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Heating, Air Conditioning, Fridge, HVAC/Fan on gas furnace doesn't shut off

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I have been having problems with the fan on my gas furnace staying on, even after the heat has stopped running. The fan is set to auto on my thermostat. I have already changed out my thermostat to eliminate that as the problem. After doing that, I checked some on the fan/limit switch. I strongly believe this is the problem. The fan will shut off when you push in on the limit switch, or just tap on the heater next to the limit switch. 1-2 days ago, after running for several hours straight, it went to working normal for about 8-10 hours, ran again solid for a few hours and then went back to working for about 8 hours. Then from the suggestion of a man who handles the HVAC at a local school, I removed the switch and gently brushed it off with a toothbrush to clean it and moved the lowest limit up 5-10 degrees, that put it at 100F. Since then (24 hours ago from the time of this post) it has been running constantly, only shutting off when I tap next to the switch. I am sure I need to replace that part, but my current one is no longer made and I am having a very difficult time finding what part they have replaced it with. The fan/limit switch itself is a Honeywell L4064T1558 3, 11 1/2" insertion, 85MA at 24VAC. The heater is is going in is a Comfortmaker Forced Air Furnace Model G-U 400125-16M (1980-1982?). If you can tell me what the new part number might be, I would greatly appreciate it so I can get one ordered. Thank you so much for your time.

Answer
hi Shelli,
Yes it sounds like a bad switch. What you have is a combination High limit and fan control switch. The high limit will turn the gas valve off if you have a fan motor failure. The fan control will start the fan when it detect heat above say 90*.

Go to hvacpartsoutlet.com and look for Honeywell #L4064B2210. I think this is the best match of your old one.

Let me know if you have questions.

Heating, Air Conditioning, Fridge, HVAC

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Jim S.

Expertise

I can answer questions on gas or electric furnaces, ac units and heat pumps. I don't work on boilers,chillers,or home appliances.

Experience

I have been in the HVAC trade for over 35 years. For the past 15 years I have owned my own business. I am semi-retired so I can answer questions in a timely manner.

Education/Credentials
I attended trade school for 5 years on weekends and evenings. This was very hard to do, but well worth it!

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