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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.

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.

This website discusses these systems and heating components in detail in articles listed at the left of these pages. If your heating system is not working properly, see NO HEAT - BOILER / FURNACE DIAGNOSIS. This website answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect residential heating 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.

© Copyright 2012 InspectAPedia.com, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use page top links to major topics or use links at the left of each page to navigate within topics and documents at this website. Green links show where you are in a document series or at this website.

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

If your heating or cooling system blower fan itself appears not to be working, see BLOWER FAN OPERATION & TESTING. Readers should also see How to Install the Fan & Limit Control, or begin this topic at FAN LIMIT SWITCH. Also see FURNACE OPERATING TEMPERATURES.

The basics of how furnaces work can be read at FURNACE OPERATION DETAILS and the key heating furnace components are introduced at FURNACES, HEATING

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

When installation is complete, disconnect the fan motor circuit at the L4064. 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. Turn off power, recconnect the fan switch, turn on power and again set thermostat to call for heat. 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 How to Set the Fan & Limit Control. 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 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.)

If your Furnace Fan Runs Continuously - (the fan won't shut off on its own)

Take a look at the fan limit control switch. Before pulling the cover off of the switch, just see if the switch includes a control such as the white push-pull switch like the white device shown at the lower left in our photo. 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. Check the printing on the switch face to see if you should push the switch in or pull it out to leave the blower on "Auto". ("Man" or "Manual" would be forcing the fan to run.) Which way to set the furnace or air conditioner blower fan switch: Usually: "Push-in" position on this (Honeywell) fan limit switch is for continuous fan operation and "Pull out" position is for automatic fan operation. There are some reasons to leave the blower fan on continuously for long periods, such as to use an air filtering system to improve indoor air quality. And in some buildings we may run the blower continuously for more even or better conditioned (heated or cooled) air distribution. Details are at BLOWER FAN CONTINUOUS OPERATION.

Frequently Asked Questions (FAQs) about warm air heating furnace fan limit switch controls

Question: 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

Mike:

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 at CAPACITORS for HARD STARTING MOTORS.

Question: Why does the blower fan keep turning on and off repeatedly after the thermostat has stopped calling for heat?

What do you do if the blower fan goes on for its cycle then shuts off only to go on again, going on and off too many times. What is wrong? - Trevor

How do you know if the [fan limit] switch is going bad because my still works but the fan seems to turn on and off too frequently? - Anonymous

Reply: Why the blower fan may turn on and off after the end of a heating cycle

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.

First let's review the normal warm air furnace operating cycle and fan limit switch settings

When the HVAC system is in HEATING mode and the thermostat calls for heat, the oil or gas burner turns on and begins to warm the heat exchanger. When the air in the supply plenum served by the heat exchanger becomes warm enough the blower fan turns on to begin circulating building air through the occupied space.

On most warm air heating systems both the burner and the blower fan unit will run continuously until the temperature at the wall thermostat rises to the thermostat set point. Then the thermostat stops calling for heat and the burner turns off.

The blower fan, however, will continue to run until the heat exchanger and the supply air plenum have cooled down. That "run-on" period scavenges otherwise wasted heat and sends it to the occupied space, and it prevents warping and possible cracking damage to the heat exchanger by cooling it down.

Finally, when the heat exchanger and supply plenum have cooled, the fan limit switch will shut off the blower fan. The system stops.

Note the three temperature settings on the fan limit switch dial in our photo just above, and in the sketch higher on this page.

1. The lowest setting (blue arrow, the left end of the dial) is the "FAN OFF" temperature. The FAN OFF setting lets the furnace blower continue to run for an interval after the furnace burner has turned off, but will shut the blower off after the heat exchanger has been cooled down to this temperature (usually 90 degF).
2. The middle setting on the fan limit switch dial (green arrow) is the "FAN ON" temperature. Ths setting turns the blower fan ON when supply temperature is rising, and is usually set around 130 degF.
3. The rightmost setting on the fan limit switch (red arrow) is the "FAN LIMIT OFF" temperature, a safety setting that turns off the burner if the system temperature becomes too high. This switch is usually set at 100 degF or higher.

The fan limit switch temperature settings and adjustment procedures are explained in detail at How to Set the Fan & Limit Control.

Common, acceptable extra furnace blower on cycles?

But because on some furnaces the heat exchanger itself and the combustion chamber below it are still hot, that residual heat continues to heat air in the supply plenum (where the fan limit switch is located). If the temperature rises enough in the supply plenum, that will activate the fan switch once again, causing the fan to turn back on again.

This happens because the fan limit switch turns the blower on when the air temperature in the supply plenum rises above an "on" set point, both to deliver warm air to the building interior AND to prevent the heat exchanger from overheating. That is, the fan is blowing to cool down the supply plenum - a step that helps prevent heat cracking or damage to the heat exchanger.

So at some warm air furnace systems the blower might turn on once, even twice after the call for heat has been satisfied. Typically, because the oil or gas burner is NOT running (because the thermostat is not calling for heat), the blower fan should not run more than a minute or two before stopping.

Signs of trouble with the furnace controls or air flow that cause the blower fan to cycle repeatedly - things to check:

If however when the thermostat is not calling for heat the blower fan comes on and runs repeatedly and for longer intervals then the fan limit switch may be defective, or there may be a problem with the airflow rate or temperature through the system and you need a service call.

Check for an improperly installed fan limit switch

If the fan limit switch was installed askew, or if the sensor element (a long bimetallic spring in a metal protective enclosure) is too long, such that either defect causes the sensor assembly to actually touch an internal steel part of the heating furnace, then the switch will not perform properly.

A "too long" fan limit switch sensor problem may occur if a previous replacement of the fan limit switch installed the wrong model - a unit that did not match the original.

Watch out: Some fan limit switches are mounted using a rigid bracket that requires tightening a set-screw (red arrow in the edited Honeywell sketch at left] to hold the limit switch in the bracket.

But a simple error of mis-locating the bracket set screw can cause the screw to contact the bimetallic spring inside the switch - a dangerous condition that can cause the switch to fail to shut off properly in response to high temperature. [1]

Also the fan limit switch should be installed in the same location on the furnace as the original switch.

If someone relocated the switch it may be in a too cool or too warm location, or in a location that does not allow the switch to reliably sense supply plenum air temperatures.

The Honeywell L4064B fan limit switch is designed for use in both line voltage and low voltage installations. But for low-voltage installation a brass jumper must be removed (red arrow at left). Honeywell warns:

Failure to remove brass jumper, if limit switch is in low voltage circuit, can cause electrical shock hazard or damage low voltage controls. [1]

Check for a missing heat insulating gasket at the fan limit switch mount

At furnaces set to higher operating temperatures, a heat-insulating gasket is required between the switch mounting contact body and the surface of the furnace.

If plenum surface temperature exceeds 190OF (88OC), heat insulating material or a mounting bracket must be used. [1]

Check out the wall thermostat for heat call cycling

Before replacing the fan limit switch, try checking the wall thermostat too. Some thermostats can develop a contact bounce, or may have a failed heat anticipator circuit, resulting in some cycling of actually calling for heat. To debug this condition try calling for heat up to a fairly high thermostat setting, say 74 degF. Then when the room temperature has reached 72 degF., disconnect the thermostat wires at either the thermostat or the furnace control. That's essentially the same as the thermostat reaching its set point and ceasing to call for heat.

Now watch the performance of the furnace blower assembly. The furnace burner should stop, but the blower fan should continue to run after the call for heat stops until heat has been extracted from the heat exchanger and supply plenum. If the blower fan cycles on and off as before, then the problem is not the wall thermostat.

Typically the HVAC tech will replace the fan limit switch in these conditions. The switch itself retails for around $100.

Reader Comment: reader suggests cleaning the fan limit switch

The question above is: Why does my blower motor keep coming back on for short/intermediate/momentary bursts/pulses immediately after the furnace cycles normally? I assume we are talking about L4064's and there equivalents/competitors
The answer was not helpful. The fan/blower limit switch may be "sticking" at the "ON" or "OFF" setting or intermediately one or the other. It is, after all, an electro-mechanical device, relying on a spiral shaped bimetal coil to turn a disc that has mechanical contact points for the on and off settings for the blower. That heat sensitive bi-metal spiral attached to a shaft passes through a bushing that simply dries out after hundreds if not thousands of cycles...then starts sticking and jumping between the "ON" and "OFF" settings instead of smoothly transitioning as it did when it was new. Do you need a new $100 fan limit switch? Maybe! But first try removing, inspecting, cleaning and applying an appropriate hign temperature lubricant. Unless the spiral bi-metal coil is broken or the on/off/limit contacts are completely worn down...you may be surprised at how much life is still left in that old limit control switch. - Kevin Sharpe

Reply: we do not recommend disassembly nor modification of heating system safety limit switches or similar controls

Kevin, you are correct that a typical Honeywell fan limit switch retails for around $100. But your advice that people try disassembling, cleaning, lubricating the switch makes me nervous.

Honeywell's own installation instructions (see our references below) mention nothing whatsoever about such steps, but the company does warn more than once that care must be taken to avoid bending internal springs and parts of the switch. If these sensitive parts are modified in any way, say by bending or even by a blob of grease left by someone who may be less meticulous, the switch loses calibration and could fail to shut off the system at high temperatures - an unsafe condition that could also damage the heat exchanger.

In our FAQs above, please take another look at "Why does the blower fan keep turning on and off repeatedly after the thermostat has stopped calling for heat?" for some discussion of why the blower might cycle on and off once or even twice after the heating furnace has stopped its usual operating cycle.

And if you discover other reasons for unexpected heater blower assembly cycling on and off after the end of a heat-on cycle, do add those comments below. Thanks - Ed.

Question: Why does the blower fan keep turning on and off repeatedly during the heating cycle (the thermostat is calling for heat)?

Our furnace seems to turn off and then back on repeatedly during the heating cycle. I've read that normally on a call for heat the burner should come on, then the blower, and both should keep running until the call for heat is satisfied. What's going on? - Anonymous

Reply: Abnormally low air temperature at the return plenum can cause blower fan short cycling

Abnormally low incoming air temperature entering the heat exchanger after the oil or gas burner has shut off may cool down the supply plenum so rapidly that the fan limit switch, working properly, turns off the blower before enough heat has been extracted from the heat exchanger itself.

If this is happening, rising heat and air (by convection) from the still-hot heat exchanger into the supply plenum may warm the plenum again and turn the blower fan back on.

Blower fan short cycling on and off too frequently during the heating cycle is often traced to this problem.

Why would we have "abnormally low incoming air temperature" at a furnace?

We've seen this problem at buildings with improper HVAC duct design, in particular where some or all of the return air to the furnace is being taken from an opening right at the furnace such as a unit located in a cold basement or crawl space(see our photo, above left).

Sometimes in an effort to improve the total airflow delivered by a furnace a technician will cut an opening in the return ducts in an unheated basement or crawl area.

Watch out: This return air opening boosts the total air output of the system but it is an improper and unsafe design. See INCREASING RETURN AIR for details.

Abnormally high air temperatures at the supply plenum can also cause blower fan to turn on and off unexpectedly

While this defect is less common than the cold return air problem discussed above, a dirty air filter or any other defect that restricts air flow through the heating system supply or return ductwork can also cause unexpected blower fan on and off cycling.

If the airflow through the heat exchanger and supply plenum is too weak the furnace oil or gas burner may be able to heat the system to the point that the fan limit switch, to protect the unit from overheating damage (warping, cracking) will turn off the burner, even though the thermostat is still calling for heat. In this condition the blower fan may continue to operate (to cool down the "too hot" heat exchanger and supply plenum).

The blower fan will continue to run until the supply plenum temperature drops to the low limit, then the fan limit switch will turn off the blower. As the thermostat is continuing to call for heat, when the furnace temperature drops, the fan limit switch will permit the burner to turn back on.

At AIR FILTERS for HVAC SYSTEMS we discuss various problem experienced by heating systems due to a dirty or blocked air filter.

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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

  FAN LIMIT SWITCH
    What is a Fan & Limit Switch?
    Sensing Furnace Temperatures
    How to Set the Fan & Limit Control
    How to Test the Fan & Limit Control
    How to Install the Fan & Limit Control
    How to Wire the Fan & Limit Control
  FAN LIMIT SWITCH TROUBLESHOOTING

• [1] Honeywell Corporation, Minneapolis, MN 55408. Honeywell has sales offices in all principal cities in the world and has manufacturing facilities in Australia, Canada, Finland, France, Germany, Japan, Mexico, Netherlands, Spain, Taiwan, United Kingdom, U.S.A. Honeywell Form Number 60-0450 7-75, residential division. Honeywell's latest product data for this type of control can be found in English at http://customer.honeywell.com/Techlit/Pdf/69-0000s/69-0117.pdf or contact Honeywell at Automation and Control Solutions Honeywell International Inc. 1985 Douglas Drive North Golden Valley, MN, 55422 - honeywell.com/building/components 07/06 RB © Honeywell 2006 Printed in Canada 69-0117-3

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.

• Hot Air Heating Furnace Basic Operating Steps
• BLOWER FAN OPERATION & TESTING how to test a heating or cooling fan that is not working
• CAD CELL RELAY SWITCH on oil fired furnaces or boilers as flame sensors & safety devices
• Draft Hoods on gas fired heating equipment, function and safety
• Draft Regulators & barometric dampers on oil fired heating equipment
• Electrical Power Switches: Where to Turn On or Off Heaters for Furnaces & Boilers, Heat Pumps or Electric Furnaces or Air Conditioners
• Fan Limit Switch on hot air furnace heating systems gas or oil fired
• Flue Gas Spill Switch on gas fired heating equipment sense combustion and protect from blocked flues
• Gas Piping, Valves, Controls: Guide to LP and natural gas meters, valves, tanks, piping, gas leaks, gas regulator assembly & other gas controls
• Oil Burners: Guide to Oil Burners for heating systems, boilers & Furnaces: basic parts, operation, maintenance, performance & money-saving tips
• Oil Tanks: Guide to Heating Oil Underground & Above ground Oil Storage Tank Leaks, Testing, Problems & Solutions, Home Buyer's / Home Owner's Guide
• Oil Piping: A guide to heating oil piping, valves, controls, leaks, repairs for heating systems
• Power Switches to turn on or off heaters for furnaces & boilers, oil & gas fired, heat pumps or electric furnaces or boilers
• Stack Relay Switches: Guide to finding, resetting, maintaining stack relays on oil fired furnaces or boilers as flame sensors & safety devices
• Thermostats & Heat Controls for furnaces & boilers, oil & gas fired, heat pumps or electric furnaces or boilers
• Zone Dampers: A guide to zone dampers for heating zone control on hot air heating system ducts

• Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

• Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
• DUST CONTAMINATION FROM HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
• Fuel Oil & Oil Heating Magazine, 3621 Hill Rd., Parsippany, NJ 07054, 973-331-9545
• Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
• Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]
• Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
• Lennox Pulse Furnace Safety Inspection/Warranty Program: Carbon Monoxide Warning
• Oil Tanks - The Oil Storage Tank Information Website: Buried or Above Ground Oil Tank Inspection, Testing, Cleanup, Abandonment of Oil Tanks
• Oil Tanks Above Ground, UL Standards, guidance for home owners, buyers, and inspectors
• Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
• Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
• Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
• National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
• The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
• Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
• The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
• Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
• "Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• "Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• "Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
• Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
• Installation Guide for Residential Hydronic Heating Systems
• Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
• The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
• Links to our list of additional information on heating system inspection, repair, maintenance
• ...