Furnace blower fan limit safety switch Q&A set #4:

This article series explains how to install, wire, test, set or repair the fan limit control switch used on heating furnaces - the same blower that may be used for both heating and cooling.

HVAC Fan Limit Switch FAQs set #4

Recently-asked questions about furnace fan limit switch controls.

These questions and answers were posted originally

at FAN LIMIT SWITCH - topic home, so you may want to start reading there.

Or check out FAN LIMIT SWITCH TROUBLESHOOTING.

Photo: the interior of a typical fan limit control switch found in the air handler of furnaces - forced warm air heating systems, discussed by Ken Martin's question below

[Click to enlarge any image]

Question: can I replace a traditional bi-metallic spring fan/limit control with a newer, more-accurate controller for my furnace?

Your explanation of limit switch was, and is, the best on the internet. Your thoroughness is unparalleled.

I recently bought a mid-century modern home with a 550 gal oil tank in the garage and a hallmark furnace and needless to say have been searching for resources.

I had 4 wires going from the thermostat hoping I wasn’t gonna have to tear out walls.

Well, I tore out the 50 year old walls and ran 2 18/8 down (for the future needs).

Here’s my question, I want to know if I can get something “better” than this mechanical limit switch. I like that it’s mechanical for safety purposes but I want more control than metal expanding.

Love your site and I can pitch in on the software side if help is needed. - Anonymous by private email 2018/02/07

Reply: probably not, for safety and functional reasons

at FAN LIMIT SWITCH - topic home, I have added details to distinguish between the traditional bi-metallic spring helical probe used by Honeywell and White Rodgers and newer snap-switches that are used by some furnace manufacturers.

You'll see that the principal difference between the disc-type snap switches and the older controls is that the older ones are adjustable.

But it's also the case that a furnace might be unsafe if you swap in a different control that does not place the temperature sensor where the furnace manufacturer wants it.

And I have not found solutions that include the adjustable features of the traditional control.

I, too, like the supposed precision of the newer controls but the decision isn't that clear. Aside from giving up adjustability, you'd probably need several, certainly at least two, snap-disc type sensor/controls, and they'd need to both be located essentially at the same sensing point.

You might also be voiding warranty.

Those old helix type bimetallic spring devices have been used in many thousands of heaters over many decades. They perform well and by offering a large sensing area they might be more resistant to problems from dust and debris than snap-discs.

Where the older spring type controls gave me trouble (when I worked on heating equipment back in the dark ages) was as stack-relays on oil fired equipment. Using the same approach but inserting the probe into the flue of an oil fired boiler or furnace meant exposing the probe to sooting. Those controls were finicky and gave trouble on occasion.

Sure there may be dust in the supply plenum too if a furnace is being run without filters or is otherwise moving a lot of building dust. But that dust is not as troublesome as oil burner soot in a flue.

In sum, I would be very cautious about swapping in a different type of safety control than what was specified by the manufacturer of your heating furnace.

On 2018-01-08 by (mod) - Why fan dont stop

anon

In the Continue-Reading suggested articles please take a look at FURNACE FAN WONT STOP where you will find a live link to an article diagnosing and discussing how to fix a fan that won't stop

On 2018-01-08 by Anonymous

Why fan dont stop

On 2018-01-06 by Mary lou

My central heat won't blow unless it is set to ON position. Whay can I do to fix. I put new wall thermostat digital control. Does same as old one.

On 2017-12-13 by (mod) - The limit switch on the blower should indeed turn the heating burner off if the temperature exceeds the upper or safe limit.

The limit switch on the blower should indeed turn the heating burner off if the temperature exceeds the upper or safe limit.

On 2017-12-13 by Jim

Will the limit switch on blower keep furnace not light or light and go back out

On 2017-12-11 by (mod) -

Eric take a look at the

FURNACE FAN WONT STOP inspectapedia.com/heat/Furnace_Fan_Wont_Stop.php

and let me know if that works for you

On 2017-12-11 by Eric

1985 Byrant, what could cause the fan to continuously run but no heat? Thanks

On 2017-11-27 by (mod) - fan limit control on your Miller oil-fired furnace

Kathy,

The fan limit control on your Miller oil-fired furnace will be just about in the location shown in the photos and sketches in the article above. You'll need to lift up and then out the upper front cover to see the furnace burner and controls.

Or you can use the little "picture" icon to the right of the Comment button to enter a comment and include a photo of your furnace with cover on and with its cover removed and I can probably circle and point to the limit switch.

The "model" you cite is for an oil burner, not for the limit switch.

The Miller Furnace company does not provide readily-accessible installation and operation manuals for their heating equipment at their website - at least not that I can find, but if you don't have the manual for your furnace you can find one at our list
MILLER International Furnace Company - https://inspectapedia.com/heat/HVAC-Manuals-Air-Conditioners-Boiler-Furnaces-J-O.php#MIller

OR you can surely contact the company to ask for one

• Miller Heating and Cooling
  Customer Service
  8000 Phoenix Parkway
  O'Fallon, MO 63368 USA
  http://millerac.com/Products/Furnaces/OilFurnaces.aspx
  
  We also provide Miller / Nordyne furnace manuals at MILLER International Furnace Company
  
  Also see NORDYNE Furnaces
  

On 2017-11-27 by Kathy

Have a Miller oil furnace, trying to locate the limit switch. Model Beckett AF9

On 2017-11-15 by gary - Old Mayflower needs replacement Honeywell#L498B5X0A3 fan limit switch.

Old Mayflower needs replacement Honeywell#L498B5X0A3 fan limit switch. I can't find one anywhere or even what to use for a replacement. Do you know a equivalent switch I can use. It has an 11 1/2 inch probe.

On 2017-11-04 by (mod) - Help I'm freezing with no heat

Tom at FAN LIMIT SWITCH INSTALLATION & WIRING you'll find wiring info and additional instruction manuals at the REFERENCES section to these articles

On 2017-11-04 by TOM - lost wiring diagram

HELP IAM FREEZING FRTEE

LOST WIRE DIAGRAM AFTER INSTALLED NEW MOTEr

On 2017-10-26 by Eiland

Unit heats n cools but it will not shut off when temp is reached. Fan comes on the moment it is turned to heat or cool.

Thermostat clicks when it reaches temp but fan n compressor keeps running. What to do

On 2017-10-10 by (mod) -

Robert

If you can find the right original equipment part, new, undamaged, it should be proper and safe to replace the bad switch.

There may also be generic switches that will work properly. The switch has to fit in the right location, have a temperature sensor of the right dimensions, and an operating range and functions that match the original one.

On 2017-10-09 by robert

I have an early 1970"s general electric imperial force air gas furnace that needs a new fan limit switch.

We had a friend come out and work on it. He said it couldnt be replaced because the part cant be found and to buy a new furnace. I met another repairman and he showed me the part on amazon at under 200 dollars. Is it safe to replace this part. I"m happy with the furnace and cant afford a new one.

On 2017-09-07 by jerry

fan motor won't turn off

On 2017-08-01 by BC

We have a very old Trane Forced Air Unit. Model # RFH121GB. The blower motor won't turn off. Checked the thermostat, switch is in "Auto" and the temperature is set to the upper limit. This unit is old, can't find any info about it at all. Help!

Thanks.

On 2017-06-16 by Doreen

Two times now I have come hone and the blower is going. It was 86 degrees. I dont have an ad its just a 7600 presidential Coleman mobile home unit. Does it blow when too hot?

On 2017-04-13 by (mod) - is PUSH - ON or PUSH-OFF always true or the manual fan controller ?

Thank you Mark. I agree that there might be some fan limit switches that do not work the way I describe.

The widely-used Honeywell Fan Limit Control used as an example in the above article includes an instruction sheet for the fan limit switch showing the same information as is actually embossed on the switch body: the marking tells us PUSH ON, PULL AUTO.

In some of our photos of this control at FAN LIMIT SWITCH (click to enlarge) you'll see PUSH MAN, PULL AUTO - which have the same functions as "ON" and "AUTO"

You can see this more clearly if you click to enlarge the illustration. https://inspectapedia.com/heat/Fan_Limit_Switch610-DFcs.jpg

This means that if you push that switch IN the fan will run continuously while if you pull the switch OUT the fan will run automatically under control of the room thermostat and furnace. In the AUTO mode the fan will not start, in a response to a call for heat, until the plenum has warmed up and the fan limit switch has rotated to the set ON temperature.

I appreciate your comments and would much appreciate knowing the fan limit control brand and model that works in the opposite direction so that we can add that information for other readers.

I'll edit the material above to add your note.

On 2017-04-13 by Mark

Editor, a question: You write to push the manual switch in for continuous blower use but in the next paragraph you say to pull it out. Huh? See below:

PUSH the white button IN to force the fan to ON or MANUAL or always-running mode.

PULL the white button OUT to put the fan back into AUTO mode - turning on and off under control of the switch and furnace temperature.

On systems where we have installed high quality air filtration to address an indoor air quality issue, and where the fan is rated for continuous duty, we may pull this switch out to keep the fan on continuously.

On 2017-04-05 by (mod) - where to find Rheem furnace manuals and guides

Breano

You're welcome to send photos and a problem description to us using the email found at our page top or bottom CONTACT link, and perhaps we can help research, comment, or suggest actions.

Also see the manuals and guides at

RHEEM Gas Furnaces, boilers, heaters

On 2017-04-05 by (mod) - Rheem gas hot air furnace. Model# RGVC 07EA ER problems

Beano said:
Hi, I have had a reoccurring problem with my Rheem gas hot air furnace. Model# RGVC 07EA ER and

SER #KN5D1041F47889671. This is a 28 year old furnace that is a horizontal attic mounted furnace.

I have had many service calls, with professional techs, costing hundreds of dollars with parts and labor to no solution.

It is rather a long description but I would like to give you all the know symptoms and all the parts that have been replaced thus far so you have as much info as I can give you.

If you need pictures of the control area I can get some and email them to you.

No one and I mean no one has conquered this problem as of yet.

On 2017-03-28 by Vivian Bailey

I have electric heat and air and my heater won't shut off at all it keeps heating until I turn the thermostat to off (it blows hot air constantly) but my air conditioner will shut off like it is suppose to. What could be wrong.

On 2017-03-05 23:34:40.855043 by (mod) -

AV

See BANGING NOISE at AIR HANDLERS or in DUCT SYSTEMS at https://inspectapedia.com/noise_diagnosis/HVAC_Noise_Descriptions.php#Duct_Bang

On 2017-03-04 by Av

Hi the furnace ignights blows hot air and in a minutes or two there is a banging from high up in the furnace and the blower motor cuts off

On 2017-02-18 by (mod): how to set the fan control in an older furnace or air handler

Brandon: Typically

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 board

Thanks 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 Warren

Spell check sorry

The motherboard will turn on and run the heat but no blower. Would like to bypass the blower to run constantly.

On 2016-12-31 by Warren

How 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 Josh

I 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 Josh

My 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 Anonymous

Sure Jerry.

In the fan limit switch article list at the end of this page just click on FAN LIMIT SWITCH INSTALLATION & WIRING to see fan limit switch wiring details.

On 2016-12-14 22:03:55.716788 by Jerry

I 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 Bill

Thanks 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

At least the high limit is working - to avoid destroying the furnace heat exchanger or killing the building occupants.

Take a look at this article https://InspectAPedia.com/heat/Fan_Limit_Switch_Test_Procedure.php FAN LIMIT SWITCH TROUBLESHOOTING to see if that helps.

Unfortunately one can't easily test the switch off of the furnace since you do not want to manually twist its dial - that risks de-calibrating or damaging the switch making it unsafe.

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 Ken

The guy replaced a 8" fan limit switch with a 5" last winter. It works but is that ok?

On 2016-11-20 by Anonymous

danjoefriedman,

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

Brett,

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 Brett

My 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 Anonymous

Some times I have to rotate the control manually to shut the blower off, not always.

...

Continue reading at FAN LIMIT SWITCH - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see FAN LIMIT SWITCH FAQs-5 - more-recent questions & answers on fan limit switch or control operation, troubleshooting, repair

Or see these

Recommended Articles

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• FAN, AIR HANDLER BLOWER UNIT
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• FAN ENERGY INDEX FEI
• FAN LIMIT SWITCH
• FAN LIMIT CONTROL SETTINGS
• FAN LIMIT SWITCH INSTALLATION & WIRING
• FAN LIMIT SWITCH TROUBLESHOOTING
• FAN RUNS ONLY ON FAN-ON / MAN
• FAN WONT STOP - LIMIT SWITCH
• FAN WONT STOP - THERMOSTAT SWITCH
• FURNACE BLOWS COLD AIR
• FURNACE FAN CYCLES AFTER HEAT
• FURNACE FAN CYCLES DURING HEAT
• FURNACE FAN STOPS EARLY
• FURNACE FAN WONT START
• FURNACE FAN WONT STOP
• WOOD FURNACE FAN LIMIT CONTROL
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• FAN NOISES, HVAC
• FAN ON AUTO MAN THERMOSTAT SWITCH
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• FAN WONT STOP - THERMOSTAT SWITCH
• FURNACE OPERATION DETAILS
• NO HEAT - FURNACE
• UNDERSIZED RETURN DUCTS - not enough return air

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