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Guide to Fan Limit Switches on Warm Air Furnace Heating Systems
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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.

The articles at this website describe the basic components of a home heating system, how to find the rated heating capacity of an heating system by examining various data tags and components, how to recognize common heating system operating or safety defects, and how to save money on home heating costs. We include product safety recall and other heating system hazards. We continue to add to and update this text as new details are provided.

Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

What is the Function of the Hot Air Furnace Fan Limit Switch?

The warm air furnace fan limit switch is a control which determines when the hot air furnace blower assembly turns on and off. The switch prevents the furnace blower from sending chilly air into the building if the oil or gas burner has not sufficiently heated up the furnace heat exchanger and supply air plenum.

The fan limit switch is also a safety control which protects the furnace heat exchanger from damage by turning the burner off on the furnace gas or oil burner if the temperature inside the warm air supply plenum (just above or just next to the heat exchanger) gets too high. This would be an unusual condition but might occur if air ducts were blocked or if someone has fouled up the system controls. The fan limit control switch is usually found on both oil and gas fired heating furnaces.

In the photographs above we show to illustrations of a hot air furnace fan limit switch as you're likely to find one at a typical furnace.

A fan limit switch on a gas fired furnace is shown in the left hand photo. You can see the black switch body just above the gas regulator valve. The dial at the right of the switch contains the fan on, off, and limit temperature settings. The white button you can see at the lower right corner of the furnace fan limit switch is a manual override which can cause the blower fan to turn on and run continuously regardless of furnace temperature.

Continuous furnace fan operation

On hot air heating or cooling systems where we have installed high quality air filtration to address an indoor air quality issue, and where the fan is rated for continuous duty, we may pull this switch out to keep the fan on continuously. Usually the fan limit switch of this type has a silver cover hiding the switch details but with the white "fan override" button projecting through the cover and visible.

A fan limit switch on an oil fired furnace is shown the right hand photo where we can see about 3/4 of the silver colored dial where the fan limit switch settings are made. The switch is about in the center of the photo, and is partially hidden by a low voltage transformer and a metal electrical junction box which are mounted at the right side of the furnace cabinet. (The soot and foil tape above the oil burner assembly also tell us that this system has been operating improperly with backpressure in the combustion chamber.)

Here are the functions of a hot air furnace fan limit switch and its settings:

• Furnace Fan Limit Switch Low Limit or "Cut In" Point: When the thermostat for a hot air heating system calls for heat it turns on the oil or gas burner at the furnace, but the fan limit switch prevents the hot air blower or fan unit from operating until the air in the warm air supply plenum just above (or perhaps next-to in a horizontal unit) has reached a warm temperature. This prevents blowing cold air on the building occupants. When the "cut-in" or "fan-on" temperature (the lower limit on the fan limit switch) has been reached, the blower will begin to operate.
• When does the furnace blower turn off in normal operation?: When the thermostat has been satisfied and turns off the oil or gas burner at the furnace, the fan limit switch will cause the blower or fan unit to continue to operate until the temperature at the supply plenum has reached or dropped below the "cut-in" or "fan-on" lower limit on the switch.
• Fan Limit Switch High or "Cut Off" point: if the temperature inside the supply plenum reaches the "high" limit on the fan limit switch the switch will turn off the oil or gas burner. This condition may not ever happen with some hot air furnace systems - on those systems the burner continues to run all of the time the thermostat is asking for heat. The "HIGH" or "MAX" on the furnace fan limit switch is also a safety device. In the unusual event that temperatures inside the supply plenum become too high, when temperatures reach the MAX setting the fan limit switch will turn off the burner to prevent damage to the furnace heat exchanger (from warping or cracking from excessive temperatures)
• Fan override switch, if present, is usually a white button that can be pulled out to cause the fan to run continuously.

How to Identify, Reset, or Adjust Hot Air Heating System (Warm Air Furnace) Controls and Switches

If the heat in your building is provided by warm air that flows out of ceiling, wall, or floor air supply registers into the occupied space, or if your heating system uses a water-to-air heating system then the air which warms the living space is probably being delivered through large or small diameter ducts, registers, air filters, and a furnace blower, and the air is being heated by a gas, oil, or electric furnace, or perhaps by a heat pump or a geo-thermal system.

This website provides description of all of the major components of warm air heating systems, how to recognize or find each component, what it looks like, what goes wrong, and how to maintain, repair or adjust the component. For details about the setting, re-setting, or function of the controls and switches commonly found on hot air heating systems see these articles:

How to Identify, Reset, or Adjust Hot Water Heating Boiler or Steam Boiler Controls and Switches

For details about the controls, components and switches commonly found on hot water 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 hot water heating system. 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.

How to Identify, Reset, or Adjust Warm Air Furnace Heating Controls and Switches

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. 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 at page left show where you are in our document or website.

• Hot Air Heating Furnace Basic Operating Steps
• Cad Cell Relays 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

Technical Reviewers

Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to the material. Content suggestions, technical corrections and content critique are invited for any of the content at our website.

• Daniel Friedman - principal author/editor of the InspectAPedia ^© Website
• Critique, contributions wanted: Contact Us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

More expert information on this topic

More Information on Inspecting and Repairing Heating Systems

• Carbon Dioxide Gas Toxicity
• Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
• Dust from HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
• 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.)

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