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 How to Install & Wire Combination Fan & Limit Controls on Warm Air Furnaces - e.g. Honeywell L4064B
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Fan limit switch wiring details: this article describes in detail the installation & wiring of 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 installed horizontally on a gas furnace. Readers should also see How to Test the Fan & Limit Control, or begin this topic at FAN LIMIT SWITCH. 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.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
Advice for Installing and Wiring the Furnace Combination Control Fan Limit Switch on Heating Systems
This example is based on advice from the Honeywell Tradeline L4064B. Check the test specifications provided by the manufacturer of your particular control.
Furnace Combination Control Operating Temperature Range & Limits
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The switch portion of this control can tolerate 190 degF. and the sensing element can handle up to 350 degF.
The control can handle 120V and 240V devices and can also be wired to control low-voltage devices. The electrical wiring used must also be rated for suitable temperature exposure (Honeywell advises wiring rated for 167degF).
The sketch at left, courtesy of Carson Dunlop, shows an improper (too high) upper limit setting - this is an unsafe fan-limit switch setup which is likely to allow the furnace to overheat, risking heat exchanger damage and dangerous carbon monoxide leaks. |
At 120V the control can switch fan motor loads at 14 Amps full load (84 Amps locked rotor load), and the limit switch (presumably an oil or gas burner) can handle up to 8 Amps full load (48 Amps locked rotor load).
When controlling 240V devices the fan or blower control can handle up to 7 Amps full load (42 Amps LRA) and the limit switch can handle 4 Amps full load (24A LRA).
Where to mount the furnace Fan-Limit Control
This furnace combination control is mounted on the furnace in a location where the bimetallic spring/probe (shown above in this article) will protrude into the warm air plenum to sense furnace air temperatures there. Each furnace manufacturer will provide instructions of where, on their system, the air temperature should be monitored for control purposes.
The reason the control manufacturer warns that the control tip should not touch any internal surfaces of the furnace is that doing so can cause improper reading of furnace air temperatures or could damage the control or prevent free movement of the bimetallic spring in response to temperature changes.
Of course if you are replacing an old control that has failed, just mount the control in the same location that held the prior unit. When replacing an old furnace limit control, make sure that the new control has a sensing tip of the same length as the unit being replaced. Otherwise the new control may not work safely.
The manufacturer provides details for surface mounting, rigid-bracket mounting, or swivel-mounting of the control. Which of these methods you choose depends on what mounting is needed to place the sensor probe in the proper location in the air plenum.
Wiring the Furnace Combination Fan & Limit Control using the Honeywell L4064B as an Example
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 This control can be wired to serve as a safety LIMIT switch on a furnace by wiring just the limit terminals on the control. When the device is used both to control a furnace fan on-and off as well as serve as a LIMIT switch, then all four terminals are used.
Fan control wiring: As Honeywell's illustration shows, the two fan terminals are on the upper and lower left side of the control.
Line voltage is wired at the bottom left push-in terminal.
Load voltage (to the fan) is wired at the upper left push-in terminal.
Limit control wiring: As the illustration shows, the two LIMIT terminals are on the upper and lower right side of the L4064B control.
The Line (power in) wire is connected to the lower right push-in terminal, and the Load (wire to oil or gas burner) is wired to the upper right push-in terminal. |
The wiring diagrams shown in more detail below are typical for wiring the furnace combination control on heating systems. Remember that all electrical wiring of furnace controls (or any other electrical devices) must comply with national and local electrical codes as well as the specifications of the control manufacturer and the furnace manufacturer.
Wires are connected to the control using push-in terminals. A wire strip gauge is provided on the left side of many versions of this control. The control used for our photos came with additional push-in terminals (Part # 137813) that can be used to convert the push-in wiring connectors to screw-terminal connectors. This is a great idea if you expect to be changing wiring from time to time. (Dr. Jess Aronstein's research has demonstrated that repeated-use or re-use of push-in type terminals on electrical receptacles does not provide a very reliable connection. This may be true for this control as well.)
Honeywell L4064B Limit Wiring When Controlling Line Voltage (120V or 240V) - Control Installation Notes
In most applications in the U.S. and Canada the Honeywell L4064B combination control is used to switch on and off 115V-120V or 240V fans and heating burners, or on some gas equipment, the burner controls (the LIMIT switch function) operates at lowe voltage. The pull-out or break-off tab discussed below at "Low Voltage Control Installation Notes" is left in place for line voltage applications.
The left-illustration shows normal wiring for this set-up. The fan or furnace blower motor is controlled by the two left connections (green dots); the furnace limit switch which will turnoff the burner if the temperature limit is reached is controlled by the two right connections (red dots). Here the furnace limit switch is controlling a line-voltage device. The colored triangles refer to notes given below.
The right-hand illustration above shows the wiring for controlling line voltage when the jumper or pull out tab has been removed. You can see that in effect the installer in effect is replacing the missing jumper by installing a common wire to both terminals on either side of the contacts where the jumper was removed. This important detail permits this control to be used to control line voltage (120V) devices even if the jumper has been removed or the paper pull-out tab on older controls has been lost.
Wiring Notes for the Combination Furnace Control L4064B:
- Be sure to add a power disconnect switch and overload (fuse or circuit breaker) protection on this circuit. Note that the jumper or paper tab has been left in place.
- Wires to control low-voltage equipment - in this case the jumper has been removed.
Additional installation details for this control in the latest form are available from Honeywell.
Honeywell L4064B Limit Wiring When Controlling Low Voltage - Control wiring Details
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To prepare the L4064B furnace control for use in a low-voltage application, simply remove the small red cardboard tab shown at left or break off the copper jumper between the two contacts in the same location on newer models of this control. .
If you make a mistake and remove this tab and then realize that you need to use this control to handle line voltage (120V or 240V) you can simply install a jumper wire as we show in the right-hand illustration just above on this page.
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The diagram at left shows how to wire the Honeywell L4064B combination furnace control when it is used to control low-voltage equipment.
This tab is found protruding from the control near the center of the bottom of its face. You'll see embossed on the control above this tab the words "Remove for Low Voltage".
Newer versions of this control have a brass jumper in the same location. The brass jumper is broken off for low-voltage use and is not replaceable once it has been removed. |
The fan or furnace blower motor is controlled by the two left connections (green dots). The furnace limit switch which will turnoff the burner if the temperature limit is reached is controlled by the two right connections (red dots). Here the furnace limit switch is controlling a low-voltage device such as a heating furnace gas valve. The colored triangles refer to notes given below.
Notes:
- Be sure to add a power disconnect switch and overload (fuse or circuit breaker) protection on this circuit. Note that the jumper or paper tab has been left in place.
- Wires to control low-voltage equipment - in this case the jumper has been removed.
Before wiring this or any control be sure to obtain the latest data from the manufacturer of the control and the furnace on which it is to be installed. Additional installation details for the Honeywell L4064B Combination Fan Limit control in the latest form are available from Honeywell.
After wiring this control make sure you've use the proper settings by reading over How to Set the Fan & Limit Control and then 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.
If your heating or cooling system blower fan itself appears not to be working, see BLOWER FAN OPERATION & TESTING.
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.
- 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
- Honeywell's latest product data for this type of fan limit control switch can be found in English at http://customer.honeywell.com/Techlit/Pdf/69-0000s/69-0117.pdf (local copy provided)
- 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. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides:
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- The Home Reference Book, a reference & inspection report product for building owners & inspectors.
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- Thanks to reader Dan Somerville for technical editing assistance 01/09/10
- 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
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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
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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
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