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.

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

Fan Limit Switch Checkout Procedure

Honeywell provides a very simple fan limit switch checkout procedure to be used after the switch is installed:

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

at FAN LIMIT CONTROL SETTINGS.

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.

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

at THERMOSTATS

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:

• Turn off electrical power to the heating system
• Reconnect the fan motor where you previously disconnected it
• Turn on electrical power to the heating system
• Call for heat: Set the building thermostat to call for heat

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

at FAN WONT STOP - LIMIT SWITCH.

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

at BLOWER FAN CONTINUOUS OPERATION.

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

at FURNACE FAN CYCLES AFTER HEAT.

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

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

at FURNACE FAN CYCLES DURING HEAT

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

at FURNACE FAN STOPS EARLY

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

at FURNACE BLOWS COLD AIR

...

...

Diagnose & Fix a Blower Fan that Will Not Start

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.

Diagnose a Blower Fan that Won't Run on AUTO - but will Run on FAN-ON / MAN

See FAN RUNS ONLY ON FAN-ON / MAN

 

Thank you to our readers for their generous comments

Thanks, Very helpful and informative web site. keep up the great info work! - (Dec 11, 2011) K. Mapes said:

very informative! thank you! (Jan 22, 2012) Anonymous said:

Reply:

Thanks for the nice note K.M. and anon; We welcome reader questions or comments - those help us see where our information may be lacking.

We are dedicated to making our information as accurate, complete, useful, and unbiased as possible: we very much welcome critique, questions, or content suggestions for our web articles. Working together and exchanging information makes us better informed than any individual can be working alone.

...

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

Or see FAN LIMIT SWITCH TROUBLESHOOTING FAQs questions & answers posted originally at this article

Or see these

Recommended Articles

• AIR HANDLER / BLOWER UNITS - home
• FAN, AIR HANDLER BLOWER UNIT
• FAN, COMPRESSOR / CONDENSER UNIT
• FAN LIMIT SWITCH - home
  • FAN LIMIT CONTROL SETTINGS
  • FAN LIMIT SWITCH INSTALLATION & WIRING
  • FAN LIMIT SWITCH TROUBLESHOOTING
  • FAN RUNS ONLY ON FAN-ON / MAN
  • FAN WONT START
  • FAN WONT STOP - LIMIT SWITCH
  • FAN WONT STOP - THERMOSTAT SWITCH
  • FURNACE BLOWS COLD AIR
  • FURNACE FAN CYCLES AFTER HEAT
  • FURNACE FAN CYCLES DURING HEAT
  • FURNACE FAN STOPS EARLY
• FAN ON AUTO MAN THERMOSTAT SWITCH
• FAN ON SWITCH ADDITION
• FAN WONT STOP - THERMOSTAT SWITCH

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

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INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING FURNACES

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