Furnace Fan Cycling

Old 11-18-02, 10:30 PM
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Smile Lennox gas furnace fan, going on & off.

Hi Tom,

We started up our Lennox Mod# G12D2-55-5 yesterday & ran into our 1st problem ever. The inside thermostat starts the flame with standing piliot lite. The flame will burn as long as the inside thermostat calls for it to. It is LPG not natural gas.

Then when the temp starts the fan, it only runs for about 5 secs & then stops. The Fan & Limit Controler is a Honeywell L4064T1723. I can watch the round plate covering the BiMetalic spring go clockwise in a jerky manor, then the fan will start & run for 5 secs, repeating this everytime it reaches the same point.

In the 5 seconds the fan runs the plate will very quickly go counter clockwise & the Fan will shut off, but the flame & pilot will continue to burn normaly. I can take a straight blade screwdriver and move or hold the plate in the clockwise direction & the fan will run, until I let it spring back counter clockwise.

The furnace is 9 years old & I am about to spend around $100.00 on a new Honeywell Fan & limit controler. So I just wanted to know that I was replacing a played out Bimetalic spring in the fan & limit control. When I move the spring in a clockwise direction is is squeky but does not bind or stick in any postition.

Does this sound to you like the fan & limit control is bad? I have checked everything that can turn the fan off in 5 seconds & everything seems ok except the fan & limit control. Can I go on the jerky climb clockwise when the flame is on, & the rapid return counter clockwise after the fan starts, turning off the fan until it goes back up due to heat & then the fan comes on again for 5 seconds.

Most of all, the moving of the settings plate clockwise will turn the fan on until I release it, as a good enough test to conclude that the Fan & limit control, is in need of replacement?

I called the Lennox dealer explaining my problem & he said to set the main blower fan speed to low. I have it set one up from low now & thought if anything, it would only let the fan run for a few seconds more before shuting off.

At this point I would like to know if any other part could do this. Or when I hold the spring from returning counter clockwise thus allowing the fan to run as long as I hold it in place, point to a worn out Bimetalic spring in the fan & limit controler.

Thanks for any advice you can give me. I would fell pretty foolish spending $100.00 on this part, if it were another $20.00 part causing this LOL.

Thanks, Marturo
Old 11-19-02, 05:18 AM
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Hello and Welcome into my Gas Appliances forum Marturo.

Based on the well written and detailed furnace problem description, I would suspect that fan switch to be defective also. The bi-metalic spring most likely has lost some of it's tension.

Since the rotation of the dial plate moves in a jerky motion but freely, you could attempt to clean the electrical contacts of the switch. I say attempt, simply because it does not always resolve the problem immediately or permenately.

Cleaning, attempting to fix or adjust the switch does not solve the problem, as I have done this experimentally before with poor results. The time spent was not worth the effort but was more for classroom instructional purposes... We all learned to replace the switch, thus avoiding future frustrations...

The fact that the fan motor runs does not indicate there is a problem with the motor. As long as the burner flames will remain on and only the fan motor cycles, indicates there should not be any air flow restriction causing the high limit to shut off the burners.

At this point it almost surely accurate to say that fan switch is defective. Replacing it should resolve the problem. Be sure to turn off the electrical power before removing that switch.

I highly recommend the part with an exact OEM replacement. I have never had good success with non OEM replacement parts. In my opinion and experiences the non oem parts are close but not exactly made to the same standards.

An excellent source for orginal replacement parts is your local retail heating dealers and agents retail part stores. Bring the make, model and serial numbers.

Regards & Good Luck my friend,
Old 11-19-02, 03:43 PM
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Smile I love it when a plan comes together

Hi Tom,

I want to thank you for reading my post, and giving me your professional opinion.

After checking with the Lennox dealer & finding that I was blessed with the most expenseve Fan & Controler switch @ $125.20. I called a heating & Air parts Wholesaler & got the exact Honeywell controler for $57.60.

Sometimes it sure pays to shop around, however at first they were not going to sell it to the public, until I said hey I own a Farm & am a Do it yourselfer @ DIY.com. At this point I thought what could it hurt LOL.

Then I heard this guy say from the Office " If he is a DIYer, go ahead and sell it to him." I don't know who you are, back in the Office, but if you are as I suspect a DIYer, Thanks for the break, a Farmer sure can use a break now and again. A very grateful & warm Marturo

Well Tom I hooked the switch back up & made the same settings I found on the old switch. Started it up & it works like a new furnace.

Thanks again Tom for all your help

Your Friend, Miguel

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