LARGER VIEW of heating furnace fan limit switchFan & Limit Switch FAQs
Diagnostic questions & answers for furnace heater fan limit switch control installation, wiring, troubleshooting, repair, replacement

  • FAN LIMIT SWITCH FAQs - CONTENTS: A Guide to Fan Limit Switches on Warm Air Furnace Heating Systems: what is the fan limit switch, how to inspect and set its controls. How to Wire the Fan & Limit Control wire and test the combination fan and limit control on a furnace. Which Way to Set the White AUTO / MANUAL Fan Control Switch. Sensing Furnace Temperatures. How to manually turn on a furnace or air conditioning blower fan. Guide to troubleshooting heating system furnace controls, limit controls, and fan controls. Causes of furnace blowing cold air at start-up of heating cycle
  • POST a QUESTION or READ FAQs about Troubleshooting the Furnace Fan Limit Switch
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Furnace blower fan limit safety switch installation & troubleshooting FAQs:

Frequently asked questions (FAQs) about heating furnace fan limit controls, the fan limit switch purpose, operation, setting, installation, wiring, and testing of furnace combination controls, also commonly called the "fan limit switch" on warm air heating systems.

The sketch at the top of this page shows the typical features of a combination fan and limit control such as the Honeywell type L4064B. [Click to enlarge any image]

Green links show where you are. © Copyright 2017, All Rights Reserved.

Forced Hot Air Furnace Fan Limit Switch FAQs

combination furnace control Honeywell L4064B with cover on

These questions and answers were posted originally at FAN LIMIT SWITCH - home. At the end of this article you'll find a complete list of air handler or fan limit control switch diagnosis and repair articles.

The warm air furnace fan limit switch (shown above and below on a gas fired warm air furnace) is a control which determines when the hot air furnace blower assembly turns on and off.

Question: My furnace stays on and running even when the thermostat is disconnected

(Mar 18, 2015) Armando Morales said:
My furnace stays on and running even when the thermostat is disconnected. Please tell me what's the problem and how to fix it.I think is the limit switch but I'm not sure.

Question: furnace won't turn off

(Sept 20, 2014) richard said:

My furnace turns on immediately when I switch the breaker on and will only turn off by switching the breaker back off. I put a new thermostat and new wire to it in. Any ideas?


Sure Armando

Check for thermostat wires shorted together or a failed fan controller.

In the More Reading article index links above see


Check for shorted thermostat wires

See HEAT WON'T TURN OFFfor helpful diagnostic procedures

furnace fan limit switch control switch (C) Daniel Friedman

Question: blower fan only works on manual

(Nov 30, 2012) Fan blower works when limit swit said:

Hi There
My fan blower in my furnace will work when I push the button on the limit switch to manual but not when I pull the button out to auto. Is it possible that I need to replace the limit switch or maybe the fan motor? Any help would be appreciated.

Thanks Nick Van Amstel

Question: blower fan wont run

6/16/14 Lee Ann said:

I have a carrier heat pump with electric heat. The blower fan will not run with the heat pump, but will run with the electric heat. Any suggestions?


Lee Ann that sounds like a control or control board problem, as you know that the blower itself will operate.

A possibility to check first is that the heat pump is actually working. If we are trying to heat a space and if the heat pump is unable to raise temperatures inside the air handler to a sufficient level, a fan limit switch might keep the blower from ever turning on - as it doesn't want to blow chilly air on people who are clamouring for heat.

Nick, see the diagnostics at FURNACE FAN WONT START

Question: furnace overheats, stops before thermostat is satisfied

(Jan 23, 2014) Rusty Agus said:

My frnace overheats and stops without reaching the desired temperature as set-up in the thermostat. Technician for the company that I have a maintenance agreement with said that the cause is the plenum being undersized. I can't believe his diagnosis. Should I get a second opinion or could you tell me the real cause of the problem? Thanks.

This question was posted originally at HEATING SYSTEMS


It's a fair question and the explanation is possible, Rusty. BUT if the system is not new, if it was not behaving this way before, then one would not expect the problem to be plenum size.

Also here's a crude rule of thumb tip: compare the cross section of both the return air plenum and supply air plenum to the cross section of the furnace heat exchanger. If they match then that's good.

Finally, I would look for some other clues that would explain the problem, such as

- reduced air flow due to a dirty air filter
- reduced air flow due to dirt on the blades of the squirrel cage fan in the air handler
- reduced air flow due to squashed, disconnected, or damaged ductwork

Question: blower won't shut off and heat is inadequate

(Jan 1, 2015) Anonymous said:
my gas furnace is not functioning properly. The blower fan is running constantly and the house temperature keeps falling, the air coming out is pretty cool. I went to look at the filter and it seems fine, but as I was down there, I noticed that the burner keeps igniting over and over, repeatedly, but the house will not warm. What is going on?


The blower fan may be set to ON or MAN at the thermostat or at the fan limit switch.

Normally when the thermostat calls for heat, if the fan is on AUTO, the fan won't turn on until the burner has already fired AND the temperature in the air plenum over the heat exchanger has reached or passed the FAN_ON temperature on the fan limit switch.

The fact that your burner is cycling on and off suggests a control problem, shorted thermostat wires, a loose connection or something similar. I'd ask for a heating service call.

(Jan 1, 2015) Anonymous said:
Hi Dan, the blower fan is just set on AUTO. I was suspecting, like you stated, from what all I have read here is that maybe a problem that I can't fix. Thank you for responding. I appreciate your help. This heating and air system is awful. I have to call a service person at least 2-3 times a year, and the house is only 6 years old.


Having to make repeated service calls suggests something fundamentally wrong. I'd talk to the service manager, politely, and ask that they send a senior, experienced tech so that you can get down to a single annual service and maintenance call.

Question: blower fan won't turn off, blows cool air at end of heating cycle

26 January 2015 jeri said:
my oil furnace turns on fine and blows hot air but when its time to expel the hot air after the tempeture has been reached, it does not, it blows cool air. What the problem?



It sounds as if the limit switch is leaving the blower on too long after the plenum has cooled down. Check the FAN OFF temperature setting or time.


Question: stuck pressure switch?

If u get a pressure switch stuck open fault - Julio 8/16/11


Julio if you are asking about a fan motor reset switch that does not reset, either there is a short in the motor or a bad switch itself. "Pressure switch" - I'm unsure what you mean.

Question: blower keeps starting & stopping at beginning & end of cycle

(Nov 13, 2012) phil said:

My blower starts and stops and starts at the beginning of the cycle and at the end as well.



Depending on air movement rates and heat exchanger temperatures and fan limiit switch settings, a blower can cool down the plenum at the start of a heating cycle, causing the blower fan to turn off. And at the end of a cycle when the heat exchanger is much hotter, the blower may cool down the plenum, shut off the fan, but residual heat in the heat exchanger can re-heat the plenum, turning the fan back on.

Try widening the ON OFF gap between the FAN ON FAN OFF settings on the limit switch by about 5 degrees. Do not change the upper fan limit safety setting.

Question: how to delay blower turning on to avoid blowing cold air

(Nov 19, 2012) Mark Brady said:

When heat is called for the furnace burners start and the blower comes on with no delay in a gas furnace. I have two similar furnaces and the other one has a temp delay. I've verified limit switch is turning. And have adjusted the on limit up and down 10 degrees from it's 120 F set point. It just seems like the blower comes on with the burners. And I'd like it to delay like it should.


Mark, I agree that we don't want to blow cold air

I suspect a bad control or a wiring error.

Just check all settings at the thermostat and limit switch


limit switch fan control set the fan override to AUTO

shut the whole system off, let it cool down, thunder it on and thermostat up. If it blows cold air immediately most likely it's time to replace the control.

Question: fan runs faster in manual than in auto mode

(Dec 7, 2012) Bill McManus said:

Had new thermastat installed last year, after which the fan always turned faster in the manual on mode than in the auto on mode. This year my heat would not come on and my plumber replaced the igniter. I told him about the fan speeds and he said he never heard of that but replaced the fan limit switch and it still runs at two speeds, no change. He said the switch was part of the no heat problem. Does this sound right to you?


Bill if your unit has a variable speed fan it may be that the controls are wired to only use the slower speed in heating mode. Often blowers run at different speeds for cooling than for heating.

Question: fan cycling when thermostats are OFF

(Jan 2, 2013) chris castagner said:

I have a question the fan on my furnace come on and off all the time even when i have both my thermostats cool, off , heat switches to off and the fan switch setting on there is only on and auto and they are both set to auto


Chris I suspect a short circuit in the thermostat wiring or a control problem -

Question: Heat won't turn off

(Jan 2, 2013) Tom said:

I have a Borg Warner unit and the blower and burner keeps running. I installed a new fan limit switch but I noticed once I turned th gas back on the system reached fan on setting the fan came on and when the unit continued to run i
turned the thermostat off and the fan continued to run. So I turned the breaker off to the fan and the burner kept on burning, it would not shut off unless I turned the pilot valve off or the main gas valve. Also the gas valve has a 2 wire connection on it. One wire I assume is coming from the thermostat and the other wire I think should be wired to the fan load limit side. I cannot figure out why the burner will keep running no matter what unless I turn the gas off.



as a place to start diagnostics. Also check for shorted thermostat wires that keep calling for heat.


Question: Olsen HWSV9500 reverse flow furnace blows cool air into the cabin

I have an Olsen HWSV9500 reverse flow furnace. My heat pipes are under my cabin in a non usulated crawl space. When my furnace kicks in, it blows cold air from the pipes into the cabin and actually cols it off for the first few minutes. When the heater turns off the fan keeps running for a while again blowing cooler air in the cabin.

This results in my furnace constantly turing on anf off just to keep my cabin heated to 20 degrees c. when it is 5 degrees outside. IS there some kind of setting on the furnance that will solve this? Thanks Rick Gagnon 9/5/11

Reply: Causes of furnace blowing cold air at start-up of heating cycle


A standard warm air heating furnace fan limit switch won't turn on the blower fan until the heating plenum has reached a sufficient temperature; this design is specifically intended to avoid blowing cold air on building occupants.

But I don't think you want to set the fan cut-in to too high a temperature; I worry about overheating the plenum if you push both limits up together (cut in and cut out) and I worry about short cycling the fan if you just push up the bottom limit.

Here are two common causes of a heating furnace blowing cold-air into the room at the start of a heating cycle:

If you have not recently messed with your thermostat wiring, then iIt sounds as if the root problem is that your ducts are inadequately or completely uninsulated.

So I'd look into better insulation of the ductwork as well as wind shielding for it.

Question: Where do I get a fan switch

I am need of a fan switch. My part number is L4064T1848 3 89169 S
I cant seem to find it. All I find is L4064B plus other numbers. What does the Letter T mean compared to B or even A I have found? - Max 10/30/11


Start with the furnace brand name and model number, that plus your serial number takes you to the manufacturer and the right aprt.

Question: Goodman GDT090-4A counterflow furnace inspection

i have a goodman gdt090-4a. counter flow. how would i get to the blower to inspect for dirt on blower? i can not find any instuctions on this on-line, please help, thank you - 11.20/11



Contact Goodman with the model number and serial number of your heater and you can obtain, probably at no cost, the installation and maintenance manual for your heating system. We give Goodman contact information in our references section below.

Question: replaced fan limit switch, but it only works intermittently

I replaced the limit switch to get the furnace back up and running. It works intermittently and the the aluminum cover where limit takes the reading gets really hot and the limit switch stops the burners and lights up after a few seconds. I know the limit switch had failed but I really havent fixed the problem.

My dad actually removed one of the air filters a couple of months ago to replace the electronic air cleaner but may have forgotten. I will wash the cells and get a new air filter to see if i can get cleaner air flow. The fan seems to be running at a similar speed to the other furnace in the house.

Any suggestions on what may be the problem? It overheats and shuts down which means i will probably fail the new limit switch. Obviously been operating a dirty furnace for a while but where could i get a problem with dirt accumulating. I also cleaned the plastic hose to the pressure switch. It had some dirt in there. Please help. - Sandy 1/31/12

we recently installed a heatpump to an existing oil furnace,but cant make fancontrol switch to work on ac mode. what am i doing wrong, because it works fine on emergency heat mode. - Anon 7/31/12

just installed a goodman 2 ton, Heat turns on upstairs appropriately but the downstairs does not. thermostat works appropriatly for the a/c. any ideas - Rob 10/25/12



Sounds as if the thermostat wiring or control wiring is incorrect or you have a loose or bad connection.

Question: replaced fan and limit controls, now stuff doesn't work

A week ago had the fan and limit controls replaced. Furnace started running fine but then fan blower did not start when thermostat in "auto" position.

If I switch to "on" position the blower fan comes on. After a while if I turn fan to "auto" position again the blower will continue to run as needed - then occasionally I can hear the furnace heat up and then shut down and heat up again but the blower does not turn on.

I then have to turn the fan setting to "on" once again to get blower to work.... Does the limit control switch need to be adjusted? - Sam 2/28/12


This sounds like you need a service call. But first check that your thermostat wiring is correct, not loose, and that the thermostat is not blocked by dust or debris.


(July 19, 2011) Tony  said:

I have an old style furnace/ac. The furnace has a honeywell combination furnace control type L4064 on it. When my ac kicks in, the blower does not kick in. I also cannot turn my fan onto the continuous mode on my thermostat at all. But if I pull the manual button on the Honeywell control, the blower will run.

Does this Honeywell control box control the fan motor in relationship to the ac as well. The blower works fine when it is in the heat mode.


That fan limit switch that forces the fan to run is a helpful diagnostic - it tells us that the problem you have is not in the fan assembly nor fan motor since those will run. So I infer there is a problem with your thermostat, thermostat wiring, thermostat transformer, or a control circuit in the air conditioner.

Start by checking the wiring.

To be more clear: the fan limit switch is a heating control that controls the fan on and off in heating mode.

But when you pull the (usually white) manual button on the limit switch, that's an electrical override that simply forces the fan to the "ON" or "run" condition. It's typically not part of the cooling circuit.

Reader follow-up:

(July 20, 2011) Tony  said:

Found the culprit, was a broken brown wire coming out of the main electrical junction box inside the cabinet of the furnace. Did a quick patch job and yohoo, blower motor works in manual mode on thermostat now. Very much appreciated !!!!!!!
Thank You from So. Wisconsin


Thanks for the feedback Tony - your comment reminds us that if a check for voltage at the blower fan does NOT find voltage and we are sure that power is on to the unit, we need to check for a disconnected or broken wire just as you did.

Question: blower fan short-cycling - keeps turning on and off

(July 26, 2011) Tyler  said:

Alright I'm trying to figure this one out, any input is greatly appreciated. I have an older Rheem 1991 unit, everything is outside gas heat, ac, and house blower in one unit. The problem I have is at times, without any warning the air handler stops all together, or cycles on and off constantly (when it cycles off there is a faint thump in the duct, but only when it short cycles).

The whole time the air handler is acting up, the compressor runs constantly, or the burners are going (I'm almost certain thats what fried the last set of heat exchangers) Earlier today, it was short cycling and i caught it.

I have an electronic White Rodgers control (changed the batteries, but its also hardwired), s

So I turned it off for twenty minutes, flipped it back on and its been running fine since. I just can't figure out why it is totally random, sometimes late at night, early morning, or over a 100 outside, no difference.

Sometimes its fine for months, other times an hour. Is it time for a new blower, control board? Anyone have this problem with an older Rheem?

Also if i do replace the blower motor, can i install a multi speed, or a higher cfm? Thanks

(July 24, 2012) RICK POLK  said:

A/C starts fine but blower motor shuts off after about 1 minute

(Aug 3, 2012) wes  said:

My blower cuts off and on constantly in 10 to 15 second intervals. It was still cooling the house till i turned it off to prevent further or more severe problems. The unit is less than 2years old. The fan wheel moves freely. Any suggestions? Carrier/Bryant



"So I turned it off for twenty minutes, flipped it back on and its been running fine since." might be a clue. If we turn an air conditioning system off and then back on too quickly, in many designs the compressor just can't get started against the high head pressure it confronts from when it was just pumping.

After you wait an hour or so the head pressure drops - pressures equalize throughout the system and the compressor has an easier time starting.

Don't flip quickly back between heat and cool or "off" and "on" for this reason.

If the problem is recurrent, however, then something else is wrong, perhaps with a control circuit.

Rick, check for a wiring or control or control board problem. Also check that the blower fan motor is spinning freely and not overheating, and that there is no loose safety door switch on the blower unit. Sometimes vibration or heat can cause the door sensor to open.

Wes, honesty is best - I'm not sure. I suspect a control board or switch problem.

Question: fan shuts off after 30 seconds, furnace cycles on and off

(Feb 8, 2014) tom said:

after gas furnace shuts off about 30 seconds later furnace kicks back on and cycles like this untill thermostat heat temp is reached,runs like this all the time


See FURNACE FAN CYCLES DURING HEAT - - Furnace Blower Fan Cycles On/Off During the Heating Cycle

to read a diagnosis of this problem Tom, and let us know if you still have questions. Thanks. Daniel

Question: heat shut off by landlord

(June 9, 2014) xenaon said:

i rent from a bad guy. He shut off the wall switch - which shut off the fan. Before, if I turned on the wall switch - the AC went on.

Now, it doesn't. Inside my apt the AC and Furnace breakers are fine. 3 hours later, the bully shut off the gas water tank downstairs - turning the switch horizontal, then setting it to off and vacation, so the pilot light is out.

I cannot relight the pilot - turn on the gas line, hold down the pilot valve - won't light.

I cannot find a way to turn on the Rheem Criterian II gas/AC unit. What else needs to be turned on? Thermostat (Honeywell) does not ignite the AC or fan or heat - but it could not possibly be the thermostat as the person who shut off the fan switch - also turned off the water heater.


Xeanon your attorney will take a look at your lease to confirm that it's a standard form that entitles you to certain heating and cooling service. You may need to share that information with your landlord.

Do not mess with the controls to the heating system other than the wall thermostat as you risk making the system unsafe.

Question: furnace fan speed varies on its own

(July 7, 2014) Anonymous said:

A/C blows fast then slow by itself.


Some AC systems use a variable speed fan under control of the unit.

If your unit is not one of those look for voltage variations, a bad fan run capacitor, or loose wiring.

Question: furnace fan runs when furnace is switched off

(July 22, 2014) Carl W Horneck said:

Why would my furnace fan turn on when my furnace switch is turned off?


Carl, if the blower is used also for central air conditioning it can run on a call for cooling when heat is off.

Also at the end of a heating cycle if the fan limit switch senses that temperatures in the heat exchanger and plenum are too high the fan will cycle or keep running to cool down the unit to a safe level.

Question: Bryant Model 355BAV furnace crackling sound

(Sept 15, 2014) Woody said:

I have a Bryant furnace - Model 355BAV. When i turn up the air conditioner / heating there is a crackling sound before the furnace kicks in. The furnace is 2 years old. The sound has been occurring consistently over the last month.


Woody I suspect we can't diagnose this sound by e-texting, but before calling the service technician, could the snap sound be occurring in an electrostatic air cleaner at the furnace? If so that may be normal. Let me know.

Question: furnace only works on MANUAL not on AUTO

(Sept 30, 2014) Paul said:

Why would by furnace work on manual but not auto?


Check for shorted or broken thermostat wires or a thermostat not calling for heat.

Question: fan does not run on call for heat

(Oct 4, 2014) Will said:

I'm testing my furnace in anticipation of the snow season. The furnace is 8 years old. I can get the fan to kick on, but only on the fan option and not when I switch it over to heat. I have checked the wiring, changed the filters and still no luck.

I'm thinking it is possibly the fan relay switch or I need to replace the thermostat??? I installed a new thermostat, but have the same results. Looking at the control board, all it is showing is a solid green light. Your thoughts.


I got the pilot light to engage. It ran for about 30 seconds then shut off. I haven't gotten it to kick on since then.


Will if the gas pilot won't stay on your system may need a new thermocouple.

Question: Older Bryant furnace fan runs constantly

(Oct 13, 2014) Trent said:

I am helping out an neighbor who has an older Bryant furnace. The fan has started running all of the time. Even after the burner has shut down. Could there be a problem with the fan limit switch.


Trent before replacing the control, check for a manual fan ON switch setting at the thermostat and at the fan limit switch. Also check for control wires shorted together.


(Oct 18, 2014) tom said:

have a upflow gas furnace for a garage, tempstar (20+ yrs old) with a L4064e limit switch.
burners turn on as it should
fan comes on as it should
then less than 1 min the fan shuts off but the burners stay on.
Can you help me with what the problem is?

its has a no frills thermostat just heat with on and off with the dial, no auto/on fan switch
also fan limit switch is missing the auto/man push button. so I think its in the push man position.


Question: factory settings for fan limit switch

(Oct 24, 2014) Alicia said:

What are the factory settings for the fan limit switch for an international furnace?

Question: Carrier furnace locks out every 3 minutes

(Nov 6, 2014) mike said:

My carrier furnace locks out every 3 minutes but the blower keeps blowing alternating warm and cold air blowing into the room. It's a carrier weathermaker. Is there a way to adjust this to turn off the blower during lockout, so I don't get cold air blowing between heat cycles.



The air handler should stop after purging heat from the heat exchanger. It should NOT be stopped early as doing so leaves a hot heat exchanger that can crack the unit, making it unsafe - risking flue gas or CO leaks.

Check that your fan control switch is not in MAN or ON

Question: heater fan cycles on and off constantly

(Nov 7, 2014) Anonymous said:

My inside heater fan goes off and on constantly


Anon see these diagnostic articles

Question: Wesco electric furnace ATR-20-204VAC

SHARKY said:

Nov. 17, 1972 i have a 1970 Wesco electric furnace model# atr-20 240vac. 4 elements total. A total electricity hog with today's power/ electric prices indeed. A great high quality fully american made electric furnace though.

These were made for 1972 electric rates for sure! Can i disconnect 1 or 2 elements to save on 50% of my power bill? Thanks and where can i get the original schematics and owners manual for this unit?

Reply: where to find parts & product information for older Wesco Electric Furnaces

Wesco is currently an electrical products distributor and is no longer listing electric furnaces among their products.

But possibly the company has information on its discontinued products, Wesco Corporation, 225 West Station Square Drive, Suite 700, Pittsburgh, PA 15219 , can also be contacted at (412) 454-2200 or at their Website:

The company does supply heating units for electric furnaces through this website:

Keep in mind that on many electric furnaces a number of the parts are generic so heaters, controls, sequencers can often be replaced with an equivalent unit. Try taking old parts to your local HVAC supplier.

Question: wood boiler forced air furnace fan only runs on low speed

11/21/2014 sandi said:

I put a wood boiler in my forced air furnace, but the fan only runs on low. How do i get it to run on high?


Sounds like a wiring error. Also check for a bad start/run capacitor on the fan motor.

Question: fan button doesn't work

(Nov 23, 2014) Anonymous said:
the fan manual button dose not work



If the manual fan switch never worked I suspect a wiring error. If the switch used to work then it needs replacement. Check first for a loose electrical connection.

Question: inadequate heat from my furnace

(Dec 1, 2014) Anonymous said:
My furnace only partially heats. What could be the problem?



At More Reading above see the diagnostic article titled NO HEAT FURNACE

Look for dirty air filter, dirty blower fan assembly, blocked, crimped, disconnected supply ducts, inadequate return air.

Question: fan/limit switch that isn't working

Re-posting without links (security)

I have a fan/limit switch that isn't working (shuts off the fan AND the burner at the high temp, causing furnace to overheat and then shut down). I have one like the black honeywell shown above, but its beige instead. Like this:

but without the hard stop in the middle of the dial.

My question is, what the heck do I replace it with? I can deduce that a Honeywell 4064:

With the same length (11.5"), but which model? It's hard to even tell the difference other than the insert length.

Any advice? Thanks for the help!



I agree it's confusing. The simple approach is to take your old limit switch to your heating supplier and let them help you find a compatible replacement.

On the Honeywell page whose link you cited there are many control numbers but the variations are typically these:

1. bulk pack (multiple controls in a package) versus individual controls

2. Control sensor length (must be the same as your old one)

3. Limit switch with or without manual FAN ON switch

Some controls may also come with different operating temperature ranges or defaults - that's what you also want to match to your old control

The old model Honeywell limit switch you have installed (per your other photo link) MAY be mounted on your furnace in a location at which the new controls with a larger base won't fit - that's why you want to take the old one to the supplier - to be certain the new one

- fits in the old space
- has the right sensor length
- the same operating temperature range (most of these are the same)
- your old control did not have a manual fan ON switch so you don't need that feature.

Question: had to replace the damper motor attached to the damper on my wood furnace. Subsequesntly, the fan motor would engage and not shut off

(Dec 31, 2014) Dan said:
I had to replace the damper motor attached to the damper on my wood furnace. Subsequesntly, the fan motor would engage and not shut off when the plenum temperature dropped off below the limit setpoint.

At this point I replaced the limit control. Now wjen n operation, the fan limit will cycle the fan motor on for one minute and ten seconds and off for two minutes fifteen seconds.

I will do this wether the thermostat is calling for heat or not until the fire burns out. It is part of a wood oil combination, two furnaces using the oil furnace fan motor, Each has it├Ęs own fan limiy conntrol, the oil side works fine. Is it possiblt the coil or relay associated with the wood fan limit is weak and causing the limit to engage



This sounds like a wiring error, a damaged limit switch, or odd cycling plenum temperature on the furnace.

Question: long on intervals for Thermo Pride Furnace

Anonymous said:
what causes a thermo pride furnace blower to run for long intervals 35", then off for 30 sec. then run again. when NOT calling for furnace heat coverage. the blower fan limit changed 3 times,



Is just the blower running or is the burner on the furnace also running?

In any event, look for a wiring error, wires shorted together, or a bad thermostat or control board.

Question: Rheem oil furnace blower works but not burners, water in the oil tank?

13 Jan 2015 Jerry Cannizzaro said:
Rheem oil furnace. Blower goes on but the burners does not. Reset works.

Heat Turns off at night when temp is set lower. In morning blower on but no heat.

Filter and nossle replaced. Repairman thinks water (moisture) may be in oil tank. Drying agent applied still the furnace does not go on when thermostat turned up. Intermittent. Occurs mainly at night but has happened during day. Please help


Jerry I'm confused by the description of the heating trouble too. Certainly you want an on-site heating service tech who's experienced.

Perhaps you can politely and calmly (so as not to chase them off) ask your heating company's service manager to send a senior tech out to get to the bottom of the problem.

Detecting water in the system is pretty easy: see


Question: furnace blower not shutting off

(Jan 14, 2015) Scott said:
The furnace blower in my (Park Model) mobile home periodically fails to shut off after reaching desired temperature.



In the More Reading Article Index links above see the diagnostics at


Question: water in the oil tank

(Jan 14, 2015) Jerry Cannizzaro said:
We have some pretty good techs working on this issue.

They did the moisture check and found moisture in the oil tank. They put in drying agent. However the burner sporadically does not turn on even though the blower is running. So the thermostat turns the blower on but not the burner. This has occurred mostly at night when we set the temp to 65.

We get up in the morning and the blower is going and the temp is 60 not 65. It has also happened during the day. If I use the reset button the burner turns on.

The burner system may work correctly for awhile but then it may not work. The oil tank is 38 years old.



If there is a lot of water in an oil tank it needs to be REMOVED by pumping-it out.

Small amounts of water or sludge can be dispersed and run through the burner using additives like 4-in-1 hot.

But I've found that the additive approach can take months or longer to clear out sludge and that it won't correct rust and debris clogging oil lines and nozzles. Where I've had that problem we

- replaced the oil line between tank and burner with a new larger diameter line
- installed double oil high capacity oil filters at the burner to be sure we could keep fuel arriving to the burner between service calls
- replaced oil filter nozzle
- replaced oil filter fuel unit internal filter screen

Question: my furnace fan keeps blowing after the thermostat temp is satisfied

(Feb 13, 2015) digitthead said:

my furnace fan keeps blowing after the thermostat temp is satisfied. If I remove the thermostat then it will turn off after a few minutes or if I turn off main power it will stop and reset after I turn power back on. This doesn't happen all the time. I replaced the fan control board but no help.

Reading the description above about under the header Primary Limit Control, Goodman Furnace ... 3 The circulator blower to remain energized continuously until limit control resets. is confusing. So I'm not sure what my issue is here.



In the More Reading article Index list above please check out


Question: fan motor just hums, doesn't blow air

(June 11, 2015) Henry said:
Fan on furnace for ac doesn,t blow it just hums. How to fix?


Your service tech may check for a bad motor starting capacitor

Meanwhile shut the system OFF to avoid further damage, overheating, or a possible fire.

Question: fan off temperature settings

(Nov 10, 2014) Anonymous said:
Hi Sir, I would like to know what is reference point to decide temperature. I mean to say how do you know which temp fan stop so I can set fan off switch to that temparature?

Please feel free to contact me at Thank you very much for your time.



I think you've got it wrong. It is the limit switch that will stop the fan.

The recommended temperature settings, designed to protect the heat exchanger from dangerous cracks due to overheating, are pre-set on the control by the manufacturer and should not be changed without consulting with the manufacturer of your furnace.

Also watch out: messing with the limit switch dial while trying to change its settings can bend internal parts, taking the switch out of calibration and making it DANGEROUS!

Question: fan manual/auto switch broken

(Dec 22, 2014) terry said:
My manual/auto button the white on a Honeywell fan limit switch and I can't it to auto

Sorry it broke and I can't switch it to auto from manual



You'll want to ask your heating service tech to replace the switch. I would not mess with it as if a spring is bent the whole system could be unsafe.

Question: How fast or responsive should the dial turn on a fan limit switch?

(Jan 2, 2015) Anonymous said:
How fast or responsive should the dial turn on a fan limit switch?



What an interesting question. I've never timed the dial turn, but I can say that when the furnace burner turns on you should be able to see the dial rotating slowly as the plenum heats up.

The movement is plainly visible but with a *cold* furnace the movement may not begin until the plenum temperature reaches the low end of the limit switch sensor range.

The time to get from fan on to fan-off varies widely as (except for cases where the burner and plenum temperature reach the fan OFF upper limit) that depends on when the thermostat is satisfied.

Question: Goodman Janitrol 1992 gas furnace cycles on and off rapidly

(Jan 13, 2013) JohnBoy said:

gas burners cycles on 10 for 20 seconds repeatly. blower has stayed on with out shutting off because it never reaches the t-stat set temperture.Is it the limit or gas valve.Also,can the pilot light on this furnace be converted to a electronic ignition. It is a goodman-janitrol made in 1992


Rapid on-off cycling sounds like a control problem and an unsafe heating system. Shut if off while waiting for the service tech. The problem maybe trivial - a loose wire, but repeated rapid on-off cycling can damage the equipment.

Question: fan won't stop

(Feb 26, 2014) Charles McMillan said:

Have a split system..set temp at 68 and after getting to desired temp the fan keeps running ....the temp went from 68-76 in a hour.


Because the temperature has exceeded the thermostat setting there's clearly a problem.
I'd start by eliminating the thermostat itself from the equation.

Just disconnect the thermostat wires - if you disconnect at the thermostat (be sure to notice color codes and connection points) then IF the heat keeps running, then

Next disconnect the thermostat wires at their other end - at the heater controller

Then if the heat keeps running the problem is at the heater controller not in the thermostat and its wiring.

Try turning off all power to the heating system; wait five minutes. Then turn power on and see if it behaves properly.

If not it's time to call for service.

You can also search InspectApedia for "HEAT WON"T TURN OFF" to see our diagnostic suggestions.

Question: aux gas heat ignites but fan doesn't come on

(Nov 25, 2014) Jane Metzger said:
My furnace works fine when set on the heat pump. When it switches to the gas auxilary the gas ignites, but the fan doesn't come on until the high heat turns off the gas.

Then the blower comes on and runs until the air starts cooling. The AC guy was here and thinks it is the control board. but I want to make sure.

We had a problem with a clogged condensation pipe last year, but there doesn't seem to be any water issues this time.



In heat pump mode heat is coming from your heat pump and the coil in the air handler - on the gas furnace the fan limit switch is probably not being called on to do a thing - since its job is solely to protect the heat exchanger in the gas fired furnace.

But when the system switches to aux heat then the gas furnace wants to operate (or you want it to) and the fan limit switch is on-duty.

I'm not smarter than your on-site AC guy but you'd think that if the control board in the heat pump is turning on the gas furnace - that is the gas furnace tries to run and its burner ignites, then at that point it's the fan limit switch that actually turns on and off the blower (in response to furnace heat exchanger temperature).

I'm not quite clear on what's happening but can offer a diagnostic question or two.

What is turning off the gas: the fan limit switch?

On most gas furnaces the blower will run continually during a call for heat and will run for a short time after the thermostat is satisfied - to cool down the furnace heat exchanger (avoiding damage to it).

The limit switch will shut down the burner if its sensor thinks that the heat exchanger temperature is dangerously high.

I'd start by checking that the wall thermostat switches are all set properly.

I'd continue by watching the fan limit switch during a heating cycle to see if its dial rotates and noting its position when the burner operates, burner stops, fan comes on or off. (the limit switch dial should start moving until the burner has begun heating the plenum)


Question: new fan limit switch installed, still no heat

(Dec 31, 2014) edsel said:
i replace a new limit fan switch and fan relay and still the heat want come on when it reaches the temperture


Look for a wiring error

Question: furnace comes on but fan does not

(Nov 5, 2014) John M. said:

Furnace burner comes on but fan doesn't. Not sure if it's fan switch or capacitor or fan motor. The fan limit switch does turn when it heats up. It has 4 leads, 2 fan and 2 limit. Can I jumper the 2 leads for fan to see if the fan runs?



I would first

- turn power off to the unit, then see if the fan assembly will rotate freely

Then observe: your heating service tech will probably check for voltage at the fan motor - if there is none we think it's a control problem. If there is voltage and the motor hums but doesn't spin it's more likely a start/run capacitor problem.


Question: flashing indicator says limit control is bad; replaced unit still flashes.

(Nov 9, 2015) stephen said:
My furnace (lennox) has stopped working it is flashing that the limit control is bad. I replaced it and is still flashing that i is bad and my blower is not engaging or spinning but it sounds like there is electricity going to it. I replaced the blower a few months ago. What do I do next?


When you replaced the blower I am guessing that means the fan motor or motor + actual blower assembly.

If the blower isn't running either the motor or a control is bad or worse, an underlying problem - maybe a short or nicked wire - damaged the new motor or its start/capacitor.

I'd start by checking the things that I've missed before like: be sure that the blower door is not open or ajar or that the door switch isn't shutting off the blower.

Question: fan limit switch hums

(Jan 1, 2016) les said:
You stand next to the switch and fan not running and you can hear the switch hum


I can only guess from my remote position, but a humming switch that doesn't actually operate suggests instead: a seized electric motor, bearing, pulley, blower assembly, OR a failed switch / relay. I would turn the system off and call a repair person.

Question: what are normal settings for limit switch to be set on a Honeywell gas furnace?

(Jan 1, 2016) nick said:
what are normal settings for limit switch to be set on a Honeywell gas furnace?



Near the start of this article (FAN LIMIT SWITCH TROUBLESHOOTING where this question was first posted) you'll find a live link for the capitalized text below:

The fan limit switch temperature settings (shown on the dial sketch) are discussed separately

Question: limit fan switch goes on and off

(Jan 4, 2016) Donnell said:
My limit fan switch run an goes off in the furnance I think it's a bad limit fan switch it comes on an goes off



In FAN LIMIT SWITCH TROUBLESHOOTING (where this question was first posted) you'll find two related articles you should review

Question: furnace blower is short cycling on and off

(Jan 20, 2016) Paul F said:
My blower is short cycling with the burner staying on. When I watch the fan limit switch (Honeywell Disc type), it rotates up as the burner heats up the supply plenum (is that what it is called?) and the fan kicks on (on setting is at ~150).

As the fan starts moving air, the switch quickly drops below 50F and the fan turns off.

The box is hotter than 100F at this time. If I put the fan in manual, I get warm air out of the ducts but the limit switch remains below 50. Is there a way to calibrate the switch? Even at the lowest off setting, it cycles off. I have ordered a new switch but I am looking for a way to keep the house warm until it arrives.

I am reluctant to leave the house with the fan on Manual and the burner going.



I would watch the limit switch - it may be that the plenum is overheating - perhaps due to low air flow rate perhaps due to a dirty blower or dirty air filter or inadequate return air

Question: blower fan not turning on and off at proper times, erratic blower fan operation on heating furnace

2016/03/24 Cory said:

Hello, the problem I'm having is ,I changed my thermostat yesterday thinking it was my problem.

When I activated the new stat and called for heat the gas valve activated and the heat turned on ,but the blower fan did not , eventually the fan did start maybe 60-90 seconds after ignition, the unit ran for 5-10 mins then shut off fan continued to run then it eventually shut off.I waited about 15-20 mins knowing that my stat was still calling for heat and not satisfied but the unit did not turn back on I appreciate any help.



As the thermostat successfully called for heat, it's not likely to be the problem.

It is normal for the blower fan to not start until the plenum above the heat exchanger at the furnace is sufficiently warm - that's to avoid blowing cool air on the building occupants.

Watch the fan limit switch as the burner runs: you should see the disk turning until it reaches the FAN ON setting. On some systems that might take a minute.

It is also normal for the fan to keep running a bit after the furnace turns off, extracting the remaining heat and avoiding damage to the heat exchanger (from overheating).

But if the blower and furnace are turning off before the thermostat is satisfied, that is the room temp is below the set temp on the thermostat, then something else is wrong.

The most common cause, I suspect, is a furnace that is overheating, perhaps because of a dirty air filter, dirty blower fan, or crimped air duct or inadequate supply air. But that wouldn't necessary cause inconsistent or "erratic" blower on-off operation. For that I'd look for a failing electrical component: a control, relay, door switch, even wire.

Check those out and keep us posted.

Reader follow-up: ruling out a dirty air filter as a a cause for blower fan cycling off and then back on

Cory said:
Thanks for the fast reply,Today I was working on the problem a little more,I checked the air filter and noticed that it was dirty so I removed it completely for the sake of getting the unit to run properly I will be replacing , I assumed the unit was cutting out on limit because of the lack of air flow the unit.

But my trouble continued unfortunately. I called for heat and the unit operated properly the burner started first followed by the blower ,the unit ran properly for 10-15 mins so I lowered the stat to make sure the unit cut out which it did , I waited several more Minutes to call for heat again and unfortunately it wouldn't come back on.

I will take your advice and look for damaged air duct or blocked blower fan to see if this is hurting my situation .Thanks again for your help


Cory from the sequence you described, if a furnace starts normally and runs 10-15 minutes normally, then won't re-start, most likely the problem is not in the air duct system.

If this is oil heat I suspect the burner needs cleaning, service, adjustment.

Reader follow-up: gas furnace blower fan erratic behavior traced to loose wire

Cory said:
The system is gas ,the system is actually very simple it consists of gas valve 24 transformer combination switch and blower motor.

Ok so I went over everything again and I ended up finding a faulty connection with my wire going to my gas valve,it is a push in and lock stlye connection so to the naked eye it looked like it was in correctly ,you live you learn I guess thanks for all the help it is appreciated.


Continue reading at FAN LIMIT SWITCH - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see FAN LIMIT SWITCH FAQs-2 - more questions & answers about fan limit switch troubleshooting


The basics of how furnaces work can be read at FURNACE OPERATION DETAILS

Or see these

HVAC Blower Fan Articles

Suggested citation for this web page

FAN LIMIT SWITCH FAQs at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman