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furnace fan limit switch control switch (C) Daniel FriedmanFan & Limit Switch Test Procedure Q&A#2

FAQs on Furnace Fan Limit Switch Testing

Fan limit switch troubleshooting test Q&A set-2.

This article series 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.

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Q&A on How to Test a Furnace Fan Limit Switch

heating furnace fan limit switchThese questions & answers about testing the fan limit switch were posted originally

at FAN LIMIT SWITCH TROUBLESHOOTING - please be sure to see the fan limit control installation, diagnosis, and repair procedures given there.

[Click to enlarge any image]

On 2018-01-15 by (mod) - pilot is lit, but burner won't ignite

I would check the fuel supply, a thermocouple, and then the gas valve

This isn't a problem with the fan limit controller.

On 2018-01-15 by John

Just replaced the fan limit switch in my furnace, pilot is lit, but burner won't ignite, fan won't turn on unless done manually, but burner still won't ignite.
replaced thermostat and voltage regulator also. Any suggestions?

On 2018-01-07 4 by (mod) - wall furnace blower fan comes on when it should not

Peter

Take a look at

BLOWER FAN WONT STOP

On 2018-01-07 by peter osborne

I have a gas wall furnace which sends heat to all rooms via ducting. However, in summer here, with yesterday temperature very high (41 degrees C or about 107 F) , the fan (not heating, just fan) comes on and blows air.

Gas pilot light was on but no heat - just cool air blows . I had to switch off at power point to stop . Please advise cause and happy to get expert to fix if necessary.

On 2017-12-31 by (mod) - unit does not turn on if temperature falls below the setting in my thermostat

Marie,

I suspect that the system is shutting down because of a temperature or burner operating defect. When you turn the system off and back on you are in effect re-setting those safety controls.

I am nervous about forcing the heating system to run when we don't know why it's turning off. If the system is being shut down by a safety control you may be causing an unsafe condition. I would ask for repair from a trained heating service technician.

On 2017-12-31 9 by Marie

My unit does not turn on if temperature falls below the setting in my thermostat but it goes on if I turn the switch on/off and keep going until it reached the desired TT setting.

On 2017-12-16 by (mod) - I am thinking the fan limit switch is shutting off burner due to high fire box temperature?

Randy

Good going. Your guess is the same as mine. In most residential forced warm air heating furnace installations the furnace burner and blower run continually until the thermostat is satisfied.

When a burner shuts off and the blower continues to run before the TT is satisfied my first guess is overheating.

That overheating could indeed be due to having adjusted the burner to run too hot - either by the wrong nozzle size or spray pattern or by the wrong combustion air setting (or less common, having pushed the oil pressure up, thus increasing the burn rate and BTUh of the burner).

Other causes of the same symptom include:

An obstruction in the air flow through the heat exchanger - a dirty air filter or blocked air duct

Or something else I've forgotten.

You can easily confirm our hypothesis that the heat exchanger is overheating.

Just turn the thermostat way up so the furnace should keep running.

Then stand there (maybe with a cup of coffee) and watch the fan limit control switch dial. The dial will begin to rotate as the furnace heats up.

First it rotates past the FAN ON setting - and the blower turns on.

Then if temperatures continue to rise in the heating supply air plenum (where the fan limit switch sensor is monitoring temperature) and as the dial continues to rotate up to the FAN LIMIT OFF position it will turn OFF the blower fan.

We'll thus know that the system is overheating.

Let us know what the next tech finds when you call the heating company service manager and poilitely ask for a repair call. Overheating the furnace heat exchanger (if that is indeed happening) can cause it to crack, making it unsafe and requiring a costly repair.

On 2017-12-16 by Randy

I purchased a home with a 10 year old oil fried forced air furnace. I just had the unit serviced by a reputable company that I have dealt with in the past. Prior to the service, the unit would start and run until the t stat was satisfied and shut off (as normal).

After service, the burner shuts off in mid cycle prior to reaching tstat set point and then restarts one or more times until set point is reached. The fan continues to run. During the service some mild accumulation was removed from the area around the heat exchanger but nothing too drastic.

The tech noted the exhaust stack (6 inch metal pipe) was inserted too far into masonry chimney and shortened same by 1-2 inches.

He also adjusted the draft damper (which now hangs open all of the time.)

I am thinking the fan limit switch is shutting off burner due to high fire box temp. Could there be another reason or was there something done by tech that may have caused this?

On 2017-12-08 08:35:56.453182 by David

Sorry, I didn't put my name on last post...I know I must sound like an idiot, but the furnace is in a tight crawlspace and the furnace is kinda sideways...instead of the "thumbnut" that holds in the "gun" being on the left side (as in all the videos I've seen), it's on the bottom...and it seems I saw one picture where the limit switch was in the AC unit...Am I crazy ?

On 2017-12-08 by Anonymous

, Thank you. So far that's been the problem. When there's a call for heat, burner turns on-fire looks good, but no fan-even when I make it come on, burner soon shuts itself down along with the fan and furnace will not start again for several minutes...

then it must reset itself because I can repeat this again. So far, I have not even located this "limit switch"-at least nothing like in the pictures or vides I've seen. Is it possible it could be somewhere else ? In the AC ?

On 2017-11-30 by (mod) - watch fan limit control dial as the furnace heats up

David,

In normal operation when the fan switch is set to Auto at the thermostat and the manual fan on switch is off at the fan limit switch, then the fan will turn on once the temperature in the Supply Air plenum above the furnace reaches the cut in temperature set on the limit switch.

You can tell what's happening by watching the burner and the limit switch when the thermostat is calling for heat.

The thermostat should cause the burner to turn on and you should hear the burner operating normally.

Then you should see the fan limit switch dial begin to rotate as the temperature in the plenum warms up.

When the pointer on the limit switch is pointing to the FAN ON setting on the dial you should see and hear the fan turn on.

If the problem is with the fan control itself the plenum will continue to heat up, the dial will continue to rotate and will reach the maximum LIMIT temperature at which it will turn the fan off.

So by watching you should be able to tell whether the problem is with the switch or with the blower or blower wiring or wiring connections.

On 2017-11-30 by David - furnace starts but fan won't run

Have 24 year old "inter-city"(now "international comfort") oil furnace. Fan will turn on w/switch at fan, but not on auto or on at thermostat.

Furnace will start, but (again) fan doesn't- then in 40-45 sec furnace stops ALONG with fan (if I have started the fan with switch near fan motor-as that's the only way fan will start). Fan limit switch ?

I wondered if the furnace is keeping the fan from starting or the furnace stopping is an effect of some fan trouble...other than an occasional nozzle replacement & one time transformer replacement-has given no trouble...ANY information would be greatly appreciated.

On 2017-11-10 by Tim

the fan is constantly running, even though the furnace side may be on cycle or off. I've recently replaced my fan motor and the limit switch. Is there something wrong with the wiring?

On 2017-10-17 by (mod) -

Marie,

That's not an issue that I've seen discussed. But you might ask your service technician to check the system start-up - something else might be making an abnormal noise, such as a spark or relay.

On 2017-10-17 by Marie

I just started my furnace for the year and I feel that the electrical « click » sound that the fan limit switch(Honeywell) makes when it tells the blower to go off/ on is much louder than it used to... anyone else?

On 2017-02-01 by Leon Rickards

This past summer I replaced my blower fan on my 1979 York/Borg Warner HVAC system... This winter, when the furnace was started, the furnace only worked in continuous mode and not the auto mode.

I replaced the Honeywell Fan Limit switch and observerd a huge spark at the relay and its failure I replaced the relay and oberved a circuit breaker trip... reset the circuit breaker and observed a spark at then new fan limit switch and fan shut down.... I switched the thermostat to manual and am allowing a continuous fan to blow.

Though the auto setting worked for the air conditioning last summer, could the blower be improperly wired for the furnace but properly wired for the A/C? And could this trip the circuit breaker and blow the limit switch and or the relay?



...

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

Or see FAN WONT STOP - LIMIT SWITCH

BLOWER FAN OPERATION & TESTING

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