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Furnace blower fan limit safety 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. The sketch at the top of this page shows the typical location of a combination fan and limit control such as the Honeywell type L4064B, a control whose installation, settings, testing, & operation are explained in detail here. A link to installation instructions for this control is found at the end of this article. Contact Us by email if you are having trouble finding the information you need. Sketch at page top courtesy of Carson Dunlop Associates.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
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 or NO HEAT - FURNACE. This website 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.
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.
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.
Frequently Asked Questions (FAQs) 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
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. 
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:
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]
Question: Clare Hecla forced-air gas furnace limit switch - fan won't stay on
We have a Clare Hecla forced-air gas furnace with air conditioner. Currently, we're running the AC and have had to replace the limit switch component. The fan will not stay off long enough and we're trying to determine the appropriate settings for the limit switch---we do not have the original furnace manual and the model number information is illegible. Any suggestions on how to determine average/appropriate settings for a limit switch? - Vicki 7/20/2011
If you replaced a fan limit switch with a factory OEM replacement part, the factory settings on the limit switch, which generally pertain only to the heating cycle, should be correct for your unit.
Question: stuck pressure switch?
If u get a pressure switch stuck open fault - Julio 8/16/11
Julio if you are asking about a fan motor reset switch that does not reset, either there is a short in the motor or a bad switch itself. "Pressure switch" - I'm unsure what you mean.
I have an Olsen HWSV9500 reverse flow furnace. My heat pipes are under my cabin in a non usulated crawl space. When my furnace kicks in, it blows cold air from the pipes into the cabin and actually cols it off for the first few minutes. When the heater turns off the fan keeps running for a while again blowing cooler air in the cabin. This results in my furnace constantly turing on anf off just to keep my cabin heated to 20 degrees c. when it is 5 degrees outside. IS there some kind of setting on the furnance that will solve this? Thanks Rick Gagnon 9/5/11
Reply: Causes of furnace blowing cold air at start-up of heating cycle
Here are two common causes of a heating furnace blowing cold-air into the room at the start of a heating cycle:
If you have not recently messed with your thermostat wiring, then iIt sounds as if the root problem is that your ducts are inadequately or completely uninsulated.
Question: New fan limit switch installed, now the burner comes on, then goes off
A service technician installed a new fan limit switch in our older Lennox G12 furnace last year. I had turned off the pilot flame for the summer. Upon setting up the furnace to run this winter, I came up with the following scenario which was copied from a service forum on the Internet in 2007 but no one ever replied to it. I do not understand how the burner limit can be reached without turning on the fan first. I have yet to look at the switch but if it is a Honeywell L4064 W version with auto fan on feature, and if the feature is not utilized, looking at how the switch is constructed, the fan should still kick on when the plenum is hot enough .... before the burner limit is reached. Any thoughts?
Check for a blower fan that is not starting (bad motor, bad fan capacitor) or blocked airflow - conditions that allow the plenum to overheat and thus shut off the fan at the HI limit.
Watch the fan limit switch dial during the system heat-up and you will have a better idea what's happening.
Question: Where do I get a fan switch
Start with the furnace brand name and model number, that plus your serial number takes you to the manufacturer and the right aprt.
Question: Goodman GDT090-4A counterflow furnace inspection
i have a goodman gdt090-4a. counter flow. how would i get to the blower to inspect for dirt on blower? i can not find any instuctions on this on-line, please help, thank you - email@example.com 11.20/11
Question: our furnace fan runs but the furnace won't stay on
Fan runs blowing air, but when you turn the heat on it lights and stays on for a couple of minutes but does not stay on and reach the thermastat temp. What type of problem might this be? thank you - Dolores 12/5/11
When the furnace starts normally won't stay on long enough to satisfy the thermostat it soundsd as if it is overheating - could be blocked airflow and overheating plenum - you need a service call- the system could be unsafe as well.
Question: our furnace won't turn off
we have temp on thermostat for gas heat set at 65. Heat just keeps running and running, never clicks off. When we shut the heat down from the thermostat, heat clicked off fine. When we turn the heat back on from thermostat and set to our desired temp, it just runs and runs and never clicks off. - Anon 12/25/11
Usually when a furnace won't turn off unless you turn down the thermostat that's a pretty good clue that the system is unable to warm the room enough to satisfy the thermostat. This can happen with an undersized system, in very cold weather, with windows open in the room, but it can also happen if something (a dirty air filter, constricted or damaged heating supply ducts) is restricting the delivery of warm air into the room.
Question: furnace burners don't ignite
Furnace fan powers up, ignitor coil heats up, but burners don't ignite. I have four (4) limit switches on the furnace. If I flick the switch on each one to reset them and restart the furnace, it works fine again for about half a day. However, when the furnace attempts to power up for a second cycle, the same thing happens again (burner don't ignite) unless I reset the switches again. Any help? (This is getting old and I'm not looking forward to the trip to the basement to reset the limit switches every 12 hours.)
Check the control wiring connections first; As you are describing a gas furnace, it sounds as if the gas valve is not opening.
Question: replaced fan limit switch, but it only works intermittently
I replaced the limit switch to get the furnace back up and running. It works intermittently and the the aluminum cover where limit takes the reading gets really hot and the limit switch stops the burners and lights up after a few seconds. I know the limit switch had failed but I really havent fixed the problem. My dad actually removed one of the air filters a couple of months ago to replace the electronic air cleaner but may have forgotten. I will wash the cells and get a new air filter to see if i can get cleaner air flow. The fan seems to be running at a similar speed to the other furnace in the house. Any suggestions on what may be the problem? It overheats and shuts down which means i will probably fail the new limit switch. Obviously been operating a dirty furnace for a while but where could i get a problem with dirt accumulating. I also cleaned the plastic hose to the pressure switch. It had some dirt in there. Please help. - Sandy 1/31/12
we recently installed a heatpump to an existing oil furnace,but cant make fancontrol switch to work on ac mode. what am i doing wrong, because it works fine on emergency heat mode. - Anon 7/31/12
just installed a goodman 2 ton, Heat turns on upstairs appropriately but the downstairs does not. thermostat works appropriatly for the a/c. any ideas - Rob 10/25/12
Question: replaced fan and limit controls, now stuff doesn't work
A week ago had the fan and limit controls replaced. Furnace started running fine but then fan blower did not start when thermostat in "auto" position. If I switch to "on" position the blower fan comes on. After a while if I turn fan to "auto" position again the blower will continue to run as needed - then occasionally I can hear the furnace heat up and then shut down and heat up again but the blower does not turn on...I then have to turn the fan setting to "on" once again to get blower to work.... Does the limit control switch need to be adjusted? - Sam 2/28/12
This sounds like you need a service call. But first check that your thermostat wiring is correct, not loose, and that the thermostat is not blocked by dust or debris.
Question: which way do we push or pull the fan switch to go between AUTOMATIC and MANUAL-ON
"(Pull out the button to force the fan to "always on". Push the white button back in to return the fan to automatic operation" - It seems opposite on the diagram you have.. have a look. It says "pull automatic". I'm pretty sure mine is the same unit. I'm having trouble with blower motor always staying on after heater runs... any ideas? Thanks! Rob - 10/17/12
Rob, thanks for the suggesitons; I have revised, corrected and also clarified the article above, adding photos, and tuning up the text. We welcome critique, corrections, or suggestions from our readers.
Question: electric heating elements in our Lennox Elite Series 29 Furnace are not coming on
have a lennox elite series 29, blower comes on but heater doesnt and i have continuity on all four heating elements. any idea whats wrong - Mark 10.28/12
Search InspectAPedia for "Electric Backup Heat Repair, or go to Diagnose & Repair Electric Heat and you'll see our diagnostic procedure. The home page for that topic is at ELECTRIC HEAT, DIAGNOSIS, REPAIR.
I HAVE A LUXAIRE TWO VENT PIPE 90%afue downflow/horizontal gas furnace HI EFFICIENCY HEATER.The intake sucked in a leaf or leaves during the hurricane into the burner box, or they leaves are either stuck in the intake pipe or somehow is blocking the proper flow of vent air thru the system.My diagnosis view port is showing a 3 flash led pattern.The secondarary vent blower continues to run but will not allow system to start.Is their anyway to clear the debris and restart the system - Tom 10/29/12
For safety you need service by a trained technician.
Question: fan control comes on every 10 seconds after initial shut down of burner and fan - about 5 times, then it stays off
Why does the fan control come on about every 10 seconds after the initial shut down of the fan it does it about 5 time befor it stays off - Ted Aston 11/6/2012
Try lowering the FAN ON temperature by 5 degrees or widening the gap between the two lower settings, FAN OFF and FAN ON by about 5 degrees.
Also look for something blocking airflow.
When the fan cycles back on while the burner remains off we figure that the heat exchanger is still so hot that even without the burner being on, the air in the plenum heats back up after the fan cuts off, causing the switch to turn the fan back on.
Question: blower turns on then immediately off, cycles on and off
On my gas fired furnace, the blower will turn on the immediately turn off then immediately turn on, it will run for the cycle then when the blower turns off, it turns off then on then off then on then off. - Phil 11/13/12
I replaced the fan and limit control. The blower will not turn on in the pull auto, I have to push on manual to get the blower to work. Did I wire it incorrectly? - James 11/13/12
Reply to Phil & James:
Phil & James:
If it doesn't, the burner will shut down at the uppermost or LIMIT setting. That's normal and is a safety control function to prevent overheating and damage to the heat exchanger.
Watch out: as you can read in Roy's comments just below, messing with a Fan Limit Control Switch can be risky - even a small bend or misalighment of a part as well as a wiring operation can cause the control to fail to operater properly - presenting some safety worries.
Question: fan cycles back on without burner operating
We have a Lennox G11 furnace, forty years old and still kicking, by cracky!
Reply to Roy:
This sounds like the same problem Ted discussed just above.
Rather than drillband spray, pulling the whole switch assembly should include the helix, no?
Reader Follow-Up: - Fan Limit Switch Problem Solved
Comment & Warning:
Excellent detective work, Roy, I'll be sure our entire conversation appears in the original InspectApedia article as it will surely help other readers. Thanks so much.
I suggest replacing the control. You might also want to see How to Test the Fan & Limit Control
Given your advice about the tape adhesive getting gummy, I'll keep an eye on it, and will think about some other way to take up the slack in the rocker arm. - R.L.
Questions & answersAbout Troubleshooting the Furnace Fan Limit Switch.
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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.