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My fan limit switch will not shut the fan off in heating (gas). The dial on the limit switch rotates till the shut off point then sticks. I have to turn it manually to shut it off.

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  • stormybluese Dec 06, 2008

    I actually have to shut off the breaker for it, in order to fix it.

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  • 334 Answers

Make sure nothing is bent and causing added pressure to keep it sticking.

You will likely need to replace the limit switch.

I don't know your model, but its probably 2 wires, 2 or 4 screws and runs about 20 to 40 bucks for the part.
Turn the power off first.

Posted on Dec 06, 2008

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Posted on Jan 02, 2017

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My furnace is an older type furnace and it was working fine yesterday but the blower won't shut off after it runs for a while it run continueally I was wondering what I could do to fix this pr


When you say 'older furnace' I am assuming old enough to where there is no circuit board. They will usually have a thermally activated 'fan-limit' switch with a bi-metal sensing device. Most older furnaces had either a Honeywell or a White-Rodgers controller and they look almost identical. With the furnace door off, look towards the top of the furnace vestibule (above the flame area) for a rectangular box approximately 3 1/2" wide by 5" long. Tap the side of the box with a screwdriver and if the thermostat setpoint has been satisfied and the gas burners have been off for more than three minutes, the fan should then shut off. The switch contacts are either stuck together OR the bimetal sensing device is weakening from thousands of cycles.

If it continues to run, squeeze the sides of the 'lid' of the box and remove the cover. Inside you'll see a round moveable dial with three pointers, marked 'fan off, fan on, and limit'. The 'limit' should remain at or near the 200 degree setting, the 'fan on' can be anywhere around 140 and the 'fan off' can be anywhere near 100 degrees. It is the 'fan off' position thats giving you the grief.

Turn the thermostat up again and sit there and watch the dial do its thing and yo'll understand better how this control works. Watch it rotate past the 'fan on' setting and observe the fan turn on. Turn the thermostat below room temperature and watch the gas shut off and the dial rotate back towards the 'fan off' setting. If it hangs up again before twisting past the setpoint, you can slightly increase the setting to compensate for the weakening bi-metal element. But just a tiny bit, not a great amount or you will have the fan shutting off with a lot of residual heat still in the heat exchanger. The idea of the delay in shutting off the fan is to remove all that residual heat and transfer it to the house.

There are other brands that utilized 'timed on' fan controls, and there are a few other variances of controllers but what's described above should cover the vast majority of older furnaces.

Remember there is ELECTRICITY inside the fan control and any adjustments should be done with the power OFF !!!!

Dec 26, 2014 | Heating & Cooling

2 Answers

Furnace keeps tripping off on limit switch. replaced limit switch still trips, blower motor seems to work fine.


It's always possible that the limit "dial" switch is sticking leading you to believe that it's only at 150º.

How old is the furnace and when was it last cleaned.

Dec 12, 2007 | Heating & Cooling

1 Answer

Furnace turnoff just before the blower go on.


You are to be commended for noticing this sequencing of events-- Good observation!

You need to adjust the Plenum control switch. (Or fan control switch)

Usually at the top front of the furnace, is a Fan control switch--
It is called the Plenum switch-- and it senses the temperature of the Air being heated by the fuel--
(Is your furnace Oil or gas?-- if Gas-- is it LP or Natural?)

This switch Has a cover that can be removed-- inside is a round dial, with some adjusting screw, and Degrees stamped on the perimeter of this round dial.
You can take the cover off, while the furnace is running-- (Just do not touch the screw terminals that have electric power to them--
You can rotate this dial, and it will turn the fan on-- (Or if the trip levers are mis-adjusted-- it might shut off the burner-- just rotate back the other way, and it will re-light etc.--
The purpose of this control is to make sure your fan is not blowing COLD air.
So-- lower the 'ON' switch tripper, down lower, so the fan will turn on sooner.
You may have to loosen a set-screw-- but usually they are just friction held-- and can be forced to slide (rotating CCW) to the lower temperature setting.
(If you get it too low, it will be blowing cooler air than you like at the registers.)

However-- to get the most heat from your furnace energy source-- you want the fan to come on as early as you can tolerate-- That will be delivering HEAT to your occupied space sooner, and giving yo more of those dollars you are spending on fuel, in the orm of heat in your house!

Tell us more

Mack B

Dec 28, 2010 | Heating & Cooling

2 Answers

The fan has auto ond on selections, the heat looks to be running fine but the fan will NOT shut off.


The thermostat needs to be set for AUTO. If you have it on auto and the fan still goes even though the heater is off then you probably have a stuck fan limit. The limit has a long probe which goes n the plenum. As the heat builds in the plenum at the start-up of a warming operation the bi-metal inside that probe slowly twists around till the temp inside is 110 degrees, the fan comes on and we have warm air in the vents. Then when we have a cozy house the heat shuts down. The fan keeps a going till the heater cools down and the bi-meta twists back around and shuts down the fan. So what goes wrong? The bi-metal simply gets hung up in that position. Tapping on the dial will sometimes release it but it will keep occurring and become aggravating. themobilian_306.jpg
These limits come in various sizes and brands but this is the most common one. If your machine is a newer model then that control board has a fried or stuck relay on it.

Nov 28, 2010 | Heating & Cooling

1 Answer

I have a honeywell L4064A limit control i need to know what the setting should on the dial for fan limits bluescorpion_67@msn.com


First thing I would suggest just to be on the safe side before you began shut power off to unit.
Remove cover on combination fan and limit.
I would set the fan on at 130-140 and fan off at 90-100 degrees. When you adjust setting grasp the outside of the dial to stop it from rotating when you are moving set point indicators. Replace cover and turn power back on. I hope this helps you. Thanks.

Nov 24, 2010 | Honeywell L4064B2236 Fan/Limit Control...

2 Answers

Coleman furnace -- fan runs, ingniter won't light,


Hi,
The chances are that the furnace was running when the power went out. This will cause the high limit switch to cutout and needs to be manually reset.
You will find that switch in the blower compartment near the blower. Usually somewher near the highest point of the cabinet.
Push the little red button firmly till it clicks.
When the power goes out while the furnace is running the heat builds up because the blower shuts down immediately and causes this limit switch to cut out.
If you have a electric tester you can also follow the low voltage circuit till you find the open switch. Ther is usually a wiring schematic on the door cover of most furnaces.

I hope that this will help you to solve your problem!

Thanks for using Fixya!!

Heatman101

Dec 09, 2009 | Coleman Propane Forced Air Heater...

1 Answer

T-stat turns gas valve on,burner lightsburns long enough for fan stat to turn on fan,high limit shuts off gas valve,fan shuts off;repeats cycle


Probable causes include crack in heat exchanger, dirty pilot, bad high limit switch or bad gas valve. A crack can allow the fan to blow the flame off of the thermocouple or flame sensor. A dirty pilot will cause the same effect. The bad limit switch could cycle the system and a bad gas valve could be shutting off gas supply to unit.

Dec 02, 2009 | Off The Wall Heating & Cooling

1 Answer

Burner lights but limit switch will not turn fan on automatically. the burner will shut down if i don't run fan manually. is it the limit switch?


Hi, Yes, you have the answer to you're problem. Now, I don't no if you have just a high limit, or a combination Limit control? I will go with the combination fan control as when they go bad, they stick in the closed position and they have normally open contacts. When the heat exchanger starts to heat on a call for heat, there is a dial on the fan control with temp. settings on it , and it will start to rotate as the heat gets higher. When it reaches its set fan on temp, it with start the fan.We no the fan works so the blower is good, so it shuts the burners down on you're high limit temp. which is factory set to about 250*F. When you turn The t-stat from auto to on in the fan side, that solves the problem temp, only. You will need to R & R the Fan control. If you need some direction on doing this please let me know and if it is a Honeywell auto/on type that is rectangular in looks, and if you remove the cover you will see the temp. dial with arrows on it. Thank you.
Best of Luck, and Sincerely,
Shastalaker7


Nov 21, 2009 | Dayton Electric Heater

1 Answer

Question on a day and night gas forced air heater


it very well sounds like your fan switch , does your fan work at all, like in ac mode on by turning the fan on/auto switch on the thermostat to ON. If the fan does come on its the more and likely the limit switch. if it doesnt come on it could be the motor or the run capacitor. the limit switch has a dial on it that as it heats up it spins clicking on at a desired set temp ussually about 120 to 150 degrees F. turning on the fan if the fan doen't come in the the next setting clicks off the gas valve turning off the burners ussually set at about 220 degrees F. now if the fan is working when the heater is satisfied there is a setting at about 90 deegrees F. that shuts the fan off hope this helps

Jan 05, 2009 | Honeywell L4064B2236 Fan/Limit Control...

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