot and turned the heat up. when I did everything started fine. Burners lit good and after about 30 sec the fan kicks on. After 20sec the fan cycles off but the burners stay lit. The fan continues to cycle on and off at different intervals of time while the burners continue to stay lit? Had a local hvac guy come by today. Looks at it and says he seems to think its the fan limit switch. He called me back later today and said that he can't locate a fan limit switch for the furnace that it must be discontinued?? So I'm stuck wondering is there a aftermarket fan limit switch to work with this furnace or is it even the fan limit switch that's malfunctioning? Or im looking at replacing the furnace which i dont want to do. Thanks for any help
for discontinued parts i always have luck finding NOS parts on e-bay
http://www.ebay.com/sch/i.html?_trks...at=0&_from=R40

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Old 10-17-2012, 12:36 PM   #17
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fan limit switch?


Just got the new cam stat fan and limit switch. Now just need to know what the settings should be. Fan Range 90-120 degrees, limit range 150-250 degrees, and differential range 25-50 degrees thanks again for all the help
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Old 10-17-2012, 03:34 PM   #18
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fan limit switch?


I'd start at 110 for the fan. And try 170 for the limit. 20 differential.
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Old 11-08-2012, 01:51 PM   #19
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fan limit switch?


UPDATE: replaced with the new limit switch..played with it for hours, still was doing the same exact thing I was having problems with before. The only way I could keep the fan going for a little bit longer was to place the fan cut off at 90 degrees and turn the differential almost all the way up. I had almost given all the way up on the furnace when I walked by the return and felt it sucking alot of air. I placed a t-shirt in front of the return about 3 inches away and the return immediately sucked the whole t-shirt to the return. At this time I let the furnace run with the t-shirt attached to the grate. The furnace ran alot longer until it reached the temperature set on the thermostat. Ok, so now im trying to figure out what to do and why its drawing so much air to cool off the plenum to make the limit switch cut the fan off. Or if thats even the problem which I'm assuming it is. I have one large return in the hallway and two small returns in the two bedrooms. I placed a thicker filter which is located in the attic at the furnace to calm down the amount of airflow entering the furnace. The furnace is working but the fan still continues to cycle on and off to re-heat the plenum, but no where near as much as it did at first. Any help on what to do is greatly appreciated.
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Old 11-08-2012, 02:06 PM   #20
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fan limit switch?


I'd have your evaporator coil cleaned.
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Old 11-08-2012, 05:24 PM   #21
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fan limit switch?


Was the blower motor replaced this summer/spring?

Could be the gas valve is not feeding enough gas to the burner.
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Old 11-08-2012, 05:33 PM   #22
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fan limit switch?


No fan wasn't replaced...just the limit switch. The problem we have been having is the burner staying lit but the fan cycling on and off. Which has gotten better since we cut some airflow flow down to the furnace
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Old 11-08-2012, 05:36 PM   #23
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fan limit switch?


Something has to have changed that it is getting more air now then it did last winter.
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Old 11-08-2012, 05:57 PM   #24
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fan limit switch?


this house was my grandmothers who passed a little over a year and half ago I recently moved into it. Never remember her complaining about the furnace but most likely she wouldn't have noticed.
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Old 11-08-2012, 06:22 PM   #25
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fan limit switch?


So it may well be a case of the furnace had a new blower motor or blower wheel installed sometime in the past and has operated this way ever since.

Try slowing the blower down.

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