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Fan limit switch troubleshooting diagnosis & repair:
Check this fan limit switch control if your furnace fan won't start (won't turn on) or won't stop (runs continuously). Guide to troubleshooting heating system furnace controls, limit controls, and fan controls.
This article 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.
We also discuss the diagnosis & repair of common furnace operating troubles such as a blower assembly fan that runs continuously - it may be a simple switch setting. The photo at the top of this page shows all of the controls and wiring terminals in a Honeywell combination fan and limit control type L4064B.
Honeywell provides a very simple fan limit switch checkout procedure to be used after the switch is installed:
[Click to enlarge any image]
When installation of a fan-limit switch is complete,
Here, extrapolating from that guidance and adding some field experience, we provide more detailed step by step procedures for troubleshooting and testing the heating furnace fan limit control switch.
This example is based on advice from the Honeywell Tradeline L4064B. Check the test specifications provided by the manufacturer of your particular control.
Honeywell suggests the following steps, which we have supplemented and photo-illustrated for clarity and safety.
The fan limit switch temperature settings (shown on the dial sketch) are discussed separately
Assure that the control has been installed and wired in accordance with the manufacturer's installation instructions using the following procedures.
Turn off electrical power to the heating system
Disconnect the fan at the fan motor or at the FAN LIMIT control terminal
Turn on electrical power to the heating system
Call for heat: Set the building thermostat to call for heat - this means that the "set" temperature on the thermostat must be above the room temperature. For test purposes you can if you like, turn the thermostat all the way "up" to its highest position - temporarily for this test procedure.
Don't forget to set the thermostat temperature back down to the desired level later.
Also if your thermostat has other switches such as "OFF", "HEAT", "COOL" - or "OFF" and "ON" be sure those switches are set to "ON" or "HEAT".
More examples and details of thermostat controls are
Watch the oil or gas burner: The furnace oil or gas burner should turn ON and subsequently the FAN LIMIT control should turn the burner back OFF when the temperature in the supply air plenum reaches the FAN LIMIT set point.
Watch the dial on the fan limit switch moving to confirm its operation.
IF the burner does NOT turn off as expected, QUICKLY turn off power to the heating system and order further diagnosis.
(This particular gas burner shows some rust marks and flame leakage worries).
Watch out: a mistake can ruin the furnace or cause a dangerous even fatal carbon monoxide leak. if the fan limit switch does NOT turn off the burner when temperature meets or exceeds the fan limit setting, IMMEDIATELY turn off the gas burner and power to the unit immediately so that you do not damage the heat exchanger by overheating it.
If the Fan Limit switch did NOT work normally, leave the system turned OFF and call for emergency heat service.
If the Fan Limit switch worked normally continue as follows:
The furnace blower FAN should come on when the plenum temperature has reached the FAN ON setting.
Again watch the dial move on the FAN LIMIT control.
The furnace blower FAN should turn OFF when the plenum has cooled down to the FAN OFF setting.
This may not happen until after the thermostat has been satisfied (or you turn it back down to a low setting) and the oil or gas burner has shut down.
(The particular furnace has a filter that is insecure in this photo and so the heating system is unsafe as the filter could be sucked into the blower assembly causing overheating and a possible motor or electrical fire.)
Details for diagnosing and fixing this problem are
But first just take a look at the fan limit control switch. If the switch includes a control such as the white push-pull switch, depending on the position of this switch (pushed-in or pulled out) the fan may be on manual override - causing the fan motor to run continuously.
Details of how to check or set the manual FAN-ON switch at the fan limit control are found
According to Honeywell, voltage transients or even other conditions around the limit switch that impact the temperatures to which it is exposed can affect the cut-on or cut-off temperature behaviors of the switch, but the company's instructions to not mention apparently excessive on-off cycling of the furnace blower fan traced to the limit switch itself.
Some conditions that cause unexpected furnace fan cycling on and off may be dangerous, risking overheating of the furnace heat exchanger which in turn risks cracks and even carbon monoxide leaks.
So it is worth checking out this problem promptly. We have several suggestions for things to check that can identify and fix problems with the heating system that cause post-heat fan cycling on and off.
Details for diagnosing and fixing a furnace blower fan that cycles on and off repeatedly after the call for heat has stopped, that is after the thermostat is satisfied, are found
If your furnace seems to turn off and then back on repeatedly during the heating cycle the problem may be with the HVAC duct system or the problem could be in the heating burner itself or its controls.
Improper blower fan on and off cycling during a heating cycle may mean higher heating costs or even unsafe equipment.
There may be other explanations for this during-heat cycling on and off of the blower fan, but in most systems that cycling is not normal - you will want to find and fix the cause.
Details for troubleshooting a blower fan that cycles on and off unexpectedly during the heating cycle are given
On a call for heat on most warm air heating systems, the furnace heater turns on, the supply air plenum air heats up, the then the blower assembly runs and will often continue to run until the call for heat is satisfied.
Watch out: overheating the heat exchanger is dangerous. If this is going on your furnace should be checked by a professional.
Details for determining why the blower fan shuts off early and fixing this problem are found
In some forced air heating systems that are not working properly, when heat called for the furnace delivers blows cold air when it should not, or it alternates blowing cold, then warm air on and off.
The root problem may be traced to the ductwork, a dirty air filter, an improperly adjusted control (that may be unsafe) or other sources.
Details of diagnosing a force air heating system blower that blows cold air when warm air should be provided, or a blower that delivers alternating warm then cold air, are found
at FURNACE BLOWS COLD AIR
29 January 2015 Matteo said:
Hi I have a slightly different problem. Furnace is about 30 years all. My blower starts late and then cycles for about 30 second intervals.
Burner turns on normally, begins to heat. Fan limit switch moves past " Fan Off", then past "Fan On", Blower doesn't start until about 10 - 20 degrees above "Fan On"
Then blower runs on for 30 seconds and shuts off for a few minutes, turns back on for 30 seconds, and shuts off for a few minutes.
Meanwhile, Fan limit Switch reaches "Fan Limit Off", burner turns off and blower still runs in short intervals until furnace has cooled. All the while no heat makes it into the house. P.S.
Fan Limit Switch was just replaced. Please help!
Some things to check:
Dirty air filter or blocked, crimped, leaky return air ducting causing inadequate air passage through the heating plenum, dirty blower fan itself not moving enough air
Loose wire connection at the limit switch or at the blower assembly
Improperly-installed fan limit switch, helix in contact with something inside the plenum or wiring error or bent control components while someone messed with the switch adjustments
Defective fan limit switch
A blower that cycles back on after turning off can also mean that air flow is the opposite (not enough air) of what I posed above. In this alternative we are seeing a too-fast, cooling down of the plenum temperature sensor (the fan limit swith helix spring) before the plenum steel itself has cooled.
A result is that the heat exchanger or plenum re-heats the air in the supply plenum thus turning the fan back on.
Use our CONTACT US link to find our email to send me photos of your system and of the control. [Photos at left and above were sent by the reader - Ed.]
I’ve attached a couple photos of the furnace for you. I've taken off the cover panels on the furnace and removed the air filter for the photos. Our conversation that I started on the site is pasted below.
The air filter is new, just changed it. The fan and probably the motor is very dirty. I plan on cleaning it this weekend. Up till about 2 weeks ago the furnace was working fine. Of course, hadn’t been using it very much and then all of a sudden we did have it on everyday.
The wiring at the Limit switch seems fine. Followed the connections and seem alright. The switch was wired like this and worked fine until recently. But I will check to make sure the helix is not in contact with anything.
The Fan limit switch, does seem to be functioning fine. The temperature does not seem to be dropping too fast. Also, when I pull the switch to turn on the fan manually, it turns right on and runs with the same problem, 30 secs or so and off for a few minutes before cycling back on.
One thing I forgot to mention, there was a slight electrical almost burning smell right when this problem first started. Could the problem be with the fan motor? Or maybe the fan motor heats up and loses it connection to the power until it cools back down? Thanks again! Matteo
Take a look at the blower itself when the unit should be starting - I see in your photos signs that there could be a pulley or belt problem: black debris is being deposited where it would spin off of the blower motor pulley.
Watch out: if your system does not have a blower door interlock switch or if you wire the switch off to watch the blower run, you could easily get fingers or a hand cut-off - stay away from an open blower compartment.
Also, when I pull the switch to turn on the fan manually, it turns right on and runs with the same problem, 30 secs or so and off for a few minutes before cycling back on.
in your note is suggestive of a fan circuit or motor problem since pulling the FAN ON switch, IF all other wiring is OK, should run the fan constantly - the limit switch won't consider temperature at all in making the fan run: I suspect there may be a bad fan motor start/run capacitor. See if there is one you can find in the fan compartment (with power off) and if so just replace it.
You might also want to read through the FAQs for this article to see what other readers have encountered when a blower fan is not operating properly.
This diagnosis and repair procedure has moved to FAN WON'T START for blower fans or air handler fans that will not start or won't run in air conditioning or heating systems.
Continue reading at FAN WONT STOP - LIMIT SWITCH or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see FAN LIMIT SWITCH TROUBLESHOOTING FAQs questions & answers posted originally at this article
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For details about the controls, components and switches commonly found on hot hot air heating systems see the articles listed below in which we explain how to identify, set, re-set, repair, replace, or avoid problems with the components of a furnace or warm air heating system.