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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. What is the Function of the Hot Air Furnace Fan Limit Switch?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.
When does the furnace blower turn OFF in normal operation?
Which Way to Set the White AUTO / MANUAL Fan Control Switch
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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. |
How the Fan Limit Switch Senses the Temperatures Inside a Warm Air Furnace
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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. |
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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
I have a Goodman PGB048075-1 furnace. The high limit switch is bad. I bypassed the limit switch and the heater worked fine, but when I tried to test the air conditioner it wouldn’t 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. [1]
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:
The primary limit control guards against overheating resulting from insufficient conditioned air passing over the heat exchanger. If the primary limit control does not function during this test, the cause must be determined and corrected. Function of this control should be verified by gradually blocking the furnace return air after the furnace has been operating (burners firing) for approximately ten minutes.
Because your furnace uses an integrated control module (an electronic device that controls all furnace operations) it is certainly possible that the control module, which monitors all of the safety circuits, is not permitting the unit to run with your modification to the safety limit control. The company describes the various safety controls on this equipment as follows:
Primary Limit Control, Goodman Furnace
The primary limit control guards against overheating resulting from insufficient conditioned air passing over the heat exchanger. If the primary limit control does not function during this test, the cause must be determined and corrected. Function of this control should be verified by gradually blocking the furnace return air after the furnace has been operating (burners firing) for approximately ten minutes. Check the control as follows:
- Allow the furnace to operate with burners firing continuously for approximately ten minutes.
- Gradually block the return air to furnace. Remove airflow blockage when limit control is activated and turns off burners. Airflow blockage causes unit overheating and will produce the following reactions:
- The gas valve to close and extinguish flame,
- The induced draft blower to be de-energized after a fifteen second postpurge, and
- The circulator blower to remain energized continuously until limit control resets.
- Remove the return air blockage to clear overheating condition. After an acceptable temperature is reached during the cool down period, the limit control will reset and allow the furnace to resume normal operation.
Watch out: WARNING TO PREVENT PREMATURE FAILURE OF HEAT EXCHANGER, PROPERTY DAMAGE, PERSONAL INJURY OR DEATH, DO NOT ADJUST THE LIMIT CONTROL (FACTORY SET).
Safety Circuit Description for a Goodman Furnace
... These checks establish that the primary limit control is functioning and will respond to a restriction in the return air, or a circulator blower failure. If the primary limit control does not function during this test, the cause must be determined and corrected.
General note on furnace safety circuits
A number of safety circuits are employed to ensure safe and proper furnace operation. These circuits serve to control any potential safety hazards and serve as inputs in the monitoring and diagnosis of abnormal function. These circuits are continuously monitored during furnace operation by the integrated control module.
Integrated Control Module on Goodman Furnace
The integrated control module is an electronic device which controls all furnace operations. Responding to the thermostat, the module initiates and controls normal furnace operation, and monitors and addresses all safety circuits. If a potential safety concern is detected, the module will take the necessary precautions and provide diagnostic information through an LED.
Primary Limit Control on Goodman Furnace
The primary limit control is located on the partition panel and monitors heat exchanger compartment temperatures. It is an automatic reset, temperature sensor. The limit guards against the overheating as a resulting of insufficient air passing over the heat exchanger.
Auxiliary limit control on Goodman Furnace
The auxiliary limit control is located either on or near the circulator blower and monitors heat exchanger compartment temperatures. The control is a temperature sensor. It guards against overheating resulting from insufficient air passing over the heat exchanger.
Rollout Limits on Goodman Furnaces
The rollout limit controls are mounted on the burner/manifold assembly and monitor the burner flame. They are manual-reset, temperature sensors. This limit guards against burner flames not being properly drawn into the heat exchanger.
Pressure Switches on on Goodman Furnaces
The pressure switches are normally-open, negative air pressureactivated switches. They monitor the airflow (combustion air and flue products) through the heat exchanger via pressure taps located on the induced draft blower. These switches guard against insufficient airflow (combustion air and flue products) through the heat exchanger.
Flame Sensor on Goodman Furnace
The flame sensor is a probe mounted to the burner/manifold assembly which uses the principle of flame rectification to determine the presence or absence of flame.
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
Reply:
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.
Below at references you will see a link to an example installation instruction sheet for the Honeywell L4064 fan limit switch
Question: stuck pressure switch?
If u get a pressure switch stuck open fault - Julio 8/16/11
Reply:
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.
Question: Olsen HWSV9500 reverse flow furnace blows cool air into the cabin
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
Rick,
A standard warm air heating furnace fan limit switch won't turn on the blower fan until the heating plenum has reached a sufficient temperature; this design is specifically intended to avoid blowing cold air on building occupants.
But I don't think you want to set the fan cut-in to too high a temperature; I worry about overheating the plenum if you push both limits up together (cut in and cut out) and I worry about short cycling the fan if you just push up the bottom limit.
Here are two common causes of a heating furnace blowing cold-air into the room at the start of a heating cycle:
- An incorrect jumper wire setting at the thermostat, particularly with an electric furnace. Quoting from Honeywell's RTH2300 thermostat installation guide: An incorrect setting is noticeable in a gas or oil heating system. When heating starts, you will initially feel cold air coming out of the vents as the fan is running before the furnace has enough time to heat up the air. [3]
- Heating ducts missing insulation. When heating ducts run through a cold space such as an attic, basement, crawl space or a poorly-insulated wall, the ducts may become quite cold between heating-on cycles unless the ducts are well insulated. When heat first turns on heat in air passing through the ducts is lost to the ductwork itself.
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.
So I'd look into better insulation of the ductwork as well as wind shielding for it.
Let us know how you progress - it'll surely help other readers.
Best
Dan
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?
1. I push the thermostat up and the heater fires up
2. After a few minutes waiting for fan to kick in, instead the fire goes out.
3. After 30 seconds , the fan comes on
4. In 15 sec, the unit fires up again and heats seemingly OK
5. After pulling thermostat back down, fire will shut down
and after a few minutes fan shuts off
Thanks,
Timo - 8/10/2011
Reply:
Timo:
If you force a furnace to run with the fan forced OFF, the furnace will rapidly reach the upper limit and the limit switch should turn it off.
WATCH OUT: I do not recommend that "test" since overheating the heat exchanger can cause cracks and expensive and dangerous damage as well as carbon monoxide hazard risks.
Finally, yes if your limit switch is working properly and the thermostat is set properly (calls for heat, fan switched to AUTO) then the fan should come on at the cut-in temp.
You can test the fan operation: on many limit switches you'll see a white knob that can be pushed or pulled to force the fan to constant ON. IF that runs the fan then you can guess that the fan motor and circuit are OK.
Reader followup:
Thanks Dan,
I will get a chance to manually test the fan tomorrow. Other possible causes suggested to me have been too high LP gas pressure and thermostat issues. Hopefully the solution is a simple one.
Thanks again,
Timo
Reply:
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
Hi,
I am need of a fan switch. My part number is L4064T1848 3 89169 S
I cant seem to find it. All I find is L4064B plus other numbers. What does the Letter T mean compared to B or even A I have found? - Max 10/30/11
Reply:
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 - chipmcclain@hotmail.com 11.20/11
Reply:
Chip,
Contact Goodman with the model number and serial number of your heater and you can obtain, probably at no cost, the installation and maintenance manual for your heating system. We give Goodman contact information in our references section below.
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
Reply:
Dolores,
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
Reply:
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.)
Thanks - Jerry 1/31/12
Reply:
Jerry,
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
Reply:
Anon,
Sounds as if the thermostat wiring or control wiring is incorrect or you have a loose or bad connection.
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
Reply:
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
Reply:
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.
- PUSH the white button IN to force the fan to ON or MANUAL or always-running mode.
- PULL the white button OUT to put the fan back into AUTO mode - turning on and off under control of the switch and furnace temperature
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
Reply:
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.
Question:
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
Reply:
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
Reply:
Ted,
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:
I can't say much about unseen wiring for a fan limit switch, but indeed in AUTO mode the blower fan should come on when the furnace plenum has heated up to the ON temperature. Summarizing:
- if the fan control switch at the FAN LIMIT Swithc AND at the Thermostat are both in AUTO and
- the thermostat calls for heat, and
- the furnace burner turns on and
- heats the heat exchanger,
then
- when the plenum temperature reaches the ON temperature level the blower unit fan should come on.
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!
However, it's recently developed a quirk: Sometimes the blower won't turn off, after the furnace cools down.
I've traced the problem to the L4064 fan-limit control unit. The fan-relay switch sometimes is not springing open like it should, but stays stuck in the closed ("fan-on") position. When this happens, I've tried jiggling the rocker arm that toggles the relay, but this doesn't help. Nor does tapping the unit lightly with a stick. The only remedy I've found is to rotate the bimetallic shaft and then let it snap back sharply.
1. Would it help to spray some compressed air around inside the unit, near the relay switch, in case there's some gunk that has collected on the contacts?
2. On the narrow side of the unit (not visible in your photos) there's a tiny imbedded screw that I imagine is part of the fan relay component inside the unit. What's the purpose of this screw? Can tweaking it solve the above problem?
Thanks for a great site! - Roy 11/6/2012
Reply to Roy:
This sounds like the same problem Ted discussed just above.
IF a fan limit switch is not turning off the furnace, I would
- inspect the switch and hot air plenum for dirt and debris and clogging
- replace the switch
I appreciate your cleverness in snapping the switch around but it's risky - forcing spring-loaded rotating parts risks subtle bending of the internal spring parts that can de-calibrate the device and make it unreliable and even dangerous.
So sure you could try cleaning the sensing portion of the bimetallic spring that projects into the hot air plenum, just use air and maybe a soft brush - don't force anything, for the same reason I gave above.
The switch is not costly - I'd conside replacing it.
Please send me (use the CONTACT US link) a sharp photo or two of the scrw that you are discussing so I can research it.
DO NOT try tweaking any screws - again I'm worried about safety.
Reader Follow-Up:
Hi,
You described my problem as the furnace not turning off. However, the furnace turns off fine. The fan is what doesn't turn off (occasionally)
Attached are two photos of the fan limiter on my furnace (Lennox G11).
Photo screw.jpg shows the screw I was referring to, the one that's probably imbedded in the fan-control switch and looks like an adjustment screw. A yellow arrow points to the screw.
Photo limiter.jpg show the fan limiter itself. This photo also has a yellow arrow, pointing to a mounting screw. This screw might be the solution to my problem.
You're probably saying, "That's a mounting screw. How could that possibly cure a fan problem?" The answer is that the fan control switch has been binding, and the binding may be due to that screw being overtightened.
This occurred to me because the fan-control switch works fine whenever I've removed the fan-limiter bakelite assembly for examination.
So yesterday, when the fan once again failed to turn off, I decided to test this theory. I got a screw driver and started to loosen the mounting screw. I hadn't gotten more than 1/32 of a turn before the fan turned off. Coincidence?
Normally, the tension on that screw is probably not a factor. But as you can see in the picture, the fan-limiter bakelite case is damaged. I had broken off the stub that's now held on only by electrical type and a bolt used as a buttress. Since the stub is no longer intact and not longer helping to keep the bakelite case rigid, the case can probably flex a little bit too much. To compensate, the mounting screw may need to be tightened only gingerly.
I'll let you know in a week or so if loosening that screw has solved the problem.
Thanks.
Reply:
Rather than drillband spray, pulling the whole switch assembly should include the helix, no?
Have you tried replacing the fan limit switch assembly? If you do so, keep the old parts for our study
Reader Follow-Up:
Daniel,
I don't remove the assembly, I simply remove the electrical module, by unscrewing its three mounting screws. The assembly case and the helical cylinder stay undisturbed and attached to the furnace.
The backside of the electrical module is solid plastic, with holes only for the three mounting screws. So simply removing the module does not give me any better access to cleaning the inside of the module.
Likewise, removing the entire assembly wouldn't add any benefit. Nor would there be any benefit to removing the helical cylinder. (As I mentioned, the helical cylinder isn't causing any problems.)
The price for a new assembly (helical cylinder and electrical module) is around $190. I haven't found a shop that will sell the electrical module individually, but I imagine it wouldn't be cheap.
Regards,
Roy
Reader Follow-Up: - Fan Limit Switch Problem Solved
Dan,
Problem solved.
To recap the problem: The furnace blower would occasionally not turn off when the furnace cooled down. I traced the problem to the heat-sensor assembly's plastic case, which contains the fan on/off switch.
The solution was obvious once I realized that pushing the rocker arm is what turns the fan switch off. Originally, I thought that releasing the rocker arm is what turns the fan switch off.
The rocker arm has a roller on the end that contacts a cam on the helical-cylinder heat sensor. Yesterday I noticed that, to turn off the fan, the rocker arm had to be pushed further than the cam was now pushing it. (The roller doesn't look like it's worn down, so the problem may actually have been due to my previously damaging and/or poorly repairing the assembly's plastic case.)
So I increased the diameter of the roller by winding one turn of electrical tape around it.
The fan now works like a charm.
It's true that, when the furnace gets running, the cam gets
pretty warm. But it's still cool enough to press my fingers
against, so I'm not worried about the masking tape going
up in flames. - Roy L.
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.
Watch out: But a safety warning: even if temperatures are low enough that you're not worried about that tape burning up, often at high temperatures the adhesive on tape becomes soft and gummy - the tape may not stay in place. If the tape comes off, the problem could be dangerous if it jams the workings of what is intended to be a safety control. A tinkerer might check for a bent rocker arm or a misaligned part, but a heating professional, concerned with safety, would almost certainly replace the entire assembly.
I suggest replacing the control. You might also want to see How to Test the Fan & Limit Control
Reader followup:
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.
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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.
- [1] 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
- [2] 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
- [3] "Quick Installatoin Guide, RTH2300 5+2 Programmable Thermostat", Honeywell Corporation, September 2009.
- Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
- John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: johncranor@verizon.net
- Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
- National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
- "Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
- Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
- The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
- Honeywell L4064 Fan Limit Control Installation Instructions [PDF file]
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides:
- Commercial Building Inspection Courses - protocol ASTM Standard E 2018-08 for Property Condition Assessments
- Home Inspection Education Courses including home study & live classes at eleven colleges & universities.
- Home Inspection Education Home Study Courses - ASHI@Home Training 10-course program.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author. - The Home Reference Book, a reference & inspection report product for building owners & inspectors.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author. - The Home Reference eBook, an electronic version for PCs, the iPad, iPhone, & Android smart phones.
Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter inspectaehrb in the order payment page "Promo/Redemption" space. - The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors.
Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space. - The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
- Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
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The Home Reference Book - the Encyclopedia of Homes
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- Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
- Links to our list of additional information on heating system inspection, repair, maintenance
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