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Furnace combination controlWire the Fan & Limit Controls on Furnaces

Camstat, Cemco, Firestat, Goodman, Honeywell & White Rodgers Fan Limit Controllers

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.

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Advice for Installing and Wiring the Furnace Combination Control Fan Limit Switch on Heating Systems

Fan limit switch showing wrong settings (C) Carson Dunlop Associates

Here we include both old and current guides to Installing & Wiring Fan Limit Switches on Warm Air Furnace Heating Systems for all major brands of fan limit control switches used on heating furnaces and other fan systems.

In this article series we also discuss the following:

The sketch at above/left, courtesy of Carson Dunlop Associates, 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. [Click to enlarge any image]

Article Contents

The following example of wiring a fan limit control switch 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

The switch portion of this Honeywell Fan Limit 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).

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

heating furnace fan limit switchThis 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.)

Fan Limit Switch Installation Manuals 1970 to Present, for Camstat, Cemco, Goodman, Honeywell & White Rodgers

Camstat Fan Limit Control Wiring

Cam-Stdat 567 series fan limit control switch at InspectApedia.com

Cam-stat FAL7C-05TD-120-A Fan & LImit control (shown above) [Click to enlarge any image]

Cam-Stat Fan Limit Controllers 567-series specifications at InspectApedia.com

Cam-Stat is a division of Supco. The Cam-Stat fan limit switch provides a dial-adjustable FAN OFF temperature ranging from 90°F to 120°F, a FAN LIMIT temperature ranging from 150°F to 250F, and typically a fixed differential of 30°F. [Click to enlarge any image]

Firestat Controller

Senaset Firestat HVAC duct fire safety device, fan limit control at InspectApedia.com

Shown above: the Senasys Firestat HVAC duct fan limit controller, a fire safety device that turns off the HVAC blower in event of a fire.

SUPCO Limit Controller Wiring Guides & Manuals

CEMCO Limit Control Wiring Guides & Manuals

Cemco Senasys Firestat wiring diagram at InspectApedia.com cited in this article

[Click to enlarge any image]

Combination Temperature Control, Columbus-Electric Mfg. ca 1970 (C) InspectApedia.com Larry C Combination Temperature Control, Columbus-Electric Mfg. ca 1970 (C) InspectApedia.com Larry C

Goodman Furnace Controls

Honeywell Fan Limit Control Wiring & Manuals

Honeywell L4064B, L4064T Fan Limit Control wiring including low voltage wires ford a gas valve, circa 1970, cited in this article (C) InspectApedia.com

Above: Honeywell L4046T Wiring diagram.

[Click to enlarge any image]

Below: Honeywell L4068A-C-E-F Wiring Diagrams from the manual given just below.

Honeywell L4064B Limit Wiring When Controlling Line Voltage (120V or 240V) - Control Installation Notes

L4064B wiring for line voltage

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 low voltage.

The pull-out or break-off tab discussed below at HONEYWELL L4064B LIMIT WIRING WHEN CONTROLLING LOW VOLTAGE 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 illustration below shows the wiring for controlling line voltage when the jumper or pull out tab has been removed.

Honeywell L4064B line voltage no tab

You can see that in effect the installer in effect is replacing the missing jumper by installing a connection 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:

  1. 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.
  2. 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 (cited below).

 

Honeywell L4064B Limit Wiring When Controlling Low Voltage - Control wiring Details

Honeywell L4064B pull out tab for low voltage wiring

Pull the tab at the bottom of the Limit Control for use with low voltage equipment

To prepare the L4064B furnace control for use in a low-voltage application (typically 24VAC), 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.

The brass jumper is the breakaway type. It must be removed when the limit is used in the low voltage circuit.

To remove jumper, break with a needlenose pliers and remove completely. Once removed, it is not replaceable.

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.

Also see HONEYWELL L4064T LIMIT CONTRL WIRING

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

Reply:

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

Honeywell L4064B low voltage installationThe 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:

  1. Be sure to add a power disconnect switch and overload (fuse or circuit breaker) protectionOn this circuit. Note that the jumper or paper tab has been left in place.
  2. 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 FAN LIMIT CONTROL SETTINGS and then be sure you test the combination fan and limit switch for safe and proper operation.

How to Check that the Fan Limit Switch is Working Properly

Excerpting from the 2017 document given below:

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, reconnect the fan switch, turn on power and again set thermostat to call for heat.

The fan should start when plenum temperature has reached fan-on setting. 

Honeywell L4068 & 4069 Fan & Furnace Controller Wiring

Honeywell L4068 Wiring Diagram at Inspectapedia.com Honeywell L4068 Wiring Diagram at Inspectapedia.com

Wiring the White Rodgers 5D-series Fan Limit Controller

White Rodgers Fan Limit Controller at InspectApedia.com

Watch out: To prevent electrical shock and/or equipment damage, disconnect electric power to system at main fuse or circuit breaker box until installation is complete. Label all wires prior to disconnection when servicing controls. Wiring errors can cause improper and dangerous operation.

Following installation or replacement, follow appliance manufacturers’ recommended installation/service instructions to insure proper operation.

Do not use on circuits exceeding specified voltages. Higher voltages will damage control and could cause shock or fire hazard. If in doubt about whether your wiring is millivolt, low or line voltage, have it inspected by a qualified heating and air conditioning contractor or a licensed electrician.

White Rodgers Fan Limit Control 5D-series wiring diagram 1 at InspectApedia.com

Above: Wiring diagram for the White Rodgers Fan Limit Control used at line voltage. [Click to enlarge any image] adapted from the White Rodgers 5D51 installation instructions cited here.

Below: Wiring diagram for the White Rodgers Fan Limit Control used with low voltage equipment. Note that the jumper is remoed for use with low voltage equipment, also adapted from the White Rodgers 5D51 installation instructions cited here.

White Rodgers Fan Limit Control Wiring Diagram for 5D51-35, -78, & -90 Installation Instructions at InspectApedia.com for low voltage equipment

Other Limit Controls, Wiring & General Information

Honeywell L4068 Wiring Diagram at Inspectapedia.comWarning on Wiring Limit Controls and Switches in Reverse Polarity

Question: is it ok to wire the limit switch backwards with wires reversed in polarity from the instructions?

2018/10/01 John Ford said:

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.

What happens if these are reversed & will it ruin the limit control switch

Reply: do not wire switches in reverse polarity: damage risk

Thanks for a critical question, John as we've all pushed a wire into the wrong connector from time to time.

Watch out: Warning about wiring fan limit switches or other controls in reversed polarity:

I don't know what specific fan limit switch you are using reverse wiring effects and I suspect that the risk of damage from wiring in reverse polarity is manufacturer and model-dependent as well as dependent on an additional detail I'll note. Here are some general remarks:

Most switches will "work" 'when wired in reverse polarity or "backwards" in that they turn-on or off the switched device as needed.

However wiring in reverse polarity can damage electrical components and can also be unsafe.

A review of THE BASICS of LIMIT SWITCHES [PDF] an excellent document on limit switches by Eaton, cited in detail below, reminds us that mis-wiring a switch as you asked can cause arcing when the switch operates; that arcing in turn can damage the switch or devices connected to it.

Excerpt:

Limit Control Wiring Polarity Observations

Polarity is a term which is used to describe the relationship between the load and line connections in a multicircuit switch.

Take a set of contacts with terminals that are made electrically common. If a line to line short circuit does not occur, these contacts are said to be connected to the same polarity.

If a line to line short circuit does occur the contacts are said to be of opposite polarity.

Some limit switches have their sets of contacts isolated from each other on the same pole. They are said to have electrically isolated outputs and are labeled as such.

Most devices, however, do not have isolated sets of contacts on the same pole and care must be taken to observe polarity.

Failure to do so can cause permanent damage to the switch, the wiring or both.

Additional remarks:

In some electronic devices leaving a live connection to the side of a circuit that's normally not energized can sometimes cause damage to a circuit or circuit board - a classic example is the effect of stray currents on the neutral wire in a circuit that damage an appliance or trip an AFCI.

When wired improperly the heating system is also unsafe. For example as you'll see in the Honeywell schematic I include below, if you reverse line and load wiring you are keeping the motor and other parts of the system always "energized" - that condition, combined with any of a number of errors or events, could shock someone.

Here is what Honeywell says, using the L4068A-series controller as an example

Connect the “hot” wire from the power supply to the upper left terminal. On L4068, connect the fan motor to the bottom terminal (marked LOAD); connect limit switch, if wired in the line voltage circuit, to the upper right terminal.

References

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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2021-02-17 - by (mod) -

Honeywell typical fan limit wiring showing black, white & red wire connections (C) InspectApedia.com Dgone

[Click to enlarge any image]

DG in addition to the example wiring diagrams found on this page, the IO manual for the Honeywell Fan Limit controller is given as a free PDF download - as well as manuals for wiring other limit switches.

Example

HONEYWELL L4068A,C,E,F & L6068A,D FAN CONTROLS [PDF] (1976) installation and wiring manual.

On 2021-02-17 by Dgone

Where does the red wire go? No thermostat

Honeywell typical fan limit wiring showing black, white & red wire connections (C) InspectApedia.com Dgone

On 2020-11-24 - by (mod) -

Thank you for the update that will perhaps help other readers as well. Keep me posted.

On 2020-11-24 by Richard Garcia

@danjoefriedman
Thanks for the reply. When you mentioned the manual it reminded me that the previous owner of my house (I've been living here one year) left her documents and I found the Carrier booklet for my model 58CMA oil furnace. Apparently I have an electronic fan timer (Honeywell circuit board ST9103) and the limit controls, two of them, are shown on the circuit diagram. Now I have to figure out the wiring for my new primary, but at least I'm not driving myself crazy trying to find a analog limit switch. Thanks again for your reply.
Richard

On 2020-11-23 - by (mod) -

Richard

Some forced-warm air furnaces use a snap switch or other control to manage the fan on-off-high limits, so you may not see the traditional Honeywell fan limit controller but you might see smaller, perhaps brown or black phenolic plastic "boxes" maybe just a couple of inches across. There ought to be one or more fan limit controls that will be mounted on or in the warm air supply plenum as that's where they have to sense temperature.

So you may be able to follow wires back from the blower motor to the plenum.

Tell me the furnace brand and model and if you don't already have the IO Manual that would show those details we may be able to provide one as we've got a zillion heater manuals as PDFs for readers to download.

On 2020-11-23 by Richard Garcia

Hello, I have a Beckett burner (AFG) forced air system that is about 20 years old. It works well. I just purchased a Honeywell primary R7284U to replace, if needed, the original R8184G just in case (it's 20 years old, too). The old primary has 3 wires and the new one is more complicated. My question: I cannot find the fan limit switch in my furnace. I've found two sensors at two places in the plenum but no fan limit switch. I looked in the lower and upper compartments and in front of the blower and found a Honeywell circuit board that a red wire from the sensors is plugged into, but no switch as shown in all the diagrams I have seen online. I need to wire the limit into the new primary when I eventually install it. I could use some help. A lot of it. Thank you very much. Richard.

On 2020-11-01 - by (mod) -

Thanks for the snap disc control wiring question, Gary.

Where a snap disc switch is simply opening or closing a circuit it's usually doing so by interrupting a single wire. In that case swapping the wires is likely to have no effect on the switch operation.

For example a switch that opens on temperature rise will simply interrupt a burner power circuit.

Watch out: it's not always the case that the wire connections make no difference on other controls or wiring connections: check the instructions that come with the specific control.



For example in some cases leaving power always present on the wrong side of some circuit boards can over time damage a board.

Another snap disc control example from Emerson is shown above. Notice that there's no identification distinction on the two wiring terminals.

On 2020-11-01 by Gary

What happens is you reverse the wires on a White Rodgers snap disc fan control?

On 2020-09-21 - by (mod) -

Anon:

White Rogers offers this spec - note the voltages given [Click to enlarge any image]

White Rodgers snap disc fan limit control switch specifications - cited & discussed at InspectApedia.com

On 2020-09-21 by Anonymous

Can you connect 120 volts to a 3L0 -190 white Rogers snap disc limit control

On 2020-01-18 by (mod) - how do you set the fan speed?

Jim,

Many home heating furnace blower fans run only at one speed.

Some have a multi-speed fan that runs at different speeds under control of the circuit board in the air handler, depending on whether or not you are in heating or in cooling mode.

Some air handler circuit boards indeed include little dip switches or jumper pins that allow the service technician to set the fan speed(s) if the fan motor supports them.

See our article BLOWER FAN SPEED SETTINGS

for step by step instructions and for links to a number of air handler manuals with more instructions on setting the fan speed

Here is an example for York air handlers that explains the details of setting fan speed on the control board

inspectapedia.com/aircond/York-Variable-Speed-Air-Handler-Instructions.PDF

Give me the brand and model of your air handler and I will look for its specific instructions.

On 2020-01-18 by jim

how do u control the speed of your fan

On 2019-05-04 by (mod) - when does the fan turn-on?

Revised

Joe

The fan comes on after the plenum is warmed. Not before - on a heating application.

Otherwise the unit would blow cold air on occupants.

Where to start: Watch the fan limit control to see if it's rotating to the FAN ON position.

On 2019-01-19 Joe T. - Carrier Day and Night FAU (Fan Air Unit)

I installed a new ICM 281 for a Carrier Day and Night FAU. When I turn on the power, the blower fan started running for 1-2 minutes, the hot surface igniter light up red hot, the burners ignited while the blower kept running. Everything seemed to work fine.

A few days later, the FAU stopped working. Shouldn't the surface igniter becomes red hot first, then the burners light and run for a few minutes, before the blower starts running to push the hot air up into the unit?

What should I test to see why the FAU stopped working?

Thanks. J.


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