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furnace fan limit switchFan & Limit Switch on Warm Air Furnaces
How the fan limit switch works

  • FAN LIMIT SWITCH - home - CONTENTS: A Guide to Fan Limit Switches on Warm Air Furnace Heating Systems: what is the fan limit switch, how to inspect and set its controls. How to Wire the Fan & Limit Control wire and test the combination fan and limit control on a furnace. Which Way to Set the White AUTO / MANUAL Fan Control Switch. Sensing Furnace Temperatures How to manually turn on a furnace or air conditioning blower fan. Guide to troubleshooting heating system furnace controls, limit controls, and fan controls. Causes of furnace blowing cold air at start-up of heating cycle
  • POST a QUESTION or READ FAQs about Troubleshooting the Furnace Fan Limit Switch
  • REFERENCES
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Furnace blower fan limit safety switch installation & troubleshooting:

This article describes in detail the purpose, operation, setting, installation, wiring, and testing of furnace combination controls, also commonly called the "fan limit switch" on warm air heating systems.

The sketch at the top of this page shows the typical location of a combination fan and limit control such as the Honeywell type L4064B, a control whose installation, settings, testing, & operation are explained in detail here. Sketch at page top courtesy of Carson Dunlop Associates.



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

Photograph of a fan limit switch on a gas fired furnace 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.

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

[Click to enlarge any image]

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 belot 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.

The oil-fired furnace fan limit switch shown 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 switch settings are made (red arrow).

Photograph of a furnace fan limit 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 switch, whose cover has been removed, is about in the center of the photo, and is partially hidden by a low voltage transformer and a metal electrical junction box which are mounted at the right side of the furnace cabinet.

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.

The fan limit control includes these key features

LARGER VIEW of heating furnace fan limit switch

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

Really? the "PUSH ON, PULL AUTO" switch is not constant across all manual fan switches, as we explain in this article.

Fan limit switch settings (C) Carson Dunlop Associates

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 burner off on the furnace gas or oil burner if the temperature inside the warm air supply plenum (just above or just next to the heat exchanger) gets too high. Sketch courtesy of Carson Dunlop Associates, a Toronto home inspection, report-writing tool, and home inspection education company.

[Click to enlarge any image]

This would be an unusual condition but might occur if air ducts were blocked 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.

furnace fan limit switch

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

A fan limit switch is shown in the left hand photo. For this example we show the Honeywell Tradeline L4064B 2228 combination furnace control. You can see the black switch body, the silver dial providing three temperature control settings shown on the face of the control dial..

That silver fan control dial, driven by a bimetallic spring that responds to temperatures inside the furnace, turns the blower fan on, off, and provides an upper limit temperature setting for safety.

The white button you can see at the lower left corner of the furnace fan limit switch is called the manual fan switch - a manual override which can cause the furnace blower fan to turn on and run continuously regardless of furnace temperature.

CLICK HERE to see a closeup of the FAN OFF, ON, and HIGH LIMIT OFF factory settings on this fan limit control.

When does the furnace blower turn OFF in normal operation?

combination furnace control Honeywell L4064B with cover on

As we show this control in our photo at left, usually the fan limit switch of this type has a silver cover hiding the switch details but with the white "fan override" button projecting through the cover and visible.

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

Fan limit switch fan AUTO or MANUAL white button (C) Daniel Friedman

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 - at left, you can see (click to enlarge) that the text embossed into the switch body says PUSH MAN instead of PUSH ON shown on the switch cover.

Pull this button 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:

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

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.

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

fan limit switch bimetallic spring

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

Goodman furnace integrated ignition controlI 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. 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. 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]

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. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

HVAC Blower Fan Articles

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Continue reading at FAN LIMIT CONTROL SETTINGS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

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The basics of how furnaces work can be read at FURNACE OPERATION DETAILS

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