Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
AIR CONDITIONING & HEAT PUMP SYSTEMS
BACKDRAFTING HEATING EQUIPMENT
BACKUP HEAT for HEAT PUMPS
BANGING HEATING PIPES RADIATORS
BLOWER FAN OPERATION & TESTING
BOOKSTORE - InspectAPedia
BTU USAGE MONITORS
CARBON MONOXIDE - CO
CIRCULATOR PUMPS & RELAYS
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DIRECT VENTS / SIDE WALL VENTS
DRAFT REGULATORS, DAMPERS, BOOSTERS
DUCT SYSTEM & DUCT DEFECTS
ELECTRIC HEAT, DIAGNOSIS, REPAIR
ELECTRIC MOTOR DIAGNOSTIC GUIDE
FAN, AIR HANDLER BLOWER UNIT
FLOODED HEATING EQUIPMENT REPAIR
FLUE SIZE SPECIFICATIONS
GAS BURNER Flame & Noise Defects
GAS PIPING, VALVES, CONTROLS
GEOTHERMAL HEATING SYSTEMS
HEAT PUMPS, DIAGNOSIS, REPAIR
HEATING COST SAVINGS METHODS
HEATING OIL PIPING TROUBLES
HEATING OIL TANKS
HEATING SYSTEM NOISES
HEATING SYSTEM TYPES
LP & Natural Gas Safety Hazards
MANUALS & PARTS GUIDES - HVAC
MIXING / ANTI-SCALD VALVES
MOTOR OVERLOAD RESET SWITCH
NOISE, HEATING SYSTEMS
ODORS FROM HEATING SYSTEMS
OIL FILTERS on HEATING EQUIPMENT
OIL FILL PIPE LEAKS
OIL SPILL CLEANUP / PREVENTION
PLASTIC HEATER VENT
PUFFBACKS, OIL BURNER
RELIEF VALVE LEAKS
Reset Switch - Heater Primary Control
RESET SWITCH - ELECTRIC MOTOR
Reset Switch - Stack Relays
SAFETY, HEATING INSPECTION
SAFETY RECALLS CHIMNEYS VENTS HEATERS
SOLAR HEATING SYSTEM DESIGNS
SOOT on OIL FIRED HEATING EQUIPMENT
STEAM HEATING SYSTEMS
THERMOSTATS, HEATING / COOLING
VIDEO GUIDES: Heating System Videos
WINTERIZE A BUILDING
WOOD, COAL STOVES & FIREPLACES
WOOD STOVE SAFETY
Fan limit switch troubleshooting diagnosis & repair: this article describes in detail the testing and diagnosis of problems with warm air heating furnace combination controls, also commonly called the "fan limit switch" on warm air heating systems. As a working example we examine the Honeywell combination fan and limit control type L4064B. We also discuss the diagnosis & repair of common furnace operating troubles such as a blower assembly fan that runs continuously - it may be a simple switch setting.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved.
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.
Honeywell provides a very simple fan limit switch checkout procedure to be used after the switch is installed:
Here, extrapolating from that guidance and adding some field experience, we provide more detailed step by step procedures for troubleshooting and testing the heating furnace fan limit control 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 supplemented and photo-illustrated for clarity and safety.
The fan limit switch temperature settings (shown on the dial sketch) are discussed separately at FAN LIMIT CONTROL SETTINGS.
How to Check the Furnace Fan Limit Control Switch Wiring
Assure that the control has been installed and wired in accordance with the manufacturer's installation instructions using the following procedures.
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 - this means that the "set" temperature on the thermostat must be above the room temperature. For test purposes you can if you like, turn the thermostat all the way "up" to its highest position - temporarily for this test procedure.
Don't forget to set the thermostat temperature back down to the desired level later.
Also if your thermostat has other switches such as "OFF", "HEAT", "COOL" - or "OFF" and "ON" be sure those switches are set to "ON" or "HEAT". More examples and details of thermostat controls are at THERMOSTATS
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 back OFF when the temperature in the supply air plenum reaches the FAN LIMIT set point.
Watch out: a mistake can ruin the furnace or cause a dangerous even fatal carbon monoxide leak. if the fan limit switch does NOT turn off the burner when temperature meets or exceeds the fan limit setting, IMMEDIATELY turn off the gas burner and power to the unit immediately so that you do not damage the heat exchanger by overheating it.
If the Fan Limit switch did NOT work normally, leave the system turned OFF and call for emergency heat service.
If the Fan Limit switch worked normally continue as follows:
Watch the furnace blower 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.
Blower Fan Won't Stop: What to Check if your Furnace Fan Runs Continuously - (the fan won't shut off on its own)
Details for diagnosing and fixing this problem are at FAN WONT STOP - LIMIT SWITCH. But first just take a look at the fan limit control switch. If the switch includes a control such as the white push-pull switch, depending on the position of this switch (pushed-in or pulled out) the fan may be on manual override - causing the fan motor to run continuously.
Details of how to check or set the manual FAN-ON switch at the fan limit control are found at BLOWER FAN CONTINUOUS OPERATION.
According to Honeywell, voltage transients or even other conditions around the limit switch that impact the temperatures to which it is exposed can affect the cut-on or cut-off temperature behaviors of the switch, but the company's instructions to not mention apparently excessive on-off cycling of the furnace blower fan traced to the limit switch itself.
Some conditions that cause unexpected furnace fan cycling on and off may be dangerous, risking overheating of the furnace heat exchanger which in turn risks cracks and even carbon monoxide leaks. So it is worth checking out this problem promptly. We have several suggestions for things to check that can identify and fix problems with the heating system that cause post-heat fan cycling on and off.
Details for diagnosing and fixing a furnace blower fan that cycles on and off repeatedly after the call for heat has stopped, that is after the thermostat is satisfied, are found at FURNACE FAN CYCLES AFTER HEAT.
How to Diagnose & Fix a Blower Fan that Cycles On/Off During the Heating Cycle - furnace short cycling
If your furnace seems to turn off and then back on repeatedly during the heating cycle the problem may be with the HVAC duct system or the problem could be in the heating burner itself or its controls.
Improper blower fan on and off cycling during a heating cycle may mean higher heating costs or even unsafe equipment.
There may be other explanations for this during-heat cycling on and off of the blower fan, but in most systems that cycling is not normal - you will want to find and fix the cause.
Details for troubleshooting a blower fan that cycles on and off unexpectedly during the heating cycle are given at FURNACE FAN CYCLES DURING HEAT
On a call for heat on most warm air heating systems, the furnace heater turns on, the supply air plenum air heats up, the then the blower assembly runs and will often continue to run until the call for heat is satisfied.
Watch out: overheating the heat exchanger is dangerous. If this is going on your furnace should be checked by a professional.
Details for determining why the blower fan shuts off early and fixing this problem are found at FURNACE FAN STOPS EARLY
In some forced air heating systems that are not working properly, when heat called for the furnace delivers blows cold air when it should not, or it alternates blowing cold, then warm air on and off. The root problem may be traced to the ductwork, a dirty air filter, an improperly adjusted control (that may be unsafe) or other sources.
Details of diagnosing a force air heating system blower that blows cold air when warm air should be provided, or a blower that delivers alternating warm then cold air, are found at FURNACE BLOWS COLD AIR
Why won't my furnace blower fan start on its own? It will run if I give it a "spin"
My gas heater isn't working correctly. My thermostat signals the heater correctly and the burner ignites, but the fan doesn't always turn on. If I open the panel and spin the squirrel cage a little, it immediately begins to blow air and runs the heating cycle. What could this be? - Mike.
Reply: Check or replace the blower fan motor start/run capacitor
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.
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
No FAQs have been posted for this page. Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Use the search box below to ask a question or to search the InspectApedia.com website.
Ask a Question or Enter Search Terms in the InspectApedia search box just below.
Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
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.