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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 FAN LIMIT SWITCH TROUBLESHOOTING.
This article series 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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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.
The switch portion of this control can tolerate 190 °F. and the sensing element can handle up to 350 °F.
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 167°F).
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).
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.
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.
[Click to enlarge any image]
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.
Additional installation details for this control in the latest form are available from Honeywell.
Pulling the tab at the bottom of the Limit Control
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.
Reader Question: I forgot to remove the brass jumper on a new fan limit switch installation
I installed a new fan limit switch but in my rush I missed removing the brass jumper for low voltage.
I tested it and it started and stopped. I can't seem to find the damage.
Do i need to buy and replace the limit switch again? I've checked the fuses and replaced both but still wont work. what else do I need to check. thanks
Dennis, I'm sorry to have to play it safe, but reading Honeywell's warning that the control could be damaged, I just wouldn't take a chance.
Watch out: A problem is that just as a bent spring can change how a switch performs, internal damage could be subtle and not visible, but the fan limit may not perform safely.
Certainly you can go through the recommended fan limit switch test procedures described above on this page to confirm that the swtich is doing what it is intended to do.
If you are still concerned I'd repalce the switch rather than take a chance or lose sleep over it.
How to Wire the Honeywell L4064B to Control Low Voltage Equipment
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.
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
Continue reading at FAN LIMIT SWITCH TROUBLESHOOTING or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs)
Question: How do I wire a furnace fan relay switch
how to wire a fan relay switch - Kenneth Sanders 10/7/2012
Kenneth, please see FAN LIMIT SWITCH INSTALLATION - separate article
Questions & answers or comments about how to hook up or wire the furnace blower fan limit control switch.
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