Fan & Limit Switch on Warm Air Furnaces - home

How the fan limit switch works

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

The fan limit switch on a heating furnace controls when the furnace blower fan turns on and off. This control also provides a safety limit that turns off the burner or furnace if temperatures at the furnace are too high.

Fan/Limit switches are used on all types of building heating furnaces.

A combined fan limit switch such as the Honeywell L4064 or the White Rodgers 5D51-90 limit control provides at least three heating blower fan temperature controls that respond to the temperature in the furnace supply air plenum:

1- FAN OFF:

the low temperature at which the furnace blower fan will STOP to prevent blowing cool air onto room occupants.

Typical factory setting: 90°F

2- FAN ON:

the temperature at which the blower fan will turn ON to begin to deliver heat to the building's occupied spaces.

Typical factory setting: 100°F

3- FAN LIMIT:

the high temperature at which the high limit switch will open (turn OFF) to stop the burner to prevent overheating of and damage to the heat exchanger. Such damage would make the furnace unsafe.

Typical factory setting: 200 °F

By keeping the fan off until the supply air plenum is warm, the heating furnace limit 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.

Watch out: when adjusting the temperature settings in this control do not press on, bend, twist nor in any other way fool around with the rotating metal disc. Doing so can bend its moving parts and spring, rendering the control both inaccurate and unsafe.

Below we show a close view of typical FAN OFF (about 90°F) FAN ON (about 140°F) and FAN LIMIT (200°F) settings on a Honeywell fan limit control.

Details about setting the temperature controls on a heating furnace fan limit switch are

at FAN LIMIT CONTROL SETTINGS.

Furnace & Blower Fan Controls at the Fan/Limit Switch

In this photo we show a close-up of the temperature dial in a Honeywell Tradeline L4064B 2228 combination furnace control.

The silver fan control dial shown in our photograph, driven by a bimetallic spring that is inside the probe that is in turn inserted into the supply air plenum, responds to temperatures inside the furnace. In response to supply air temperature this control turns the blower fan on, off, and provides an upper limit temperature setting for safety.

The wiring for this control is

at HONEYWELL FAN LIMIT SWITCH WIRING

Types of Fan / Limit Controls: adjustable vs. snap-disc

Below, adapted from shopping suggestions from a Google search for "fan limit control" we show types of furnace fan and limit switches. We describe each of these furnace fan limit control switches on this page.

In the first limit control switch at the left in our illustration you see that a snap-disc type limit control is mounted on extending legs so that it can be placed at the furnace-manufacturer's required location in the supply air plenum.

The other snap disc switches in our illustration are all surface mounted and are often matched to specific brands and models of heating equipment.

The White Rodgers adjustable fan/limit controller shown below provides a toggle switch at the top of the control. That switch allows the user to select MAN (fan will be always on) or AUTO.

This White Rodgers fan limit control sports an 11-inch long insertion probe that projects into the supply air plenum.

The wiring for White-Rodgers universal fan limit switches is

at WHITE RODGERS FAN LIMIT SWITCH WIRING

Also see WHITE RODGERS CONTROLS & MANUALS - all manuals

Watch out: for safety and for proper furnace temperature and fan operation it is essential that the fan limit control's temperature sensing device be located where the furnace manufacturer has specified and that the probe be of the proper length.

A probe that touches other metal parts or that is too short or too long will not work properly and the heating system may be unsafe.

You will see that the there are two popular Fan/Limit control switch designs:

• Adjustable Fan / Limit control:
  
  A traditional-design adjustable combination fan and limit control (shown above) operates using a bi-metallic spring wound into a helix and inserted into the supply plenum.
  
  The Fan switch "makes" or closes to permit the fan to run when temperature rises to the FAN ON setting.
  
  The High Limit switch "breaks" or opens to turn the burner off if the plenum temperature reaches the high limit setting.
  
  Two common brands/models of adjustable fan limit controls of this design are the Honeywell L4064B-series Fan and Limit Controllers White-Rodgers Fan/Limit Control Model 5D51-series.
  
  An adjustable fan-limit controller typically is designed for furnaces operating at temperatures from 100°F to 250°F, has a FAN OFF lower limit that can be adjusted from 50°F to 200°F, a FAN ON switch adjustable from 65°F to 215°F and a fan HIGH LIMIT stop set at 200°F.
  
  There are adjustable "stop" tabs that can prevent adjustments of Fan Off and Fan On beyond a particular point. The Fan-Off stop is factory set to 100°F and the Fan-On stop is factory set to 125°F.

• Fire Stat type temperature limit controls: (shown above & below)
  
  The Firestat heating system duct control shown above is another type of fan limit switch that is typically installed int eating system ductwork at locations where the duct temperature can be monitored to be sure it stays below a critical level.
  
  If the duct temperature exceeds the Firestat setting then the switch is used to turn off the building's heating system or other equipment. The adjustable model has a Setpoint Temperature Range of 100°F to 250°F.
  
  See these
  
  FIRESTAT LIMIT CONTROL INSTRUCTIONS [PDF] (2018) from Firestat, CEMCO (Division of Senasys), 1435 International Dr., Eau Claire WI 54701 USA, Tel: 1-715-831-6356 Email: info@senasys.com - retrieved 2023/12/04, original source: firestat.net/high-temperature-limit-control-installation/
  
  FIRESTAT TC SERIES DUCT LIMIT CONTROL PRODUCT SPECIFICATIONS [PDF] (2010) Senasys, op. cit, - retrieved 2023/12/04, original source: h restat.net/wp-content/uploads/2022/04/TC-Spec-2010.pdf
  
  Excerpt: The TC Duct Limit Controls contain a Helix Bi-Metal that senses rapid increases in duct temperature such as fire. The Bi-Metal causes the normally closed contacts to open and turn off the power. Available in either adjustable or fixed temperature models.

• Fixed-Temperature Snap-Disc Furnace Controls
  
  Newer "snap-disc" type switches (included below), typically with one or at most two functions, that open or close an electrical circuit in response to temperature.
  
  The temperature response point for snap disc controls is generally not adjustable.

Some gas furances use one or more "snap disc" devices that monitor temperatures and act as the necessary controls,or another solid state limit control or manual reset limit control switch rather than a conventional fan limit switch. Both types of controls are shown on this page.

Above: a single-purpose snap-disc type limit switch from Nordyne, the Nordyne L150F.

A single purpose snap-disc type "limit switch" such as this one will "OPEN" to turn off the burner if temperature reaches or exceeds the switch's rated temperature, in this case 150°F.

Below is the limit switch from a 1988 Amana gas furnace.

Watch out: do not assume that you can easily swap out one type of furnace fan and limit control for another. Improper sensing of temperatures or control of the blower fan or burner can make a heating system unsafe.

Manufacturer's installation & adjustment instructions for each of the heating fan and limit switch controls discussed in this article series are found at References or Citations at the end of of each of our articles.

• Cam-stat® type Fan Limit Control Switch
  
  such as the A-1 Components BC 7070 Blower Controller, shown above. This limit control, model BC-7070-A/M is produced by Zettler or A-1 Components.
  
  This CAMSTAT type fan limit controller provides adjustable blower-on and blower-off times (time delay at the start or end of a heat-on or thermostat-controlled blower-in cycle) between 0-240 seconds.

For the BC7070 shown above here is a wiring diagram and very brief manual

• A-1 Components, BC-7070-A//M "ON/OFF" TIME DELAY RELAY FAN LIMIT CONTROLLER MANUAL [PDF], A-1 Components, Henry Technologies, Inc., 701 S. Main St., Chatham IL 62629, USA Tel: 217-483-2406, Websites: www.henrytech.com or www.a-1components.com
  
  Product description: This fan limit switch is also referred to as a Cam-Stat control or a time-delay relay because it allows setting a timed delay in fan-on or fan-off cycles.
  
  More about this and other furnace controls can be read
  
  at MANUALS for HEATING & A/C SYSTEM CONTROLS

Ten Steps in the Sequence of Operation of a Furnace Fan Limit Control

Ten steps in operation of a forced-air heating system and the role of the furnace fan limit switch or limit control are described here.

Photo above: a General Electric Fan Limit control on an older gas furnace, courtesy of InspectApedia reader Zika. A typical GE Limit Switch model number was 3AHL5K2B8 but we don't have the model ID for the switch in our photo.

Like many other furnace plenum or duct temperature limit controls, these fan limit switches were sold with sensors at one of three typical lengths: 5-inch, 7.5-inch, or 11-inches.

The proper sensor limit length is, in some installations, important for the control to sense temperature correctly, and of course a too-long switch might be damaged or made unsafe by touching the opposite side of a plenum or duct surface.

The fan limit switch is a control which determines when the hot air furnace blower assembly turns on and off. In general, the sequence of operation of a forced-warm-air furnace heating system is this:

1. The thermostat calls for heat,
   
   either because room temperature at the thermostat has fallen below the thermostat setting OR because the occupants turn up the thermostat to a higher temperature setting.
   
   The thermostat, acting like a simple on-off switch, tells the furnace, through low-voltage electrical wires connected to a control on the furnace, to turn "on".
2. The burner or heat source at the furnace starts
   
   or ignites. Heat may be from a gas burner, oil burner, heat pump, electric heating elements, solar source, or another fuel or energy source.
3. The burner in the furnace heats a heat exchanger
   
   through which building air will pass to be heated. But the blower fan does not turn on right away because we don't want to blow cold air on the building occupants.
4. The fan limit switch senses the temperature increase
   
   in the furnace. The fan limit control includes a sensor (not visible in the photographs) that projects into the supply air plenum - on gas furnaces this is typically an area above the furnace heat exchanger.
   
   The sensor, often a simple bimetallic spring, responding to heat rising from the heat exchanger that is being heated by the burner, mechanically rotates the temperature dial component of the fan limit switch.
5. The blower fan starts when the fan limit switch turns it on
   
   When the temperature dial on the fan limit switch has rotated enough to represent a pre-set temperature (the FAN-ON temperature setting) the the fan limit switch turns on the furnace blower fan.
6. The blower fan moves warm air into the occupied space
   
   The blower fan draws cool air from the occupied space, through return ducts, pushing that air up through the furnace heat exchanger where it is heated, and onwards through supply ducts to supply registers through which warm air is blown into the occupied area.
7. The furnace burner and the blower fan will continue to run
   
   (usually) as long as the thermostat is calling for heat, so warm air is continually delivered into the occupied space.
8. The thermostat stops calling for heat
   
   When the thermostat is "satisfied" - that is when the temperature of room air around the thermostat has reached the thermostat's set temperature - the thermostat will stop calling for heat - by in essence opening or turning "off" the switch that in turn tells the furnace that no more heat is needed (right now).
9. The blower fan extracts remaining heat
   
   The blower fan continues to run for a minute or several minutes longer, extracting remaining heat out of the furnace heat exchanger and supply plenum, both for efficiency (avoiding wasting that heat) and to avoid possible damage to a heat exchanger that might otherwise become too hot.
10. The blower fan stops

Where is the Fan / Limit Switch Found on a Furnace?

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

The gas fired forced air furnace fan limit switch in the photo below is pointed-to by the red arrow.

The oil-fired furnace fan limit switch shown in our photo below is in use on an oil-fired warm air furnace where we can see about 3/4 of the silver colored dial where the FAN LIMIT CONTROL switch settings are made.

Watch out: Don't let your focus on a specific control device blind you to other important signs of the condition of a heating appliance.

In the left side of the same photo above, soot and foil tape above the oil burner assembly also tell us that this system has been operating improperly with back pressure in the combustion chamber. Such a system might be unsafe.

Really? Well not always. A heating system duct temperature limit control might also be installed at critical locations in ductwork, typically in a commercial HVAC system or as part of a building automation system. See our description of the Firestat

at FAN LIMIT CONTROL TYPES. Below I'll include two other examples of heating furnace fan limit control switches.

Above: your furnace might use a "snap disc" devices that monitor temperatures and act as the necessary control.

Below: this Amana gas furnace fan limit control switch doesn't look anything like the common Honeywell fan limit controls shown above.

Both the snap disc or the Amana gas furnace limit control, if present, would normally be found somewhere on the surface of the hot air plenum on your furnace.

Safety Features of the Furnace Fan / Limit Control Switch

Click to enlarge the fan limit control switch sketch below to read the names of its various parts and controls.

The fan limit control includes these key features

• A manually-operated blower fan on-off switch
  
  (on some, not all fan/limit controls)
• A temperature sensor in the furnace,
  
  either a probe with bimetallic spring or a snap-disc type temperature sensor. We described both of these sensors earlier in this article.
• A FAN control switch
  
  that turns the blower fan on or off in response to the furnace air temperature
• A LIMIT control switch
  
  that turns the burner or heat source off if the furnace becomes dangerously hot

Sketch below is provifded courtesy of Carson Dunlop Associates, a Toronto home inspection, report-writing tool, and home inspection education company.

Watch out: The fan limit switch is also a safety control which protects the furnace heat exchanger from damage (such as heat exchanger cracking due to overheating) by turning the heat source or burner off at the furnace should the temperature inside the warm air supply plenum (just above or just next to the heat exchanger) gets too high.

If a furnace heat exchanger overheats there is risk that the heat causes a crack that in turn could leak dangerous combustion gases or potentially fatal carbon monoxide into the occupied spaces in the building.

Overheating at the furnace heat exchanger is an unusual condition but it does indeed occur if any condition obstructs air flow through the heat exchanger.

Obstructed air flow can occur because of a dirty air filter, blocked or disconnected air ducts, or if someone has fouled up the system controls.

Details: In normal warm air furnace operation, by moving building air across the heat exchanger, the blower is warming air that will be delivered into the occupied space, but at the same time this process is keeping the heat exchanger from reaching too-high a temperature.

If a furnace blower fan fails to start but the furnace heat source (gas or oil fired burner) is running, the heat exchanger would be come overheated and may warp and crack.

The fan limit switch is designed to prevent this damage by shutting off the burner if plenum temperatures reach the high limit.

A fan limit control switch is found on both oil and gas fired heating furnaces of all types.

When does the furnace blower turn OFF in normal operation?

As we show this control in our photo at left, usually the adjustable-type fan limit switch of this type has a silver cover hiding the switch details.

If a manual fan ON/AUTO control is provided you will see (on Honeywell controls) the white "fan override" button or for White Rodgers controls a sliding metal toggle switch projecting through the cover of the control. .

Provided that the fan switch is in AUTO position, 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 just until the temperature at the supply plenum has reached or dropped below the FAN OFF lower limit on the switch - the blue arrow in our photo above.

Then the control will turn the blower fan OFF.

When does the furnace oil burner, gas burner, or other heat source turn OFF in normal operation:

On many warm air heating systems, at least during cold weather, the burner or heat source will continue to run all of the time that the building thermostat is asking for heat, and will stop running as quickly when the thermostat is satisfied.

If the furnace oil or gas burner is very high capacity, or if the furnace fan/limit controls have been set to cause this effect, the burner may on some systems cycle on and off periodically while the warm air blower continues to run.

Which Way to Set the White AUTO / MANUAL Fan Control Switch to Control the Blower Fan

Blower Fan On-Off Switch on the Fan Limit Control

On the silver cover of the fan limit control switch shown in our photo above the cover embossing indicates that PUSHING the button IN forces the fan ON mode.

"PUSH ON" is functionally the same as PUSH MAN (embossed on the switch body itself as we show at left) - it means that you are manually setting the fan to remain on continuously.

On the identical fan limit control shown with its cover off (our photo with the colored arrows), it's hard to see without enlarging the photo, (click to enlarge) but there is text embossed into the switch body that says PUSH MAN instead of PUSH ON shown on the switch cover.

Pull this button (white arrow in the photo) out to cause the fan to run automatically (AUTO) - meaning that the switch itself will turn the blower fan on and off in response to furnace plenum air temperature. Summarizing:

• PUSH the white button IN to force the fan to ON or MANUAL or always-running mode. The blower fan will run continuously.
  
  Some control guides call this "Summer Use" or "Summer Fan" using the blower fan to circulate air when not in the heating season.
  
  
• PULL the white button OUT to put the fan back into AUTO mode
  
  - turning on and off under control of the switch and furnace temperature.

Really? Ok so the "PUSH ON, PULL AUTO" switch is not present on every manual fan switch.

Some limit control models do not include this white button or switch to give manual control over the fan.

On the switch shown, pushing the switch IN puts the fan in MAN or ALWAYS ON while pulling the white switch OUT restores the fan to AUTO operation

- it will turn on or off according to the supply air temperature and the FAN OFF, FAN ON settings on the control.

On the limit control in our photos imprints on the control label it PUSH MAN, PULL AUTO.

Thanks to reader Rob for pointing out the confusion about the fan control AUTO - MANUAL switch positions on the fan limit control.

Below my pencil points to a close-up view of the white blower fan control switch on this fan limit controller.

On 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.

Watch out: don't confuse the FAN ON / AUTO / MANUAL settings on the fan limit control switch (found in or on the furnace air handler) with similar controls found on many room thermostats and described separately

at FAN ON AUTO MAN THERMOSTAT SWITCH

How the Fan Limit Switch Senses the Temperatures Inside a Warm Air Furnace

Above we've already discussed the controls and settings of the fan limit switch. What we haven't explained is how the switch senses temperatures in the furnace. The fan limit switch contains a bi-metallic spring (shown at left) which is inserted into the warm air plenum of the heating furnace.

As the air in the furnace plenum warms up the bimetallic spring expands, turning a gear which turns the fan limit control dial (shown in the photo above).

As the fan limit switch control dial rotates, mechanical "fingers" on the back of the dial operate electrical contacts inside the switch to turn the fan on or off and at the upper limit to turn off the furnace oil or gas burner as well.

When you move one of the little sliding temperature set points on the face of the dial you're moving the position of the mechanical fingers on the back of the dial.

Bypassing the Fan Limit Switch

Question: I bypassed the limit switch and the heater worked fine, but when I tried to test the air conditioner it would not work

I have a Goodman PGB048075-1 furnace. The high limit switch is bad. I bypassed the limit switch and the heater worked fine, but when I tried to test the air conditioner it wouldn’t work.

Is that because I need to replace the high limit switch in order for the air conditioner to work. Or will it work with the limit switch bypassed. I already know the high limit switch is bad.

But just want to know why the heater works but not the air conditioner with the limit switch bypassed. - Brian

[Photo, left, the integrated ignition control circuit board from a Goodman furnace. [1]

Short Answer: your bypass of one safety control may be detected by the primary controller, resulting in system operation shutdown.

Watch out: By-passing the limit control is very dangerous and can also destroy the equipment.

A competent onsite inspection by an expert often finds additional clues that help accurately diagnose a problem with furnace controls, but we agree that it makes complete sense to start with a known, obvious failure - in this case you point to the limit switch.

We have read a few other Q&A's on bad limit switches on the Goodman Furnace model PGB048075-1, though not all of them actually tracked back to a bad switch.

Though your question focuses on why the A/C won't run in cooling mode with your limit switch "bypassed", there are a few things to check right away:

Watch out: bypassing any HVAC equipment safety control such as a limit switch is dangerous, risking overheating and unsafe conditions including a building fire.

Also bypassing the limit switch and can result in permanent damage to the equipment (such as heat exchanger warping and cracking), fire or carbon monoxide hazards, or other failures that leads to having to replace the unit.

Quoting from a Goodman installation manual:

Watch out: WARNING TO PREVENT PREMATURE FAILURE OF HEAT EXCHANGER, PROPERTY DAMAGE, PERSONAL INJURY OR DEATH, DO NOT ADJUST THE LIMIT CONTROL (FACTORY SET).

The Goodman Furnace model PGB048075-1 is a mid-efficiency natural gas furnace with an AFUE rating of 80%. Manuals are available from Goodman and other online sources.

I attach a copy of a 2004 Goodman Furnace manual that may be of some help. Page 26 of that manual describes checking the primary limit control. You'll notice that nowhere does Goodman endorse any wiring modifications such as bypassing the control.

The furnace manual includes an excellent furnace troubleshooting chart that decodes the meaning of the diagnostic lights on the primary control LED. This might help you make sure that you've correctly identified the trouble with your unit.

The Goodman company describes the safety controls on this furnace line as follows

Warning: for general guidance the below is quoted from the furnace manual described at References. Details for your model may vary:

The primary limit control guards against overheating resulting from insufficient conditioned air passing over the heat exchanger

If the primary limit control does not function during this test, the cause must be determined and corrected. Function of this control should be verified by gradually blocking the furnace return air after the furnace has been operating (burners firing) for approximately ten minutes.

Because your furnace uses an integrated control module (an electronic device that controls all furnace operations) it is certainly possible that the control module, which monitors all of the safety circuits, is not permitting the unit to run with your modification to the safety limit control. The company describes the various safety controls on this equipment as follows:

Primary Limit Control, Goodman Furnace

The primary limit control guards against overheating resulting from insufficient conditioned air passing over the heat exchanger. If the primary limit control does not function during this test, the cause must be determined and corrected.

Function of this control should be verified by gradually blocking the furnace return air after the furnace has been operating (burners firing) for approximately ten minutes. Check the control as follows:

1. Allow the furnace to operate with burners firing continuously for approximately ten minutes.
2. Gradually block the return air to furnace. Remove airflow blockage when limit control is activated and turns off burners. Airflow blockage causes unit overheating and will produce the following reactions:
   1. The gas valve to close and extinguish flame,
   2. The induced draft blower to be de-energized after a fifteen second post purge, and
   3. The circulator blower to remain energized continuously until limit control resets.
3. Remove the return air blockage to clear overheating condition. After an acceptable temperature is reached during the cool down period, the limit control will reset and allow the furnace to resume normal operation.

Watch out: WARNING TO PREVENT PREMATURE FAILURE OF HEAT EXCHANGER, PROPERTY DAMAGE, PERSONAL INJURY OR DEATH, DO NOT ADJUST THE LIMIT CONTROL (FACTORY SET).

Safety Circuit Description for a Goodman Furnace

... These checks establish that the primary limit control is functioning and will respond to a restriction in the return air, or a circulator blower failure. If the primary limit control does not function during this test, the cause must be determined and corrected.

General note on furnace safety circuits

A number of safety circuits are employed to ensure safe and proper furnace operation. These circuits serve to control any potential safety hazards and serve as inputs in the monitoring and diagnosis of abnormal function. These circuits are continuously monitored during furnace operation by the integrated control module.

Integrated Control Module on Goodman Furnace

The integrated control module is an electronic device which controls all furnace operations.

Responding to the thermostat, the module initiates and controls normal furnace operation, and monitors and addresses all safety circuits. If a potential safety concern is detected, the module will take the necessary precautions and provide diagnostic information through an LED.

Primary Limit Control on Goodman Furnace

The primary limit control is located on the partition panel and monitors heat exchanger compartment temperatures. It is an automatic reset, temperature sensor.

The limit guards against the overheating as a resulting of insufficient air passing over the heat exchanger.

Auxiliary limit control on Goodman Furnace

The auxiliary limit control is located either on or near the circulator blower and monitors heat exchanger compartment temperatures.

The control is a temperature sensor. It guards against overheating resulting from insufficient air passing over the heat exchanger.

Rollout Limits on Goodman Furnaces

The rollout limit controls are mounted on the burner/manifold assembly and monitor the burner flame. They are manual-reset, temperature sensors. This limit guards against burner flames not being properly drawn into the heat exchanger.

Pressure Switches on on Goodman Furnaces

The pressure switches are normally-open, negative air pressure activated switches. They monitor the airflow (combustion air and flue products) through the heat exchanger via pressure taps located on the induced draft blower. These switches guard against insufficient airflow (combustion air and flue products) through the heat exchanger.

Flame Sensor on Goodman Furnace

The flame sensor is a probe mounted to the burner/manifold assembly which uses the principle of flame rectification to determine the presence or absence of flame.

References - where to get a Goodman furnace manual

• Goodman Manufacturing Company, L.P., 2550 North Loop West, Suite 400, Houston, TX 77092, www.goodmanmfg.com See Goodman Mfg Gas Fired Central Furnaces INSTALLATION & OPERATING INSTRUCTIONS GAS FIRED WARM AIR FURNACE AMV8 [we attached a copy to our emailed reply to this reader]

See GOODMAN HVAC MANUALS & ERROR CODES for access to Goodman heating and air conditioning equipment manuals and installation instructions.