Furnace Fan & Limit Switch Q&A

Fan limit switch damage FAQs

Fan Limit Control Switch Damage or Failure FAQs

Watch out: any damage to the fan limit control switch raises critical life safety hazard concerns because the switch could fail to prevent overheating, a cracked heat exchanger, and fatal carbon monoxide poisoning, especially with gas fueled furnaces.

Can turning the fan limit control dial cause damage?

I have a Armstrong Ultra 80 oil furnace with a Honeywell Fan and Limit Control 11inch L4064B2210

It says "DO NO TURN WHILE SETTING TEMPERATURES" on the temperature set dial. Is it possible to damage the unit by accidentally turning the dial while setting temperatures? On 2017-10-22 by Dave Waddle

by (mod) - yes - the manufacturer is very clear: DO NO TURN WHILE SETTING TEMPERATURES

Dave

Thanks for a very important question/ comment.

By "turning" what the manufacturer means is that if you physically rotated the actual moving dial on the control you may damage it, making it and the heating system it controls unsafe. That's because behind that dial is a temperature-sensitive bimetallic spring that responds to temperature changes in the heating plenum.

Watch out: If you rotate the dial itself you're bending that spring, changing its response to temperature.

That means that the control might let the furnace overheat, risking a damaged heat exchanger, flue gases, even fatal carbon monoxide poisoning to be produced or leaked into the building, or there could be risk of a fire.

That's why when the service tech is actually making adjustments to the fan limit control settings she takes care to move just the adjusters and to keep the rotating dial itself from being moved.

I wouldn't worry if you accidentally touched the dial or moved it 1/8" or so, but more rotation has the risks I outlined.

Certainly if you rotated the dial 45 degrees or more I would be concerned enough that at the very least I would put the control through the full verification test that is provided both by the manufacturer and in this article series, and I'd want independent temperature tests to be sure that the control is still properly calibrated and that it is responding properly.

Or I'd replace it.

by Dave Waddle

Thanks for the advice, I turned it more than half a rotation and now it's kicking off on high limit too soon and the fan is running too long. I'm replacing it and will be more careful when setting the new one.

Fan limit control damaged by furnace overheating?

IS it possible for a fan limit control unit on an oil fired forced air furnace to malfunction if furnace got overheated? On 2019-03-20 by M. Bustin

by (mod) - will an overheating furnace damage the fan limit control

MB

Yes but ...

That sounds backwards to me in that it would more-likely be a defective or improperly-adjusted fan limit control that would lead a furnace to overheat

Watch out: overheating a furnace can crack the heat exchanger and thus risk leaking dangerous or even fatal flue gases into building air.

If you think that happened to your furnace have it inspected / tested for safety - by an expert technician.

See FAN LIMIT SWITCH TROUBLESHOOTING 

Bryant gas furnace fan shuts off then comes back on - on and off over and over -

I have an old (30ish years) Bryant gas furnace. All works fine until the fan shuts off.

It then spins back up for a few seconds, shuts off, spins back up for a shorter period of time (~1s) and then back off.

I suspect the fan control switch is dirty and taking a long time to unwind as it cools, but I'm not sure where it is to confirm and identify the model in case I need to replace it instead of cleaning it. On 2021-11-03 by Andy Haninger

by Inspectapedia Com Moderator

@Andy Haninger,

Our list of Recommended Articles found above includes an article you will want to review:

FURNACE FAN CYCLES AFTER HEAT

as that includes most causes (and diagnostics) for the case you describe. You'll see that the problem could be a dirty heat exchanger, wrong fan speed, or something else;

Watch out: we do NOT recommend trying to clean or fool with the fan limit sensor itself as doing so can bend it out of calibration and make it unsafe.

by Andy Haninger

@Inspectapedia Com Moderator,

Thank you. I've read through this page and tried what I could. I have still not been able to locate my fan control switch and haven't felt comfortable enough to take out and clean the heat exchanger or blower fan.

I did adjust my pilot light slightly lower as someone else suggested that too large of a pilot could cause enough heat to start the fan. I didn't expect this to work, though, and didn't see any change.

I did recently come home to find the blower running, but no flame in the furnace. The pilot had blown out. The thermostat is a basic Honeywell and said Heat On, so it had apparently called for heat. But why was the fan on if the pilot wasn't there to ignite the furnace? There is no way the furnace got up to the temp needed to start the fan.

I set the thermostat from Heat to Off and waited for the fan to turn off. After a minute or two, it did it's usual Off, On two secs, Off, On one sec, Off. But why did it wait to turn off? No heat, no fan, right?

I've noticed that when I set the thermostat's fan control from Auto to On, I do not hear a relay in the thermostat clocking like I do when it calls for heat, but the fan does turn on and off correctly.

I'm wondering if the thermostat is failing in some way. It would at least be a fairly simple and cheap thing to try.

As another test, I've tried pulling the thermostat from the wall after a call for heat, but before the fan has turned off.

The fan kept running for a minute or two and then did the off-on-off-on-off thing.

Again, setting the fan from Auto to On and back to Auto causes the fan to turn on and off as it should.

by Inspectapedia Com Moderator

@Andy Haninger,

If you disconnect the thermostat but the heat exchanger is still hot, it's proper and normal for the fan controller to run the fan a bit longer -

The explanation is at FURNACE FAN CYCLES AFTER HEAT (link in earlier response)

When that cool-down cycle is "on-off" as you describe it may be indeed a dirty heat exchanger or air velocity issue or one of the problems and diagnoses suggested on that page, so it's worth a look.

When you can't find the fan limit control it might be because you're looking for a device that looks like the one at the top of this page while your furnace uses a smaller, less-obvious switch - a snap-disc type controller.

The home page for this topic

FAN LIMIT SWITCH can help you identify the type of fan control you have and to find it.

by Andy Haninger - Stemco 430-1448 HH12ZA252C limit switch

Thank you again. Here's a picture of what I believe to be the limit control switch I've been overlooking.

The model number is Stemco 430-1448 HH12ZA252C (the shadows blocked part of it, so I'm at least partly going by an Amazon listing for the part).

I'm not terribly enthused about getting a part that was "removed from a working unit and guaranteed to work" so hopefully if this is my culprit, a new part is available.

by Inspectapedia Com Moderator

@Andy Haninger,

That's right-on. That type of switch is still sold, including in generic form by other manufacturers; you'd take the old part to your local heating supplier.

In our home page for this topic

FAN LIMIT SWITCH

see the discussion titled:

Types of Fan / Limit Controls: adjustable vs. snap-disc

where we discuss CamStat and other universal replacement controls.

A similar example is shown below, the Carrier & Bryant HH12ZB250 

This particular model Opens at 250°F (121°C), and Closes at 210°F (99°C).

by Andy Haninger

Thank you for all of your help with this project. So far I've had no luck getting this limit switch out. The issue (I think) is that the hex head screws holding it in place are rounded off at the top or are really secured in place.

I don't really want to try to fix what ain't (really) broke and I don't really want to put any repair money into it. I'm tired of lighting the pilot The system is 33+ years old, which is a good run. I know it's had trouble before. The blower motor was replaced 7 or so years ago

I'm going to start getting estimates on a new system that will allow me to move on to worrying about something else.

To make things more frustrating, after I poked around inside the furnace this past weekend, it's working perfectly fine. It's still wasting gas like it's 1988, but it's turning on and off as expected.

I did cut the breaker to it before poking around. Maybe it just needed a reboot?

I don't like to give up like this, so will try to follow up if I have any more info or regrets after replacement.

Take care.

by Inspectapedia Com Moderator - bad electrical connection or bad temperature sensor

@Andy Haninger,

That suggests there could be a loose or corroded electrical connection or, though less likely, a bit of debris that fell off of a temperature sensor.

Thanks for the follow-up. I understand what you're saying.

Tapping the limit control to see if it runs: loose or corroded electrical wire connection? Bad limit control?

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 2016-07-24 by Rick

Reply by (mod) -

Tapping the switch and seeing it run suggests that you've got a bad or shorted thermostat wire or that the control itself needs replacement.

When to replace a fan limit switch that may be defective

Around the first of January the oil furnace overheated because the burner was running but the blower didn't come on.

I replaced the limit switch with the same Honeywell L4064 and it worked fine for two months.

Today it overheated again. I shut off the furnace at the thermostat and turned down the setting below room temperature and turned the furnace switch back on and

after the furnace had cooled down a bit the blower came on and short cycled a few times and shut off when it was cool. I turned the thermostat back up and the burner lit right up and a few minutes later the blower did just as it should.

I did not touch the limit switch at all. Should I replace the limit switch again or is there possibly some other problem. thanks On 2019-03-04 by Wayne

by (mod) - furnace is overheating and shutting off

It sounds to me as if there is either been wiring damage or damage to the switch itself. The fan limit should prevent overheating and overheat damage to the furnace by shutting the system down if the plenum chamber is good too high.

Assuming that you have the correct temperature settings on the fan limit switch that should work.

There are of course other reasons that the system would overheat such as a dirty air filter or blower or reduced air flow through the heat exchanger for any reason. More examples are found at

FURNACE FAN STOPS EARLY