Guide to Combination Fan & Limit Switches on Warm Air Furnace Heating Systems
InspectAPedia® -
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 and test the combination fan and limit control on a furnace
Honeywell Combination Furnace Control type L4064 explained in detail
How to manually turn on a furnace or air conditioning blower fan
Guide to troubleshooting heating system furnace controls, limit controls, and fan controls
Questions and Answers About Troubleshooting the Furnace Fan Limit Switch
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.
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The photo at the top of this page shows all of the controls and wiring terminals in a Honeywell combination fan and limit control type L4064B. A link to installation instructions for this control is found at the end of this article. Sketch at page top courtesy of Carson Dunlop
The basics of how furnaces work can be read at FURNACE OPERATION DETAILS and the key heating furnace components are introduced at FURNACES, HEATING. This website discusses these systems and heating components in detail in articles listed at the left of these pages. If your heating system is not working properly, see NO HEAT - BOILER / FURNACE DIAGNOSIS.
This website answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect
residential heating 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.
What is the Function of the Hot Air Furnace Fan Limit Switch?
The warm air furnace fan limit switch (shown at above left on a gas fired warm air furnace) is a control which determines when the hot air furnace blower assembly turns on and off.
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.
A fan limit switch (shown at above right on an oil-fired warm air furnace and in our sketch at left) where we can see about 3/4 of the silver colored dial where the fan limit switch settings are made.
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.
The 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 photo at above left shows the same type of combination furnace control in use on a gas-fired heating furnace.
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
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.
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.
On the fan limit control, the white button pulled out to force the blower fan to run continuously or pushed back in to cause the fan to run automatically when the furnace is warm.
(Pull out the button to force the fan to "always on". Push the white button back in to return the fan to automatic operation).
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.
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.
When does the furnace blower turn OFF in normal operation?
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 until the temperature at the supply plenum has reached or
dropped below the "cut-in" or "fan-on" lower limit on the switch.
When does the furnace oil burner, gas burner, or other heat source turn OFF in normal operation:
On most heating systems 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 soon as 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.
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.
Questions and Answers About Troubleshooting the Furnace 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. 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:
Allow the furnace to operate with burners firing continuously
for approximately ten minutes.
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:
The gas valve to close and extinguish flame,
The induced draft blower to be de-energized after a fifteen
second postpurge, and
The circulator blower to remain energized continuously until
limit control resets.
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 pressureactivated
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]
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For details about the controls, components and switches commonly found on hot hot air heating systems see the articles listed below in which we explain how to identify, set, re-set, repair, replace, or avoid problems with the components of a furnace or warm air heating system.
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
[1] 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
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
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
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.
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
"Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"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,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
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.)
Links to our list of additional information on heating system inspection, repair, maintenance
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