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furnace fan limit switch

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
Our site offers impartial, unbiased advice without conflicts of interest. We will block advertisements which we discover or readers inform us are associated with bad business practices, false-advertising, or junk science. Our contact info is at InspectAPedia.com/appointment.htm.

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.

Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2010 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Step by Step Photo Guide to Testing the Warm Air Furnace Combination Control- Fan Limit Switch

LARGER VIEW of heating furnace fan limit switchThis 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







  • Heat control switch

    Turn off electrical power
    to the heating system










  • Furnace blower motor


    Disconnect the fan
    at the fan motor or at the FAN LIMIT control terminal








  • Turn on electrical power to the heating system

  • Honeywell wall thermostat

    Call for heat
    : Set the building thermostat to call for heat












Gas burner on furnaceWatch 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

  • Furnace blower assembly 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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Technical Reviewers & References

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

FURNACES, HEATING
  DUCT SYSTEM DEFECTS
  DUCT & AIR HANDLER ODORS
  FAN LIMIT SWITCH
    What is a Fan & Limit Switch?
    Sensing Furnace Temperatures
    How to Set the Fan & Limit Control
    How to Test the Fan & Limit Control
    How to Install the Fan & Limit Control
    How to Wire the Fan & Limit Control
  FURNACE CONTROLS & SWITCHES
    CAD CELL RELAYS
    STACK RELAYS
    DRAFT HOODS
    SPILL SWITCHES
    DRAFT REGULATORS
  FURNACE OPERATION DETAILS
  HEAT EXCHANGER LEAKS
  STACK RELAY SWITCHES

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.

  • Hot Air Heating Furnace Basic Operating Steps
  • Cad Cell Relays on oil fired furnaces or boilers as flame sensors & safety devices
  • Draft Hoods on gas fired heating equipment, function and safety
  • Draft Regulators & barometric dampers on oil fired heating equipment
  • Electrical Power Switches: Where to Turn On or Off Heaters for Furnaces & Boilers, Heat Pumps or Electric Furnaces or Air Conditioners
  • Fan Limit Switch on hot air furnace heating systems gas or oil fired
  • Flue Gas Spill Switch on gas fired heating equipment sense combustion and protect from blocked flues
  • Gas Piping, Valves, Controls: Guide to LP and natural gas meters, valves, tanks, piping, gas leaks, gas regulator assembly & other gas controls
  • Oil Burners: Guide to Oil Burners for heating systems, boilers & Furnaces: basic parts, operation, maintenance, performance & money-saving tips
  • Oil Tanks: Guide to Heating Oil Underground & Above ground Oil Storage Tank Leaks, Testing, Problems & Solutions, Home Buyer's / Home Owner's Guide
  • Oil Piping: A guide to heating oil piping, valves, controls, leaks, repairs for heating systems
  • Power Switches to turn on or off heaters for furnaces & boilers, oil & gas fired, heat pumps or electric furnaces or boilers
  • 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.
  • ...
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