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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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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 | Air Conditioners

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 | Air Conditioners

1 Answer

Hi I have an xl80 the burner lite and heats up till at desired temp, after burner goes out the fan keeps on running 15 minutes or so. If I go down and shut off power switch to furnace then the fan shuts...


Hi, sounds like you have a problem with the heat anticipator in the thermostat. If you replaced the fan limit, and with it running that long after the burners shut down, it would be in your stat. If it fires up after resetting breaker, was the stat set to call for heat? If not, the stat is faulty. If you remove the front to get to the stat wires, take a jumper wire and jump from RH or RC to W. Heat will start. Let it run for a minute or so and remove wire. If the fan shuts down with in a min. or so, you will know it was the stat. Good Luck and let me know the out come!
Shastalaker7
A/C, & Heating Contractor

Feb 11, 2011 | Air Conditioners

1 Answer

How do I set the heat at 75 with no fan mode. I don't want the fan to come on once the room reaches 75


Hi, there is no way that this can be done. The fan is controlled by several different parts for the furnace to run properly. If you are saying that the fan continues to run after it reaches its set point, this is normal if it only runs for about a minute and 1/2. This is to cool down the heat exchanger. If it continues to run all the time after reaching its 75 degree set-point, you have a fan relay that is sticking internally, or a high limit switch that has failed and depending on how old the unit is, it may be what is called a combination fan/limit control that has dirty or burned contacts which will keep the fan on. What I am trying to say, is when your unit reaches the temperature you want, the fan should only run a short time as the limit controls are there to shut it down from a setting on the fan control that is preset by the factory. If you can give me more information on your unit such as model, serial, name of unit, and if it is a gas fired central heat and air unit, or heat only, a heat-pump ect. would be of a big help to determine what controls you have on this unit that is keeping the fan on. Leave this info in the comment thread for my user name, and I will get back to you.
Sincerely,
Shastalaker7
A/C, Heating, & Commercial Refrigeration Contractor

Jan 12, 2011 | Air Conditioners

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 | Air Conditioners

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 | Air Conditioners

1 Answer

I have an early 80's mobile home with an "intertherm gas atmospheric furnace" pilot light is lit but can't get the furnace to "fire". Seems as though the limit control is stuck open. ...


If your pilot light is lit then I would first check your thermostat. Make sure it is on heat setting and fan is on auto, turn stat up to where it should be calling for heat. If you don't get burner to light remove front cover from thermostat and see if it has a mercury switch, if it does move temperature selector up and down and see if you can see a spark in mercury switch vial. If y ou cannot see a spark remove screws that attach t-stat to base.Remove carefully and disconnect wires that go to r and w terminals on stat base. Temporarily connect these two wires together. This should make burner light if you have power to thermostat. If burner does light disconnect wires and replace thermostat. Also if you have a volt meter check across these two wires and see if you have power there. Should be around 24 volts a.c. If your thermostat has a fan on setting try that. If fan runs you know you have power to furnace The gas valve needs 24 volts to operate and inside the electric control box in furnace there is a transformer to step down 110-120 supply voltage to 24 volts. If transformer is working you should be able to hear a low buzz or humming sound. You can also check across gas valve terminals when t-stat is set for heat, you should read about 24 volts. If you have 24 volts to gas valve and it is not opening then you need to replace gas valve. The main purpose of the limit switches is to turn blower on when temperature in heat exchanger reaches fan on temp,shut off blower after burner shuts off and heat in heat exchanger reaches fan off temperature, and limit if temperature exceedes set point without blower coming on. I hope this helps you. Thanks

Nov 25, 2010 | Air Conditioners

2 Answers

When I set tstat to call for a/c, compressor outside comes on. Blower on furnace does not. When I manually rotate fan limit switch on furnace wall, blower comes on, blows cold air, and then goes off when...


blower relay bad or no call from stat on green wire to run fan jump red on stat to green if fan comes on stat bad if no look for fan relay turning the well switch bypasses the relay hope this helps

May 24, 2009 | Air Conditioners

1 Answer

150000 1974 national gas forced air furnance does the fan come on the same time the gas comes on. ...


not familiar with that particular model but on virtually every type and style of furnace i have worked on in the last 21 years. the fan is delayed. usually the burner lights then 3 to 5 minutes later after the heat exchanger has gotten nice and hot the fan comes on and blows nice and hot air. on old furnaces there was and is a switch usually on the upper right side of the furnace bulkhead that has a spring like device that heats up and rotates till it makes a set of contacts that turn the fan on. if it gets to hot the set of contacts breaks the power to the gas valve and shuts down the burner. as the furnace cools down on shut off there is a third set of contacts that open to turn off the fan so you don't get a draft. sometimes the stat can be wired up wrong, and it will bring the fan on right away ,and air will be cool and drafty till furnace catches up.

Nov 12, 2008 | Air Conditioners

2 Answers

Rheem furnaace blower stays on


On a furnace the fan is controlled by a computer and a sensor ot a oldstyle fan thermostat. Basically it waits till the furnace heats up a little and then turns it on and after it shuts off it will run till the thermostat cools back down may take a few mins. In cool mode the thermostat controls the fan so it shuts down immediatly. This is why the thermostat asks you if its electric or gas heat. Should have a switch somewhere for this.

Nov 19, 2007 | Air Conditioners

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