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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.
Installation instructions for this control are found at FAN LIMIT SWITCH INSTALLATION. Contact Us by email if you are having trouble finding the information you need. Sketch at page top courtesy of Carson Dunlop Associates.
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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.
A fan limit switch (shown at above right on an oil-fired warm air furnace and in our sketch at left) where we can see about 3/4 of the silver colored dial where the fan limit switch settings are made.
The fan limit control switch, whose cover has been removed, is about in the center of the photo, and is partially hidden by a low voltage transformer and a metal electrical junction box which are mounted at the right side of the furnace cabinet.
The soot and foil tape above the oil burner assembly also tell us that this system has been operating improperly with back pressure in the combustion chamber. The photo at above left shows the same type of combination furnace control in use on a gas-fired heating furnace.
Note that "Push ON, Pull AUTO" is not constent across all manual fan switches, as we explain below.
Watch out: The fan limit switch is also a safety control which protects the furnace heat exchanger from damage (such as heat exchanger cracking due to overheating) by turning the burner off on the furnace gas or oil burner if the temperature inside the warm air supply plenum (just above or just next to the heat exchanger) gets too high. Sketch courtesy of Carson Dunlop Associates.
[Click to enlarge any image]
This would be an unusual condition but might occur if air ducts were blocked or if someone has fouled up the system controls.
Details: In normal warm air furnace operation, by moving building air across the heat exchanger, the blower is warming air that will be delivered into the occupied space, but at the same time this process is keeping the heat exchanger from reaching too-high a temperature.
If a furnace blower fan fails to start but the furnace heat source (gas or oil fired burner) is running, the heat exchanger would be come overheated and may warp and crack.
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 switch is shown in the left hand photo. For this example we show the Honeywell Tradeline L4064B 2228 combination furnace control. You can see the black switch body, the silver dial providing three temperature control settings shown on the face of the control dial..
That silver fan control dial, driven by a bimetallic spring that responds to temperatures inside the furnace, turns the blower fan on, off, and provides an upper limit temperature setting for safety.
The white button you can see at the lower left corner of the furnace fan limit switch is called the manual fan switch - a manual override which can cause the furnace blower fan to turn on and run continuously regardless of furnace temperature.
Click here to see a closeup of the FAN OFF, ON, and HIGH LIMIT OFF factory settings on this fan limit control.
As we show this control in our photo at left, usually the fan limit switch of this type has a silver cover hiding the switch details but with the white "fan override" button projecting through the cover and visible.
Provided that the fan switch is in AUTO position, when the thermostat has been satisfied and turns off the oil or gas burner at the furnace, the fan limit switch will cause the blower or fan unit to continue to operate just until the temperature at the supply plenum has reached or dropped below the FAN OFF lower limit on the switch - the blue arrow in our photo above. Then the control will turn the blower fan OFF.
When does the furnace oil burner, gas burner, or other heat source turn OFF in normal operation:
On many warm air heating systems, at least during cold weather, the burner or heat source will continue to run all of the time that the building thermostat is asking for heat, and will stop running as quickly when the thermostat is satisfied.
If the furnace oil or gas burner is very high capacity, or if the furnace fan/limit controls have been set to cause this effect, the burner may on some systems cycle on and off periodically while the warm air blower continues to run.
On the silver cover of the fan limit control switch shown in our photo above the cover embossing indicates that PUSHING the button IN forces the fan ON mode.
"PUSH ON" is functionally the same as PUSH MAN (embossed on the switch body itself as we show at left) - it means that you are manually setting the fan to remain on continuously.
On the identical fan limit control shown with its cover off - at left, you can see (click to enlarge) that the text embossed into the switch body says PUSH MAN instead of PUSH ON shown on the switch cover.
Pull this button out to ause the fan to run automatically (AUTO) - meaning that the switch itself will turn the blower fan on and off in response to furnace plenum air temperature. Summarizing:
Thanks to reader Rob for pointing out the confusion about the fan control AUTO - MANUAL switch positions on the fan limit control.
On systems where we have installed high quality air filtration to address an indoor air quality issue, and where the fan is rated for continuous duty, we may pull this switch out to keep the fan on continuously.
Above we've already discussed the controls and settings of the fan limit switch. What we haven't explained is how the switch senses temperatures in the furnace. The fan limit switch contains a bi-metallic spring (shown at left) which is inserted into the warm air plenum of the heating furnace.
As the air in the furnace plenum warms up the bimetallic spring expands, turning a gear which turns the fan limit control dial (shown in the photo above).
As the fan limit switch control dial rotates, mechanical "fingers" on the back of the dial operate electrical contacts inside the switch to turn the fan on or off and at the upper limit to turn off the furnace oil or gas burner as well.
When you move one of the little sliding temperature set points on the face of the dial you're moving the position of the mechanical fingers on the back of the dial.
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 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.
Bypassing the 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]
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Frequently Asked Questions (FAQs)
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: 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: 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:BLOWER FAN WON'T START
(July 19, 2011) Tony said:
I have an old style furnace/ac. The furnace has a honeywell combination furnace control type L4064 on it. When my ac kicks in, the blower does not kick in. I also cannot turn my fan onto the continuous mode on my thermostat at all. But if I pull the manual button on the Honeywell control, the blower will run. Does this Honeywell control box control the fan motor in relationship to the ac as well. The blower works fine when it is in the heat mode.
That fan limit switch that forces the fan to run is a helpful diagnostic - it tells us that the problem you have is not in the fan assembly nor fan motor since those will run. So I infer there is a problem with your thermostat, thermostat wiring, thermostat transformer, or a control circuit in the air conditioner. Start by checking the wiring.
(July 20, 2011) Tony said:
Found the culprit, was a broken brown wire coming out of the main electrical junction box inside the cabinet of the furnace. Did a quick patch job and yohoo, blower motor works in manual mode on thermostat now. Very much appreciated !!!!!!!
Thanks for the feedback Tony - your comment reminds us that if a check for voltage at the blower fan does NOT find voltage and we are sure that power is on to the unit, we need to check for a disconnected or broken wire just as you did.
Question: blower fan short-cycling - keeps turning on and off
(July 26, 2011) Tyler said:
(July 24, 2012) RICK POLK said:
(Aug 3, 2012) wes said:
My blower cuts off and on constantly in 10 to 15 second intervals. It was still cooling the house till i turned it off to prevent further or more severe problems. The unit is less than 2years old. The fan wheel moves freely. Any suggestions? Carrier/Bryant
Rick, check for a wiring or control or control board problem. Also check that the blower fan motor is spinning freely and not overheating, and that there is no loose safety door switch on the blower unit. Sometimes vibration or heat can cause the door sensor to open.
Wes, honesty is best - I'm not sure. I suspect a control board or switch problem.
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