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AIR CONDITIONING & HEAT PUMP SYSTEMS
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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 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
GAS 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 Plexvent / Ultravent RECALL
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
ZONE VALVES, HEATING
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. 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.
Green links show where you are. © Copyright 2014 InspectApedia.com, All Rights Reserved.
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
Other reasons that an air handler blower fan assembly does not run include
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.
Reader Question: my furnace blower fan will not engage - the burner starts but not the fan
I have recently started using my furnace (20+ years old furnace), and the blower fan will not engage. The pilot and gas burners start, but the fan does not. I have ensured that the fan is set to "auto", on the thermostat. Is there any procedure I can do to troubleshoot the issue? I have seen several topics, however none seem to be helpful. - J.B. 12/31/2013
A competent onsite inspection by an expert usually finds additional clues that would permit a more accurate, complete, and authoritative answer than we can give by email alone. But here are some suggestions to start troubleshooting the problem of a furnace blower fan that does not start:
Step 1: I would start by confirming that the blower fan will operate. Your system probably has a manual FAN ON switch on the thermostat, or if not, there may be a FAN ON push pull switch right on the fan limit switch in the air handler. If you can make the blower run then we know the problem is not with the blower motor or drive belt or assembly and we'd go to step two.
See FAN AUTO ON Thermostat Switch for the switches on the thermostat; for a manual FAN ON switch found on some limit switches in the blower or air handler assembly itself see CONTINUOUS BLOWER FAN OPERATION
If the fan won't run at all,
see FURNACE FAN WONT START - the text section just above this Q&A.
Step 2: I would continue diagnosis by watching the behavior of the fan limit switch as the furnace heats up; presuming nobody has messed with the switch to try to change its settings, it should act as described in the article above beginning at
Continue reading at FAN WONT STOP - LIMIT SWITCH or select a topic from the More Reading links shown below.
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
Question: blower keeps starting & stopping at beginning & end of cycle
(Nov 13, 2012) phil said:
My blower starts and stops and starts at the beginning of the cycle and at the end as well.
Depending on air movement rates and heat exchanger temperatures and fan limiit switch settings, a blower can cool down the plenum at the start of a heating cycle, causing the blower fan to turn off. And at the end of a cycle when the heat exchanger is much hotter, the blower may cool down the plenum, shut off the fan, but residual heat in the heat exchanger can re-heat the plenum, turning the fan back on.
Try widening the ON OFF gap between the FAN ON FAN OFF settings on the limit switch by about 5 degrees. Do not change the upper fan limit safety setting.
(Nov 19, 2012) Mark Brady said:
When heat is called for the furnace burners start and the blower comes on with no delay in a gas furnace. I have two similar furnaces and the other one has a temp delay. I've verified limit switch is turning. And have adjusted the on limit up and down 10 degrees from it's 120 F set point. It just seems like the blower comes on with the burners. And I'd like it to delay like it should.
Mark, I agree that we don't want to blow cold air
I suspect a bad control or a wiring error.
Just check all settings at the thermostat and limit switch
TT set to HEAT, FAN AUTO
limit switch fan control set the fan override to AUTO
shut the whole system off, let it cool down, thunder it on and thermostat up. If it blows cold air immediately most likely it's time to replace the control.
Question: blower fan only works on manual
(Nov 30, 2012) Fan blower works when limit swit said:
Thanks Nick Van Amstel
Nick, see the diagnostics at FURNACE FAN WONT START
Question: fan runs faster in manual than in auto mode
(Dec 7, 2012) Bill McManus said:
Had new thermastat installed last year, after which the fan always turned faster in the manual on mode than in the auto on mode. This year my heat would not come on and my plumber replaced the igniter. I told him about the fan speeds and he said he never heard of that but replaced the fan limit switch and it still runs at two speeds, no change. He said the switch was part of the no heat problem. Does this sound right to you?
Bill if your unit has a variable speed fan it may be that the controls are wired to only use the slower speed in heating mode. Often blowers run at different speeds for cooling than for heating.
Question: fan cycling when thermostats are OFF
(Jan 2, 2013) chris castagner said:
I have a question the fan on my furnace come on and off all the time even when i have both my thermostats cool, off , heat switches to off and the fan switch setting on there is only on and auto and they are both set to auto
Chris I suspect a short circuit in the thermostat wiring or a control problem -
Question: Heat won't turn off
(Jan 2, 2013) Tom said:
I have a Borg Warner unit and the blower and burner keeps running. I installed a new fan limit switch but I noticed once I turned th gas back on the system reached fan on setting the fan came on and when the unit continued to run i
Tom see FAN WONT STOP - LIMIT SWITCH
as a place to start diagnostics. Also check for shorted thermostat wires that keep calling for heat.
Question: burner won't re-start
(Jan 6, 2013) Tiffany said:
Something is wrong with my furnace. The burner will ignite and the fan will come on and go off when the set temperature is reached. However when the temperature drops below the set temperature the burner will not re ignite and the fan will not come on again until I turn the temperature setting all the way down and the switch to off and let it remain off for a few minutes then set the switch to the desired temperature back to where I wanted it and the switch back to heat. Anybody have any thoughts as to what could be the problem? Until about a week ago it was working normally then it started doing this.
Sounds as if the system is overheating and may be unsafe. It's time to call a professional.
Question: gas cycles on and off rapidly
(Jan 13, 2013) JohnBoy said:
gas burners cycles on 10 seconds...off for 20 seconds repeatly. blower has stayed on with out shutting off because it never reaches the t-stat set temperture.Is it the limit or gas valve.Also,can the pilot light on this furnace be converted to a electronic ignition. It is a goodman-janitrol made in 1992
Rapid on-off cycling sounds like a control problem and an unsafe heating system. Shut if off while waiting for the service tech. The problem maybe trivial - a loose wire, but repeated rapid on-off cycling can damage the equipment.
Question: furnace fan won't start
(Jan 22, 2013) Erica Hartley said:
My furnace fan does not come on at all. My husband reached in and made sure it was not locked up. What could this be
(Jan 29, 2013) Steve said:
I instaled a new L4064 because my fan would not come on. Now the burner ignightes and after it warms up the fan comes on, problem is, the fan continuously.I am not 100% sure that I didn't cross up the RED and the YELLOW wires. Could that mistake cause this problem? There are three adjustments, I left them as they were right out of the box because they were the same as the old settings. I called an expert, he said to move the one on the left closter the the one in the center ( center set at about 140-150) so I mover the one on the left from about 100 to about 120 degree and the fan keeps running.This is an area of my home that I rent to someone. I told them to let me know if the heat was working as they are used to. I have heard nothing from them for 4 days, so I am assuming that they are comfortable and that they are not getting cold air from the furnace, but I am not sure about that.
Erica and Steve we diagnos this condition at FURNACE FAN WONT START
Question: no manual fan - on button
(Feb 19, 2013) Carmen said:
I pushed the button in (manual) on my fan limit switch but I am unable to pull it out (auto) because there is not a white button on it. Now when the thermostat is at the set temperature the burners do not go out. What can I use/do to switch it back to auto.
Carmen some fan limit switches do not include the FAN ON manual control.
Question: burner won't re-start
(Feb 2, 2014) Anonymous said:
(Jan 6, 2013)Elliott said:
Anon I don't have a clear picture of what's going on from just your note. It sounds as if the heater is going off on a safety reset. If this is the case, you might get it going again TEMPORARILY by a power-off, wait, and power-on cycle. But such as system is unsafe and needs professional inspection and service.
Question: blower runs but there is no heat.
(Feb 21, 2014) Anonymous said:
blower will come on but heat will not
Anon this sounds like the problem is not the blower but the heater - see our NO HEAT diagnostic articles.
But what's odd is that the blower would come on at all if there is no heat: typically the limit switch won't turn on the blower until the plenum has reached a minimum temperature - to avoid blowing cold air on occupants.
Question: fan won't stop
(Feb 26, 2014) Charles McMillan said:
Have a split system..set temp at 68 and after getting to desired temp the fan keeps running ....the temp went from 68-76 in a hour.
Just disconnect the thermostat wires - if you disconnect at the thermostat (be sure to notice color codes and connection points) then IF the heat keeps running, then
Next disconnect the thermostat wires at their other end - at the heater controller
Then if the heat keeps running the problem is at the heater controller not in the thermostat and its wiring.
Try turning off all power to the heating system; wait five minutes. Then turn power on and see if it behaves properly.
If not it's time to call for service.
You can also search InspectApedia for "HEAT WON"T TURN OFF" to see our diagnostic suggestions.
Question: hot blower housing
(Mar 31, 2014) Darren said:
My Intertherm E3 electric furnace and blower function normally. However when both the blower and elements are off the blower housing becomes to hot to touch. When I cycle the blower the housing cools down but immediately heats back up once the fan is off. I can visually see that the elements are off when this occurs. Any suggestions? Thanks.
Darren I'm not expert on this equipment but what you describe doesn't sound safe. I'd think that when the blower is OFF the heating elements should be OFF for safety and to avoid damaging the equipment. There may be a problem with the limit switch or with wiring.
I'd contact your heating service company or the company - and I'd turn the thing off until this is resolved.
Intertherm Heating and Cooling
Question: how to turn on the gas A/C unit
6/10/14 xenaon said:
i rent from a bad guy. He shut off the wall switch - which shut off the fan. Before, if I turned on the wall switch - the AC went on. Now, it doesn't. Inside my apt the AC and Furnace breakers are fine. 3 hours later, the bully shut off the gas water tank downstairs - turning the switch horizontal, then setting it to off and vacation, so the pilot light is out.
I cannot relight the pilot - turn on the gas line, hold down the pilot valve - won't light.
I cannot find a way to turn on the Rheem Criterian II gas/AC unit. What else needs to be turned on? Thermostat (Honeywell) does not ignite the AC or fan or heat - but it could not possibly be the thermostat as the person who shut off the fan switch - also turned off the water heater.
X I'm a bit nervous about getting between you and the landlord on how to control your heating and cooling, but watch out for unsfae DIY wiring of the controls.
Question: blower fan wont run
6/16/14 Lee Ann said:
I have a carrier heat pump with electric heat. The blower fan will not run with the heat pump, but will run with the electric heat. Any suggestions?
Lee Ann that sounds like a control or control board problem, as you know that the blower itself will operate.
A possibility to check first is that the heat pump is actually working. If we are trying to heat a space and if the heat pump is unable to raise temperatures inside the air handler to a sufficient level, a fan limit switch might keep the blower from ever turning on - as it doesn't want to blow chilly air on people who are clamouring for heat.
Check the FAQs just above, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
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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.