Guide to Inspecting & Testing Combination Fan & Limit Switches on Warm Air Furnace Heating Systems
- Honeywell L4064B InspectAPedia® -
A Step by Step Photo Guide to Testing Combination Fan Limit Switches on Warm Air Furnace Heating Systems
Honeywell Combination Furnace Control type L4064 explained in detail
Guide to troubleshooting heating system furnace controls, limit controls, and fan controls
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Here we describe in detail the testing 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 type L4064B.
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.
Step by Step Photo Guide to Testing the Warm Air Furnace Combination Control- Fan Limit Switch
This example is based on advice from the Honeywell Tradeline L4064B. Check the test specifications provided by the manufacturer of your particular control.
Honeywell suggests the following steps, which we have edited and photo-illustrated for clarity and safety:
Check wiring: Assure that the control has been installed and wired in accordance with the manufacturer's installation instructions
Turn off electrical power to the heating system
Disconnect the fan at the fan motor or at the FAN LIMIT control terminal
Turn on electrical power to the heating system
Call for heat: Set the building thermostat to call for heat
Watch the oil or gas burner: The furnace oil or gas burner should turn on and subsequently the FAN LIMIT control should turn the burner OFF when the temperature in the supply air plenum reaches the FAN LIMIT set point.
Watch the dial moving to confirm its operation. IF the burner does NOT turn off as expected, QUICKLY turn off power to the heating system and order further diagnosis.
(This particular gas burner shows some rust marks and flame leakage worries).
Turn off electrical power to the heating system
Reconnect the fan motor where you previously disconnected it
Turn on electrical power to the heating system
Call for heat: Set the building thermostat to call for heat
Watch the fan: The furnace blower FAN should come on when the plenum temperature has reached the FAN ON setting. Again watch the dial move on the FAN LIMIT control.
The furnace blower FAN should turn OFF when the plenum has cooled down to the FAN OFF setting.
This may not happen until after the thermostat has been satisfied (or you turn it back down to a low setting) and the oil or gas burner has shut down.
(The particular furnace has a filter that is insecure in this photo and so the heating system is unsafe as the filter could be sucked into the blower assembly causing overheating and a possible motor or electrical fire.)
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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
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
Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
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.
"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,
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.)
InspectAPedia® Home & Site Map - Building & Environmental Inspection, Testing, Diagnosis, Repair, & Problem Prevention Advice: In-depth research & advice on diagnosing, testing, correcting, & preventing building defects & indoor environmental hazards. Unbiased information, no conflicts of interest.
The Mold Information Center: What to Do About Mold in Buildings, When and How to Inspect for Mold, Clean Up Mold, or Avoid Mold Problems
Environmental Inspection, Testing, & Diagnosis On-Site IAQ, Gas, Air Testing, Mold Investigation, Sick Building Diagnosis, Lab Services, & Remediation Plan Preparation - indoor air quality testing, problem source determination, supporting lab work, written remediation plan addressing removal of environmental and other hazards and prevention of their recurrence.
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