fan limit switch?

jbrown0902

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

jbrown0902

Also...blower works fine when cut on manually from thermostat and had no problems with the AC working this summer.

how

A picture of the furnace with the doors off would tell me if your fan/limit control was bimetal or a board. Did your tech give you the number of the fan/limit control he was looking for?
If it's a bimetal fan/limit control, some possibilities are
*your heating fan speed is too fast and is shutting off the temp fan control
*The temp nibs on the fan/limit control are out of adjustment
*the fan/limit control is faulty
or
*the gas pressure is too low.

diyorpay

My furnace acts similarly after being off for warm season. After a few cycles, returns to normal with no fan cutouts.

I would wait before ordering a switch and see if it improves.

biggles

might be either one of these...the first one check the pin setting for fan ON might of moved during the season http://bestbuyheatingandaircondition...ry_Code=fl-5fl local Grainger has fan controls should be open today worth the trip...check out where they are around you... or this style non-adjustable http://bestbuyheatingandaircondition...ry_Code=fl-tdf ... both work on metal reacting to heat and closing/opening NOTE heating fan isn't thru stat but these 2 controls if jammed use FAN/ON with heating mode but will end up drafting when stat satisfies..control hasn't moved since last winter heat cycle..lets hear back what you might have...you have 2 limits there.... FAN and HI-LIMIT burner

jbrown0902

This is what I have

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biggles

that control behing the terminal strip is the fan limit control and senses the burner section behind the panel where it is mounted.... it is is old as dirt and the range is low on the setting and temp range making me think you have a disc type someplace else....this control runs the fan on/off from the heat build up and cool off during a cycle....can you give us that number off that limit control...looks like FAL??? something.it definitly can be replaced with another we'll get you there... searched it...here http://www.comfortgurus.com/product_...oducts_id/1509 with pix at the bottom of the info page to view.if you can't get this one just google "fan limit switch" then view images to see all the types that will link you to the suppliers

jbrown0902

OK I just went and got the number off of the fan limit control its FAL3C-05TD-120-A and it was set between 85 and 100 degrees the hvac guy was messing around with the fan tempt cut off on the limit switch dont know of he put it back where it should have been. Also noticed tonight when trying the heat again that the small black box with a small clear red dot on it furthest top right in the picture makes a rattling noise when the blower does kick on? Thanks for all the help!

how

Try turning that fan control clockwise to a lower temp. to see if that changes your fan's short cycling. They often do weaken with age and require adjustments.

I think the other control with the red dot might be an isolation relay which is an electromagnet switch that allows a 24 V current to control a 120 V circuit.

beenthere

Need a new Cam Stat. There still made.

biggles

is the schematic still living on the back of that cover ID that relay and get back on here...100F is low compard to other furnaces with a fan limiting switch...must be the design of that furnace.the higher you set it (which isn't much more) the discharge air into the space will be hotter,and the stat will satisfy.tweek it up to 120F check how the fan acts then...if you can read the discharge air above the furnace in the supply duct as close to the heat exchanger to get an idea at 100F then change the controller to 120F+ full CW..that red dot relay only has 4 wires to it COIL and pair of contacts isolating something...the other relay with the colored wires to the terminal point out is the fan relay in cooling for Hi fan...NOTE heating is always either LO or MED fan speed.if you have a volt meter might read that black or brown wire to ground when the fan is running to see what drops off or go across those 2 when the control opens the fan will shut off or the meter will read something...

jbrown0902

This is the best shot of the schematics I could get. Will be ordering the new fan limit switch shortly...still don't know if the relay with red dot on it is going up also....seems to be making more of a weird rattling noise when the blower kicks on then yesterday when trying it. Don't know if that's normal or not. Tried lowering the fan temperature cut off to see if it would do anything different...no change

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how

Normally turning the temp control of a fan control low enough should make the fan run forever. I'm assuming that either something is faulty with the fan control which is making it non adjustable or another cause is cutting power to the fan motor. I have seen fan controls set up with 24V which then ran through an isolation relay. Just because you've mentioned several times how noicy that relay is, does a wire from the fan control go to that relay?

ben's plumbing

you can use this as a guide.....johnstone supply...4-5 different one still made to fit your furnace..ben sr

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Missouri Bound

Sure sounds like a limit switch. I"m surprised a furnace 20+ years old only has that issue.

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