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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

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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]

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Forced Hot Air Furnace Fan Limit Switch FAQs

combination furnace control Honeywell L4064B with cover onThese 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: 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: http://www.wesco.com

The company does supply heating units for electric furnaces through this website: www.wescodirect.com

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?

Reply:

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

Reply:

Anon

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?

Reply:

Anon

In the ARTICLE INDEX found among the Continue Reading links at the end of this article 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!
Cheers,
Jon

Reply:

Jon

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

Reply:

Dan

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,

Reply:

Anon:

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

Reply:

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

inspectapedia.com/oiltanks/Oil_Tank_Water_Contamination.php

Then see NO HEAT - FURNACE

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.

Reply:

Scott

In the More Reading ARTICLE INDEX links above see the diagnostics at

FAN WONT STOP - LIMIT SWITCH

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.

Reply:

Jerry

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.

Reply:

Digit

In the More Reading ARTICLE INDEX list above please check out

FAN WONT STOP - THERMOSTAT SWITCH

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?

Reply:

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 wachobank7@gmail.com. Thank you very much for your time.

Reply:

Anon

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

Reply:

Terry,

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?

Reply:

Anon

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 seconds...off 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

Reply:

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.

Reply:

Charles,
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.

Reply:

Jane

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)

See FURNACE FAN WONT START

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

Reply:

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?

Reply:

John

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.

See FURNACE FAN WONT START

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?

Reply:

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 lesterstocker@yahoo.com

Reply:

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?

Reply:

Nick

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
at FAN LIMIT CONTROL SETTINGS.

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

Reply:

Donnell,

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.

Reply:

Paul

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.

Reply:

Cory,

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

Reply:

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 recent questions & answers about fan limit switch troubleshooting

Or see FAN LIMIT CONTROL SETTINGS

Or see FAN LIMIT SWITCH TROUBLESHOOTING

Or see NO HEAT - FURNACE

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

Or see these

HVAC Blower Fan Articles

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

INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING FURNACES

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