Guide to Temperature Settings on Combination Fan & Limit Switches for Furnaces
- Honeywell L4064B
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Furnace blower fan limit switch temperature settings: This article describes in detail the setting of furnace combination controls, also commonly called the "fan limit switch" on warm air heating systems. The photo at the top of this page, courtesy of Honeywell Tradeline Controls, labels all of the controls and wiring terminals in a Honeywell combination fan and limit control type L4064B. This website 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.
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How to Choose the Proper Settings for a Warm Air Furnace Combination Fan & Limit Control Switch
Here are the functions and and normal settings for the warm air heating furnace control switch such as the Honeywell Tradeline L4046 and 4046B combination fan and limit control:
As we explained above, the furnace fan limit control turns the furnace blower on and off at the proper times. Below we detail the functions and normal settings for five controls found inside this device.
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Warning: when adjusting the set points on the combination furnace control be sure to hold the round faceplate dial steady with one hand while moving the sliding fingers or pointers in its face. Otherwise you may bend, strain, or damage the temperature sensing element (the bimetallic spring and related parts). Bending the element could make the control fail to sense temperature accurately and thus could be dangerous.
Also do not "turn" the silver dial by hand to try to make the control turn on or off. This too can damage the control, making it unsafe. |
The basics of how furnaces work can be read at FURNACE OPERATION DETAILS and the key heating furnace components are introduced at FURNACES, HEATING. 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 or NO HEAT - FURNACE.
1. Furnace FAN OFF Setting
The furnace combination fan and limit control FAN OFF setting (blue arrow in our photo at left) 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 and the heat it contained has been sent to the occupied space.
The FAN OFF setting is the left-most metal finger protruding through the round silver plate of the fan limit control. Move the FAN OFF setting lever to the temperature at which the fan is to stop in order to prevent sending cool air into the occupied space.
From the factory the FAN OFF finger is usually set to about 90 degF.
At the end of a furnace-on heating cycle, after the gas or oil burner shuts down, the furnace blower will continue to operate for a time.
This continued fan operation achieves the following:
- The remaining heat from the hot furnace heat exchanger is salvaged and sent into the occupied space
- The furnace heat exchanger is cooled down more uniformly by the air flow over its surfaces in order to help reduce the chances of heat exchanger cracking
- The temperatures in the occupied space remain more even
2. Furnace FAN ON Setting
When an adequate warm temperature has been reached inside of the furnace warm air plenum chamber the FAN ON switch turns (green arrow in our photo at above left) on the furnace blower to deliver warm air to the occupied space. The FAN ON setting on this control makes sure that the blower fan does not turn on too soon (even though the building thermostat has asked for heat) so that the furnace will not blow cool air into the occupied space. This setting also prevents the blower fan from cycling on and off too frequently during a heating cycle.
The FAN ON setting is the metal finger second from left-most, protruding through the round silver plate of the fan limit control. This finger is set to a temperature range from a minimum of 20 degF above the FAN OFF set point, to a maximum o f 30 degF below the LIMIT OFF set point.
From the factory the FAN ON is usually set to about 130 degF.
3. Furnace LIMIT OFF Setting
The LIMIT OFF indicator setting (red arrow in our fan limit switch photo at above left) is a safety control that will turn off the oil or gas burner if temperatures inside the warm air plenum exceed a safe level. This is the highest temperature setting on a furnace combination control. It is set to the furnace warm air temperature at which this safety switch is to turn off the oil or gas (or other) burner or heat source. On the Honeywell L4064B the LIMIT OFF is set to a temperature between 100 degF and 250 degF.
If the temperature inside the supply plenum reaches the "high" limit set at the LIMIT OFF finger, the switch will
turn off the oil or gas burner. This condition may not ever happen under normal conditions with most hot air furnace systems - on those systems the burner continues to run all of the time the thermostat
is asking for heat. The LIMIT OFF or "HIGH" or "MAX" on the furnace fan limit switch is a safety device.
In the unusual event that temperatures inside the supply plenum become too high, when temperatures reach the LIMIT OFF 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)
From the factory this setting out of the box is 200 degF.
4. Furnace LIMIT STOP Setting
The LIMIT STOP control is an extra safety device to make it difficult for an amateur to set the LIMIT OFF to an unsafe or too-high temperature. This control setting could be changed by inserting a pen tip or similar object into the round hole visible in the LIMIT STOP plate near the right end of the temperature dial and slog on the silver plate of the fan limit control.
The LIMIT STOP is set at the factory to 200 degF. You should not change this setting as to do so maybe dangerous.
5. Furnace FAN ON Manual or "override" Switch
Fan override switch, also called a "manual fan switch" or "fan on switch" if present, is usually a white button that can be set to cause the furnace blower fan to run continuously or to run automatically driven by the temperatures sensed by the combination fan and limit control switch.
The fan override switch is indicated by the white arrow in our photograph.
FAN ON SWITCH Settings:
Manual-On furnace fan: Push the white fan override button "IN" to set the fan to its MANUAL position to force the furnace blower to run continuously. This setting can be used to move (and presumably to filter) building air through the heating duct system at any time of year.
Some experts opine that running the fan continuously will make heat in the building more uniform and more comfortable. (This will not be true in every building and for every heating system and in some cases can cause blowing of uncomfortably cool air into some rooms during the heating season.)
- Automatic furnace fan: Pull the white fan button "OUT" to the AUTO to cause the fan to cycle on and off automatically in response to temperatures sensed by and set on the fan limit control dial.
After completing these control settings be sure you test the combination fan and limit switch for safe and proper operation. See How to Test the Fan & Limit Control for details.
At FAN LIMIT SWITCH TROUBLESHOOTING we discuss troubleshooting furnace blower fan operating problems such as a blower that runs continuously or one that turns on and off to frequently.
Frequently Asked Questions (FAQs)
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- 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
- Honeywell L4064 Fan Limit Control Installation Instructions [PDF file]
- 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.
- "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,
- 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.)
- 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
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
 The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
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- Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
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