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Mark Henley
Mark Henley, Journeyman Technician
Category: HVAC
Satisfied Customers: 3217
Experience:  UA Journeyman Pipefitter , HVAC, Refrigeration, DDC controls. 26 years.Commercial & residential
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My furnace fan won't shut off automatically.

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My furnace fan won't shut off automatically. The thermostat still turns the heat on and off but the fan doesn't go off when the heat goes off. It's a gas forced air furnace.

The fan should run for a short period of time after your gas valve shuts off. Although, it sounds as if your fan never shuts off regardless. Can you confirm that your fan is set to auto on your thermostat for me?

Customer: replied 7 years ago.

There is no auto switch or setting on the thermostat. It's an old Lennox furnace installed when the house was built (1973). The fan has an electric manual switch wlith a fuse so I can turn it off manually but then if the thermostat kicks on the heat and I had turned off the fan manually I have to turn it on manually. The thermostat box has a wheel inside with numbers on it which I think is the amount of time for the heat to run or for the amount of time for the furnace to wait before kicking on the heat when the thermostat is turned up. I had this problem once about three years ago; then again once last month; now this is the third time in the last 8 or 10 days. I had to leave the fan on all night the last time in case the heat kicked on during the night. In the morning when I turned the fan off manually and thn turned it back on the fan stopped and seemed to be reset until today again. The only thing on the outside of the thermostat is a lever which slides left or right to turn the heat up or down.

Do you have a brand and model of the thermostat so I can look it up and see what we are working with?
Customer: replied 7 years ago.

Thanks for your patience. Here's what I found out. the Lennox thermostat has this info on the inside back panel: on the far left the letters FY. then in the middle the numbers T834A 1154 and then on the right the number 2.

The furnace itself is also a Lennox with the following info on the panel inside by the pilot button.

Line 1 -- TF555

Line 2 --###-##-####1316

Line 3 -- REG. SET 3.5 IN.

Line 4 -- 25 volt 60HZ, .45A

While I was checking for this info I switched the fan to the on position and the fan did NOT start running, so at least tonight it will only kick on when the thermostat kicks the heat on.

I'm sorry that if you can figure out what's going on you won't make much on this one for all the time it's taking you. But I do appreciate it.

I noticed that you said the fan will work when the heat has a call for heat. If your fan does this, what is your concern with your current situation.
Customer: replied 7 years ago.

My concern is that when the heat shuts off the fan does not and the fan will run for hours. I don't know how long because when the fan won't go off I have been shutting it off at the switch and every few hours will turn it on to see if it's still running.

I would like to know how to get the fan back in sync with the furnace heat output and would rather not have to have the fan running hours on end when it's not circulating heated air.

Hi ,

This sounds like the fan and limit controller is stuck on. If you open the furnace cover you may find a device like the one in the photo. It may have a cover on it. If you can pull the cover off of it you can turn the dial to shut the fan off. Be careful. Could you check if you have one ?


Limit controller

Customer: replied 7 years ago.

It's kind of like the photo. No cover on a dial-like wheel which has a lever that can be moved to different positions on the wheel which are labeld with numbers from .18 to .8.

Do those numbers control the amount of time the fan is on and are they parts of an hour or a day?

Thanks to Kenton for getting me this far and thanks to you for taking over.


I'm not familiar with exact numbers but they should correlate to minutes that the fan will run after the gas valve shuts down. If the fan is running right now and the gas valve is off, you may be able to tap on that control and get the fan to shut down. Try that


Customer: replied 7 years ago.

The fan is not currently running. when I went to get the model number info on the furnace I switched the fan on and it did not run, though several other times today I had switched it on and it started running right away. So I guess it got timed out somehow.

I will try what you suggest when and if this happens again, which it probably will. But can you tell me why tapping on the wheel inside the thermostat would shut the fan off?

also, when i went to check the inside of the thermostat i noticed that the wheel has two little notches in it, one at .25 and one at .3 which lock the wheel in place when I move the lever. Should one of those positions be used? None of the other numbered positions have notches that lock the wheel into place.

Also, would turning the thermostat down while the heat and fan are both on cause the fan to keep running when the heat goes off? That's the only thing I've done that might possibly have some effect. Also, should I as a matter of course avoid changing the temperature selection on the thermostat when the furnace is running and the fan is on?

am i correct that turning the fan off manually and leaving it off, like overnight, is a bad idea because the furnace might kick on and not having the fan on then would be a hazard?


No , we have a misunderstanding. The photo I showed you would be of a part in the furnace ( remove cover of furnace ) Now I know what the numbers you were talking about are , they are the anticpator settings in the thermostat.

This part is located in the furnace that I am referring to. No need to tap on your thermostat.

As far as the temperature settings on the thermostat, you shouldn't have to avoid changing temperature settings on it at all.


Customer: replied 7 years ago.

Yes, I did misunderstand. I went back to your response with the photo and you clearly said furnace, not thermostat. I just removed the plate on the fan box in the furnace. It does not look like the photo you showed me. The box has two chambers -- the bottom chamber connects the wall switch to the furnace; the top chamber connects the furnace to a pair of wires that goes through the wall (and probably connects to the house's electrical system and circuit breaker?).

There didn't appear to be any switches or dials, just the wires. There is a cylindrical object about an inch and a half long that's covered in heavy orange paper that has this info in it: SEC 24V 20VA Class 2. Looks like some sort of fuse to me but then what do I know. It doesn't look like something I should be messing with but appears to be held in place on ne side with a set screw.

Yeah that's the transformer, there is some type of fan limit on the furnace, Lennox did use a few different ones. I may not be able to guide you to it but I am quite sure that is your problem as the symptoms are classic. You may want to invest in a service call as the part will need to be changed anyway. They should have one on their truck.


Customer: replied 7 years ago.

Okay. If you can guide me to it, I'll know much more and be able to watch for when I have the work done.

So, guide me to it if you can.

Also, please confirm that what I most likely need replaced is a part called a fan limiter. Less likely I'll get ripped off if I know what I need done and what to call it rather than that "thingamabob."

Also, I know you can't give me an estimate but can you ballbark the cost of the part and the amount of labor involved, though if it's less than an hour I understand that I'll either have to pay for an hour or a standard "show up" charge. Thanks.


In general it will be located in the upper part of the furnace and be mounted on the wall that that the burners go into so if you pull the cover off it should be right there in front of you. Lennox also used some that are a different style but they were called the same thing a , fan & limit control.

The price could be as much as 150 dollars and it really shouldn't be more than hours labor to put it in complete. It could be a 300 dollar repair.

Here is a picture of another Lennox style control.

fan and limit control

You won't see the probe only the brown part.


Customer: replied 7 years ago.

Bless your heart! I found it from your description that the heaters go into it. the cover just popped off and the dial is sideways to the first photo you sent but looks pretty much the same.

Now that I found it, what should I do to it for right now?

I just want to say thanks for your help and your patience walking me through all this, especially since you are so generous to share your expertise and all this time. You are a real credit to this web site, especially for a first time user like me.


Well , to verify it is the problem all we really have to do is wait until it happens again and just slightly tap on it to shut the blower down. Short of replacing it there is no way to repair it.

Thank you

Mark Henley, Journeyman Technician
Category: HVAC
Satisfied Customers: 3217
Experience: UA Journeyman Pipefitter , HVAC, Refrigeration, DDC controls. 26 years.Commercial & residential
Mark Henley and 3 other HVAC Specialists are ready to help you
Customer: replied 7 years ago.

I just want to say a million thanks for your expertise and patience.

Guess maybe I "unstuck" the fan and limit control when I tapped on it last night because this morning I turned up the thermostat and let it run through several cycles and the fan worked fine.

If it sticks again, I'll know what to have repaired.

You were just great! I contacted customer service last night to find out how I could pay you a little more as a token of my appreciation. It will be only a token because as far as I'm concerned you and the help you provided are invaluable to people like me. and I can only recognize with praise the inordinant amount of time you spent to help me. I unequivocally and highly recommend you and will give the web site feedback to that effect.

You took away my feeling of helplessness because now I know what the problem is and can discuss repairs intelligently instead of being an ignorant, defenseless customer.

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