furnace fan limit switch control switch (C) Daniel FriedmanGuide to Inspecting & Testing Combination Fan & Limit Switches on Warm Air Furnace Heating Systems - Honeywell L4064B

  • FAN LIMIT SWITCH TROUBLESHOOTING - CONTENTS: Step by Step Photo Guide to Testing Combination Fan Limit Switches on Warm Air Furnace Heating Systems. 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.
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Fan limit switch troubleshooting diagnosis & repair:

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.

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Step by Step Photo Guide to Testing the Warm Air Furnace Combination Control- Fan Limit Switch

LARGER VIEW of heating furnace fan limit switchHoneywell 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,

  1. Disconnect the fan motor circuit at the L4064.
  2. Turn on power and set thermostat to call for heat. Burner should come on and limit controller should shut burner off when plenum temperature reaches the limit set point.
  3. Turn off power, recconnect the fan switch, turn on power and again set thermostat to call for heat.
  4. Fan should come on when plenum temperature has reached fan-on setting. [1]

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

How to Check the Furnace Fan Limit Control Switch Wiring

Assure that the control has been installed and wired in accordance with the manufacturer's installation instructions using the following procedures.

Heat control switch

Turn off electrical power to the heating system

Furnace blower motor

Disconnect the fan at the fan motor or at the FAN LIMIT control terminal

Honeywell wall thermostat

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

WGas burner on furnaceatch 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 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:

Furnace blower assembly Watch the furnace blower fan:

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.)

Blower Fan Won't Stop: What to Check if your Furnace Fan Runs Continuously - (the fan won't shut off on its own)

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

How to Diagnose & Fix a Blower Fan That Keeps Cycling On/Off After Call for Heat Has Stopped

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

How to Diagnose & Fix a Blower Fan that Cycles On/Off During the Heating Cycle - furnace short cycling

Unsafe return air input at furnace (C) D FriedmanIf 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

How to Diagnose & Fix a Blower Fan that Stops Before Thermostat is Satisfied

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

How to Diagnose & Fix a Blower Fan that Blows Alternating Hot then Cold Air

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

Reader Question: Fraser Johnson Furnace blower starts late then cycles at 30 second intervals; Blower doesn't start until about 10 - 20 degrees above "Fan On"

Cyling blower fan diagnosis (C) InspectAPedia Cyling blower fan diagnosis (C) InspectAPedia

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!


Cyling blower fan diagnosis (C) InspectAPedia

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.]

Reader follow-up: 30 Jan 2015 Matteo wrote:

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

Reply: look for a burned start/run capacitor at the blower fan assembly

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.

How to Diagnose & Fix a Blower Fan that Will Not Start

Why won't my furnace blower fan start on its own? It will run if I give it a "spin"

My gas heater isn't working correctly. My thermostat signals the heater correctly and the burner ignites, but the fan doesn't always turn on. If I open the panel and spin the squirrel cage a little, it immediately begins to blow air and runs the heating cycle. What could this be? - Mike.

Reply: Check or replace the blower fan motor start/run capacitor


Often when an HVAC fan won't start on its own but will start and keep spinning when you give it a push that means that the start-capacitor for the unit has failed. It's an inexpensive part that can be installed or replaced by your HVAC service tech or by a homeowner who has expertise in electrical wiring, using a VOM, and following simple wiring instructions.

See details about this component

Other reasons that an air handler blower fan assembly does not run include

This article series answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

Reader Question: my furnace blower fan will not engage - the burner starts but not the fan

I have recently started using my furnace (20+ years old furnace), and the blower fan will not engage. The pilot and gas burners start, but the fan does not. I have ensured that the fan is set to "auto", on the thermostat. Is there any procedure I can do to troubleshoot the issue? I have seen several topics, however none seem to be helpful. - J.B. 12/31/2013


A competent onsite inspection by an expert usually finds additional clues that would permit a more accurate, complete, and authoritative answer than we can give by email alone. But here are some suggestions to start troubleshooting the problem of a furnace blower fan that does not start:

Step 1: I would start by confirming that the blower fan will operate. Your system probably has a manual FAN ON switch on the thermostat, or if not, there may be a FAN ON push pull switch right on the fan limit switch in the air handler. If you can make the blower run then we know the problem is not with the blower motor or drive belt or assembly and we'd go to step two.

See  FAN AUTO ON THERMOSTAT SWITCH for the switches on the thermostat; for a manual FAN ON switch found on some limit switches in the blower or air handler assembly itself

If the fan won't run at all,

see FURNACE FAN WONT START - the text section just above this Q&A.

Step 2: I would continue diagnosis by watching the behavior of the fan limit switch as the furnace heats up; presuming nobody has messed with the switch to try to change its settings, it should act as described in the article above beginning

HVAC Blower Fan Articles


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.



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