Furnace blower fan limit safety switch installation & troubleshooting FAQs set #2:
More 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.
Page top illustration: the interior controls and standard settings for a typical furnace or forced-warm-air fan imit control switch.
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Also be sure to read FAN LIMIT SWITCH TROUBLESHOOTING. At the end of this article you'll find a complete list of air handler or fan limit control switch diagnosis and repair articles.
Photo: the exterior of a typical Honeywell fan limit control switch found in the air handler of furnaces - forced warm air heating systems.
[Click to enlarge any image]
On 2017-02-18 by (mod): how to set the fan control in an older furnace or air handler
FAN allows running the fan full-time for more even heat distribution; or there may be different fan speeds - for cooling and heating;
HEAT is where your speed selector should be if you are in heating mode. That ought to give the right fan speed for heating.
COOL is where you'd set the selector if air conditioning is running through the same air handler - which you may not even have installled.
Get the control brand and model and we (or you) can probably find the instruction manual.
You might also search InspectApedia for NO HEAT or NOT ENOUGH HEAT to read more diagnosis and repair advice.
On 2017-02-18 by Brandon
On my propane furnace i have a speed selector on the control panel inside the furnace. There's three sections. Fan, Cool, heat. My cool lever is all the way on high and the heart lever is all the way on low.
Does this mean i get less heat in my house? It seems like my thermostat is 8 to 10 degrees under what i have it set to, all tbe the time. If i move the heat lever up and the cool lever down will this improve the amount of heat coming into my living space?
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.
On 2017-01-01 by (mod)Good going on diagnosis, Warren.
On 2017-01-01 16:42:19.815976 by Warren - successful diagnosis of fan problem - need new control boardThanks for the feedback. We have the fan running now but no heat. I will pick up a new board this week and replace it.
On 2016-12-31 18:55:40.489581 by (mod)
Warren, SOME but not all fan limit switches contain a FAN-ON switch that will keep the fan on constantly. If your limit switch lacks that feature you might be able to install one that has it.
I'd be reluctant to simply hot wire the fan motor directly.
On 2016-12-31 18:23:51.358829 by WarrenSpell check sorry
On 2016-12-31 by WarrenHow do I bypass the furnace blower so that it will work constantly and heat the house. My mother board will. It turn on the blower. Can't get a new one until next week.
On 2016-12-29 by (mod) - when does the fan keep running after a heat-on cycle?
Josh, you're right that a furnace blower may keep running to purge heat from the heat exchanger at the end of a heat-on cycle. However that may not always occur. If the thermostat turns off the furnace while the heat exchanger is not at a high temperature the fan may simply stop immediately. So there's not necessarily a problem.
On 2016-12-29 by JoshI thought the fan was supposed to run a little after the thermostat kicks the burner off to get rid of that heat and the limit switch should turn the fan off? I have an older furnace btw
On 2016-12-28 by (mod) - fan shuts off early?The heat exchanger's residual heat is re-heating the air in the supply plenum, turning the fan back on. Usually I trace this to a dirty air filter, clogged heat exchanter, or dirty blower fan - air moving through the heat exchanger and ductwork don't remove heat at a normal rate.
On 2016-12-28 by JoshMy fan is shutting off as soon as the thermostat is satisfied then fan will turn back on once the limit switch heats up to the fan on setting! Why?
On 2016-12-14 by AnonymousSure Jerry.
On 2016-12-14 22:03:55.716788 by JerryI had to replace the ductwork on my blower furnace fan and I accidentally unwired the limit switch so now my fan continuously runs could someone tell me which wire is the fan limit switch or how The furnace should be wired it's an oil furnace
On 2016-12-06 by BillThanks for the info. I'll inspect the relay for possible problems.
On 2016-12-05 by (mod) - watch the fan limit switch dial rotate as the plenum heats up
Just to be sure we're on the same page, you're watching the heater dial rotate as the supply plenum heats-up. You should see the dial rotate until the dial's FAN ON position or temp setting is satisfied.
At that point the fan should run. It won't run until the plenum has heated up. Since you're seeing the furnace limit switch shut the burner off at the HI-OFF position or LIMIT position then we know the plenum temp must have passed the FAN ON position.
There's probably not much besides a fan relay to look at.
On 2016-12-05 by Bill
I followed the steps for testing the fan/limit control and it appears to work fine and shutting the gas valve off when reaching the high limit setting but when reconnecting the fan motor lead the fan does not turn on.
What other component is related to calling for the fan to run that would be after the fan/limit control. I was thinking the fan relay may be suspect but the fan does run in the manual mode.
On 2016-12-05 by (mod)Bill
On 2016-12-05 by Bill - old GE forced air furnace fan limit switch replaced
I have an old GE forced air furnace(40yrs old)and had to replace the fan/limit control Honeywell M#L4064B. The reason for replacing the control is because the fan would not run when reaching the fan on set point. The fan does run manually.
The problem still exists after replacing the fan control. At this point, because of the age of the furnace, I'm finding it near impossible to locate a schematic for this heater. Any ideas as to why the fan does not start when called for? When the burner reaches the high limit the gas valve does close.
On 2016-12-01 by KenThe guy replaced a 8" fan limit switch with a 5" last winter. It works but is that ok?
On 2016-11-20 by Anonymous
I'm an electrician, and my first thought was, it must be something wrong with the switch
. BUT, as I am not an HVAC expert, I didn't know if replacing the switch would do the job. I was concerned there might be a deeper problem. But I'm prety sure I'm right.
On 2016-11-19 by (mod) - fan limit switch may be unsafe
This sounds to me like either a loose wire or a worn, burned contact inside the limit switch.
The switch could be unsafe as well. The fact that you can get the switch to behave by toggling the AUTO ON switch - assuming you're doing this on the limit switch - would lead me to replace the control.
On 2016-11-19 by BrettMy blower fan keeps running. When I toggle the manual and auto on my limit a few times, it will shut off until the thermostat calls for more heat. then the blower fan comes back on and won't shut off again. unless I toggle the limit a few times again. Thoughts?
On 2016-11-14 by AnonymousSome times I have to rotate the control manually to shut the blower off, not always.
On 2016-10-28 by (mod)You're right, David: if the system is overheating it can crack the heat exchanger, risking fatal carbon monoxide poisoning.
On 2016-10-26 by DavidMy Ducane furnace is shutting down after 5 minutes of operation on high heat limit. Jumped out the switch and it ran longer. Didn't let it run to long though it may damage something
On 2016-10-14 by LeeHave an old hallmark natyral gas furnace the just replaced blower on. Since replacement of blower motor am not able to stop blower from coming on and going off again and again. Could it be the fan settings? Please help me set the fan settings properly
On 2016-10-11 by JohnBlower cycles on and off even when heat and AC are off and fan is set to auto.
On 2016-07-24 by (mod) - tapping the limit control to see if it runs: loose or corroded electrical wire connection?Tapping the switch and seeing it run suggests that you've got a bad or shorted thermostat wire.
On 2016-07-24 by Rick
AC blower would not come unless tapped on fan/limiter switch. I replaced limiter switch, now fan will not come on unless I set fan limiter pull switch to manual(constant run) mode. The old switch did not have a push/ pull mode switch.
Do I have new problem or still the original problem, that I can now override with my newly acquired mode switch? On manual setting, fan will not shut off, even with thermostat turned to "off".
On 2016-07-07 by (mod)Nancy this is an issue to review first with your landlord, your rental contract, and with the landlord's legal obligations - that may vary depending on where you live.
On 2016-07-07 by Nancy Pierce - Bryant air conditioner, Goodman furnace - I'm getting sick - SAFETY WARNING
Im on rent my landlord has the fan blowing in the summer. The air conditioner doesnt work. Im getting dust and the smell of gas in the house and heat through thr cold air vents as well as vents heat normally comes through.
The air unit is a Bryant and the furnace is a goodman.
Something is terribliy wrong here cause im getting nauseated, my throat swells closed and i have to seek medical attn. Im afraid i wont wake one day because of the way the furnace is set up.
On 2016-02-23 by (mod)
Sorry Eric I'm not familiar with this board. I agree that it looks as if you've pinpointed a possible hardwired or jumpered 90 minute cycle setting. I would download a copy of the owners' manual for the furnace and if necessary a separate guide for the specific control board if that is not an OEM part in your furnace.
There the options and controls will be detailed. It may be you need simply to remove a jumper.
Keep us posted.
On 2016-02-23 by Eric - Goodman B12260-08 board
I have a York Diamond 80 Furnace that come on every 90mins on the clock to pump air through the house. It usually come on about 10-15 mins then shuts itself off. Then after another 90 mins it would come on again.
I notice there is a Goodman B12260-08 board that has some jumper switch on it with number 60,90,120. I try to adjust that but does not seem there is any impact....Any ideas?
On 2016-01-16 by (mod)Can, yes. Should? No, subverting this safety control can destroy a furnace or can kill building occupants.
On 2016-01-15 by Dee from michiganCan you over ride a combination bower limiter switch
On 2016-01-06 9 by (mod)
Andre, please use the page bottom CONTACT link to send us photos for comment or posting.
Indeed there are several versions of fan limit switch and they do not all look alike. Several of them appear as photos in this article series but I'd be glad to see and comment on your system and some sharp close-ups of each of its controls.
On 2016-01-06 by Andre - how do I post a picture of my fan limit switch on a 1960s Bryant QuietlineCan I post a picture here? I have been trying to locate the fan limit switch on my 1960s Bryant Quietline, but it seems to look different than the ones shown on this website.
On 2015-11-26 by (mod)Mister
On 2015-11-24 by misterw - mobile home unit blows only in summer positionSame problem as Michelle, got an old miller unit for mobile home, blows in the summer position and not winter, worked fine in winter position until today. Any tips\help appreciated
On 2015-11-21 by (mod)Peter
On 2015-11-20 by Peter
I have an old warm air furnace that uses White-Rodgers Type 5A75-5 Style V2QP1Y Fan and Limit controller.
This model is discontinued and I cannot find them anywhere. Is there a direct replacement either from White-Rodgers are from any other major brand?
On 2015-10-23 by (mod) - check fan limit dial settings
Sounds as if the air flow rate is cooling down the heat exchanger / plenum to shut off the fan to avoid blowing cold air on to people who want heat; be sure the air filter is installed; check the air flow rate.
Check that the limit switch settings are to factory spec.
Don't bend the switch plate or spring or you'll force it out of calibration and may make the system unsafe.
On 2015-10-23 by GlenI just installed a new blower fan and now the fan control starts the fan then dials back and shuts it off in abt 17 seconds. Then dials up and starts the fan again.
On 2015-10-23 by michelleMy furnace has a winter, summer, and off switch. The blower blows in the summer position but not the winter position. So it blows cool air not heat, but the blower gets real hot.
On 2015-10-17 by AnonymousCan't say from s o little informatiom. Use the page bottom CONTACT link to send some clear photos to the InspectAPedia.com editor and we'll be able to give advice.
On 2015-10-17 by AnonymousWhat is the black switch on oil miller furnace for should it be on or off mode
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