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Fan limit switch troubleshooting diagnosis & repair: this article describes in detail the testing and diagnosis of problems with warm air heating furnace combination controls, also commonly called the "fan limit switch" on warm air heating systems. As a working example we examine the Honeywell combination fan and limit control type L4064B. We also discuss the diagnosis & repair of common furnace operating troubles such as a blower assembly fan that runs continuously - it may be a simple switch setting.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
The photo at the top of this page shows all of the controls and wiring terminals in a Honeywell combination fan and limit control type L4064B.
This website discusses these systems and heating components in detail in articles listed at the left of these pages. If your heating system is not working properly, see NO HEAT - BOILER or NO HEAT - FURNACE. This website answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
If your heating or cooling system blower fan itself appears not to be working, see BLOWER FAN OPERATION & TESTING. Readers should also see How to Install the Fan & Limit Control, or begin this topic at FAN LIMIT SWITCH. Also see FURNACE OPERATING TEMPERATURES.
Honeywell provides a very simple fan limit switch checkout procedure to be used after the switch is installed:
Here, extrapolating from that guidance and adding some field experience, we provide more detailed step by step procedures for troubleshooting and testing the heating furnace fan limit control switch.
If your Furnace Fan Runs Continuously - (the fan won't shut off on its own)
Frequently Asked Questions (FAQs) about warm air heating furnace fan limit switch controls
Question: Why won't my furnace blower fan start on its own? It will run if I give it a "spin"
My gas heater isn't working correctly. My thermostat signals the heater correctly and the burner ignites, but the fan doesn't always turn on. If I open the panel and spin the squirrel cage a little, it immediately begins to blow air and runs the heating cycle. What could this be? - Mike.
Reply: Check or replace the blower fan motor start/run capacitor
Question: Why does the blower fan keep turning on and off repeatedly after the thermostat has stopped calling for heat?
What do you do if the blower fan goes on for its cycle then shuts off only to go on again, going on and off too many times. What is wrong? - Trevor
How do you know if the [fan limit] switch is going bad because my still works but the fan seems to turn on and off too frequently? - Anonymous
Reply: Why the blower fan may turn on and off after the end of a heating cycle
According to Honeywell, voltage transients or even other conditions around the limit switch that impact the temperatures to which it is exposed can affect the cut-on or cut-off temperature behaviors of the switch, but the company's instructions to not mention apparently excessive on-off cycling of the furnace blower fan traced to the limit switch.
First let's review the normal warm air furnace operating cycle and fan limit switch settings
When the HVAC system is in HEATING mode and the thermostat calls for heat, the oil or gas burner turns on and begins to warm the heat exchanger. When the air in the supply plenum served by the heat exchanger becomes warm enough the blower fan turns on to begin circulating building air through the occupied space.
On most warm air heating systems both the burner and the blower fan unit will run continuously until the temperature at the wall thermostat rises to the thermostat set point. Then the thermostat stops calling for heat and the burner turns off.
The blower fan, however, will continue to run until the heat exchanger and the supply air plenum have cooled down. That "run-on" period scavenges otherwise wasted heat and sends it to the occupied space, and it prevents warping and possible cracking damage to the heat exchanger by cooling it down.
Finally, when the heat exchanger and supply plenum have cooled, the fan limit switch will shut off the blower fan. The system stops.
Note the three temperature settings on the fan limit switch dial in our photo just above, and in the sketch higher on this page.
The fan limit switch temperature settings and adjustment procedures are explained in detail at How to Set the Fan & Limit Control.
Common, acceptable extra furnace blower on cycles?
But because on some furnaces the heat exchanger itself and the combustion chamber below it are still hot, that residual heat continues to heat air in the supply plenum (where the fan limit switch is located). If the temperature rises enough in the supply plenum, that will activate the fan switch once again, causing the fan to turn back on again.
Signs of trouble with the furnace controls or air flow that cause the blower fan to cycle repeatedly - things to check:
If however when the thermostat is not calling for heat the blower fan comes on and runs repeatedly and for longer intervals then the fan limit switch may be defective, or there may be a problem with the airflow rate or temperature through the system and you need a service call.
Check for an improperly installed fan limit switch
If the fan limit switch was installed askew, or if the sensor element (a long bimetallic spring in a metal protective enclosure) is too long, such that either defect causes the sensor assembly to actually touch an internal steel part of the heating furnace, then the switch will not perform properly.
A "too long" fan limit switch sensor problem may occur if a previous replacement of the fan limit switch installed the wrong model - a unit that did not match the original.
Watch out: Some fan limit switches are mounted using a rigid bracket that requires tightening a set-screw (red arrow in the edited Honeywell sketch at left] to hold the limit switch in the bracket.
But a simple error of mis-locating the bracket set screw can cause the screw to contact the bimetallic spring inside the switch - a dangerous condition that can cause the switch to fail to shut off properly in response to high temperature. 
Also the fan limit switch should be installed in the same location on the furnace as the original switch.
If someone relocated the switch it may be in a too cool or too warm location, or in a location that does not allow the switch to reliably sense supply plenum air temperatures.
The Honeywell L4064B fan limit switch is designed for use in both line voltage and low voltage installations. But for low-voltage installation a brass jumper must be removed (red arrow at left). Honeywell warns:
Check for a missing heat insulating gasket at the fan limit switch mount
At furnaces set to higher operating temperatures, a heat-insulating gasket is required between the switch mounting contact body and the surface of the furnace.
Check out the wall thermostat for heat call cycling
Before replacing the fan limit switch, try checking the wall thermostat too. Some thermostats can develop a contact bounce, or may have a failed heat anticipator circuit, resulting in some cycling of actually calling for heat. To debug this condition try calling for heat up to a fairly high thermostat setting, say 74 degF. Then when the room temperature has reached 72 degF., disconnect the thermostat wires at either the thermostat or the furnace control. That's essentially the same as the thermostat reaching its set point and ceasing to call for heat.
Now watch the performance of the furnace blower assembly. The furnace burner should stop, but the blower fan should continue to run after the call for heat stops until heat has been extracted from the heat exchanger and supply plenum. If the blower fan cycles on and off as before, then the problem is not the wall thermostat.
Typically the HVAC tech will replace the fan limit switch in these conditions. The switch itself retails for around $100.
Reader Comment: reader suggests cleaning the fan limit switch
The question above is: Why does my blower motor keep coming back on for short/intermediate/momentary bursts/pulses immediately after the furnace cycles normally? I assume we are talking about L4064's and there equivalents/competitors
It is, after all, an electro-mechanical device, relying on a spiral shaped bimetal coil to turn a disc that has mechanical contact points for the on and off settings for the blower. That heat sensitive bi-metal spiral attached to a shaft passes through a bushing that simply dries out after hundreds if not thousands of cycles...then starts sticking and jumping between the on and "OFF" settings instead of smoothly transitioning as it did when it was new.
Do you need a new $100 fan limit switch? Maybe! But first try removing, inspecting, cleaning and applying an appropriate hign temperature lubricant. Unless the spiral bi-metal coil is broken or the on/off/limit contacts are completely worn down...you may be surprised at how much life is still left in that old limit control switch. - Kevin Sharpe
Reply: we do not recommend disassembly nor modification of heating system safety limit switches or similar controls
Kevin, you are correct that a typical Honeywell fan limit switch retails for around $100. But your advice that people try disassembling, cleaning, lubricating the switch makes me nervous.
Honeywell's own installation instructions (see our references below) mention nothing whatsoever about such steps, but the company does warn more than once that care must be taken to avoid bending internal springs and parts of the switch. If these sensitive parts are modified in any way, say by bending or even by a blob of grease left by someone who may be less meticulous, the switch loses calibration and could fail to shut off the system at high temperatures - an unsafe condition that could also damage the heat exchanger.
Question: Why does the blower fan keep turning on and off repeatedly during the heating cycle (the thermostat is calling for heat)?
Our furnace seems to turn off and then back on repeatedly during the heating cycle. I've read that normally on a call for heat the burner should come on, then the blower, and both should keep running until the call for heat is satisfied. What's going on? - Anonymous
Reply: Abnormally low air temperature at the return plenum can cause blower fan short cycling
Abnormally low incoming air temperature entering the heat exchanger after the oil or gas burner has shut off may cool down the supply plenum so rapidly that the fan limit switch, working properly, turns off the blower before enough heat has been extracted from the heat exchanger itself.
If this is happening, rising heat and air (by convection) from the still-hot heat exchanger into the supply plenum may warm the plenum again and turn the blower fan back on.
Blower fan short cycling on and off too frequently during the heating cycle is often traced to this problem.
Why would we have "abnormally low incoming air temperature" at a furnace?
We've seen this problem at buildings with improper HVAC duct design, in particular where some or all of the return air to the furnace is being taken from an opening right at the furnace such as a unit located in a cold basement or crawl space(see our photo, above left).
Sometimes in an effort to improve the total airflow delivered by a furnace a technician will cut an opening in the return ducts in an unheated basement or crawl area.
Watch out: This return air opening boosts the total air output of the system but it is an improper and unsafe design. See INCREASING RETURN AIR for details.
Abnormally high air temperatures at the supply plenum can also cause blower fan to turn on and off unexpectedly
While this defect is less common than the cold return air problem discussed above, a dirty air filter or any other defect that restricts air flow through the heating system supply or return ductwork can also cause unexpected blower fan on and off cycling.
If the airflow through the heat exchanger and supply plenum is too weak the furnace oil or gas burner may be able to heat the system to the point that the fan limit switch, to protect the unit from overheating damage (warping, cracking) will turn off the burner, even though the thermostat is still calling for heat. In this condition the blower fan may continue to operate (to cool down the "too hot" heat exchanger and supply plenum).
The blower fan will continue to run until the supply plenum temperature drops to the low limit, then the fan limit switch will turn off the blower. As the thermostat is continuing to call for heat, when the furnace temperature drops, the fan limit switch will permit the burner to turn back on.
At AIR FILTERS for HVAC SYSTEMS we discuss various problem experienced by heating systems due to a dirty or blocked air filter.
Question: our fan limit control shuts down the furnace before the thermostat temperature was satisfied
I tested the fan/limit switch by removing the fan connection on the switch. The burners fired up, went through the cycle and cut off just as it should. I reattached the fan, the system kicked in and the fan came on. The furnace ran for about 20 minutes, burners cycled on and off normally, but then the system shut down before satisfying the desired temp. ? Howard Curtis 1/4/12
Howard make sure you are considering the right "temperatures".
The set temperature at the room thermostat is the desired room temperature. As long as the room temp is below the set temperature, (with a slight technical exception around the heat anticipator circuit) the thermostat will continue to call for heat.
On a call for heat on most warm air heating systems, the furnace heater turns on, the supply air plenum air heats up, the then the blower assembly runs and will often continue to run until the call for heat is satisfied.
But in some cases, such as blocked airflow due to a dirty air filter, the supply plenum could reach an abnormally high temperature and the fan limit switch will then shut off the burner for safety. In such cases, the furnace heats up to the FAN OFF high limit before the room thermostat is satisfied.
Watch out: overheating the heat exchanger is dangerous. If this is going on your furnace should be checked by a professional.
Question: setting the fan limit switch FAN OFF down 5 degrees seemed tofix extra fan-on cycling - is this OK?
I notice that the fan on the heater came on briefly after the heating cycle concluded. I checked this article and decided to adjust the fan switch shut off temperature from 90
degrees to 85 degrees. So far the problem of an additional, short running fan cycle has
Thanks for the comment Larry. Indeed in the article above we discuss the concern of blower fan cycling on and off one or more extra times at the end of a heating cycle. If that small shut-off temperature fixed the problem that's a great tip for other readers.
Question: forgot to remove brass jumper on a new fan limit switch installation
I installed a new fan limit switch but in my rush I missed removing the brass jumper for low voltage. Tested it and it started and stopped. I can't seem to find the damage .Do i need to buy and replace the limit switch again? I've checked the fuses and replaced both but still wont work. what else do i need to check. thanks
Dennis, I'm sorry to have to play it safe, but reading Honeywell's warning that the control could be damaged, I just wouldn't take a chance.
A problem is just as a bent spring can change how a switch performs, internal damage could be subtle and not visible, but the fan limit may not perform safely.
Certainly you can go through the recommended fan limit switch test procedures described above on this page to confirm that the swtich is doing what it is intended to do.
If you are still concerned I'd repalce the switch rather than take a chance or lose sleep over it.
Question: My furnace turns off at 20C even thermostat is set to a High Limit of 35C.
My furnace turns off at 20C even thermostat is set to max 35C. New thermostat was installed I have looked and monitor the fan limit switch. The silver dial is moving between ON and OFF position and never get to the Limit position. Just wondering if there is something wrong with Fan Limit Switch and how do I reset the Fan limit switch. Appreciate - Dungthieu@optusnet.com.au 5/27/2012
Question: How do I wire a furnace fan relay switch
how to wire a fan relay switch - Kenneth Sanders 10/7/2012
Kenneth, please see How to Install the Fan & Limit Control - separate article
Question: the furnace comes on, heats, but the fan then cycles off and back on - what's wrong?
My Grandmother old furness comes on and heats but the fan cycles off and on. If I put the unit to run it works great heats the house. Is something I can trouble shoot with and electrical back ground or should I call a pro? - Harry Doucet 10/8/2012
I haven't seen my exact problem. My furnace will come on, run it's whole cycle but where You think it's going to shut completely off, the fan or motor or whatever keeps running & within about 30 seconds the furnace goes thru another cycle where I hear the click to tell me the flames have come on then I hear the blower click on then the furnace runs it's usual 12 min cycle before I hear the click of the flames go out leaving the blower on & then when the blower is supposed to shut completely off it doesn't & so again in 30 seconds I hear everything click & it starts up another cycle.
And sometimes I can Stop it by turning the thermostat down slightly & if that doesn't stop it I have to shut off the Power Source by the Wall Main Furnace Switch, Not the Breaker, but the Furnace Wall Switch.
Would this still be the Limit Switch if it Doesn't completely Stop? I was hospitalized for 1 month & shut my thermostat off & when I came home & put the thermostat up to 63 where I usually keep it abouts, this is when all the trouble started with the furnace Not shutting down completely but it was Fine prior to that. And every summer I shut the thermostat off so I'm not using gas & this has never happened before.
My furnace is made by Intertherm for a Mobile Home & it's a Natural Gas Furnace. Thank You, Kimberlee :) PS... I'm asking You first bcuz as a single Mom I always seem to get ripped off bigtime by repair men! - Kimberlee 11/6/2012
Reply: what to check if the furnace fan seems to cycle back on shortly after it shuts off
Harry and Kimberlee,
This same furnace-fan-runs-again phenomenon may also occur (before the burner ignites) followed by the burner igniting as well while the fan is running. This second case occurs when both the fan has re-cycled on due to what we just explained combined with the thermostat switching back to calling for heat. This process is more likely to occur in very cold weather or in a very drafty building.
Question: forced air heating furnace blows COLD, HOT, COLD, then OFF - what's up with its fan limit control?
Two story 75 year old classic cape cod home. Gas forced air furnace is Lenox G16 24 years old, in good condition. With Honeywell fan limit switch, outside fresh air intake and draft motor. We have owned home 12 years. (has AC). Owner not a technician but understands mechanical/heating concepts.
Furnace operating problem
When heat called for and fan comes on it blows cold air, then heat, then cold, then off. It has always done this. Limit switch is at off 100, on 125 off 150.
Changing set points on limit switch does not seem to change pattern. I do not dare force it. This winter local utility provider replaced burned out fan motor with same HP, number of speeds, etc. Since the draft motor was noisy we replaced that as well. Upon disection of draft motor is was covered with white crusty minerals probably from the water from the humidifier mounted on the nearby cold air return. When we first owned this house we have removed the humidifier (and installed a whole house water filter).
The fan limit switch sensor which protrudes into the burn area is crusted and therefore "locked" into a former position and now unadjustable. What do you think? - J.C. 4/15/2013
Reply: stuff to check when the fan is not turning on and off at reasonable temperatures
Thanks for the interesting question and the excellent photo. Unfortunately the photo had a virus - we had to clean it up so what we display here is a bit different and at at lower resolution that your version.
I don't know what's happening with your heating system but I can, from your note make a few diagnostic suggestions:
Watch out: you are quite right not to force the set point adjustments on a fan limit switch. The assembly is intended to be adjusted within a safe range by the heating service tech by moving the stops, but if you bend something the switch is damaged and unsafe, and if you set the switch incorrectly (for example fan coming on too late or turning off too early, or setting the upper limit too high) there is risk of overheating the heat exchanger, cracking it, risking fatal carbon monoxide poisoning at worst, or perhaps a costly heat exchanger or furnace replacement job. I like Dan Holohan's expression "Keep your hands in your pockets" - my mom used to make me sit on my own hands. (No not recently.)
Take a look at the duct system itself for things that can make it cool down quickly blowing cold air sooner than one might expect, such as a long uninsulated run, missing insulation, air leaks at the return side.
Check that the air filter is clean and that the ductwork is not obstructed. (This is just on general principle).
Now about that pesky fan limit control, you could
Keep us posted - what you learn will help other readers diagnosing a fan limit switch problem with their own furnace.
Questions & answers or comments about how to test the fan limit switch and how to diagnose and fix problems with the furnace fan limit control. .
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
For details about the controls, components and switches commonly found on hot hot air heating systems see the articles listed below in which we explain how to identify, set, re-set, repair, replace, or avoid problems with the components of a furnace or warm air heating system.