Furnace blower fan limit safety switch installation & troubleshooting FAQs set #2:

More frequently asked questions (FAQs) about heating furnace fan limit controls, the fan limit switch purpose, operation, setting, installation, wiring, and testing of furnace combination controls, also commonly called the "fan limit switch" on warm air heating systems.

Forced Hot Air Furnace Fan Limit Switch FAQs set #2

Recently-asked questions about furnace fan limit switch controls.

These questions and answers were posted originally at FAN LIMIT SWITCH - topic home, and is a continuation of questions and answers about the fan limit switch found at FAN LIMIT SWITCH FAQs.

Also be sure to read FAN LIMIT SWITCH TROUBLESHOOTING. At the end of this article you'll find a complete list of air handler or fan limit control switch diagnosis and repair articles.

Photo: the exterior of a typical Honeywell fan limit control switch found in the air handler of furnaces - forced warm air heating systems.

[Click to enlarge any image]

 

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.

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: 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: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.

Reply:

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.

To be more clear: the fan limit switch is a heating control that controls the fan on and off in heating mode.

But when you pull the (usually white) manual button on the limit switch, that's an electrical override that simply forces the fan to the "ON" or "run" condition. It's typically not part of the cooling circuit.

Reader follow-up:

(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 !!!!!!!
Thank You from So. Wisconsin

Reply:

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:

Alright I'm trying to figure this one out, any input is greatly appreciated. I have an older Rheem 1991 unit, everything is outside gas heat, ac, and house blower in one unit. The problem I have is at times, without any warning the air handler stops all together, or cycles on and off constantly (when it cycles off there is a faint thump in the duct, but only when it short cycles).

The whole time the air handler is acting up, the compressor runs constantly, or the burners are going (I'm almost certain thats what fried the last set of heat exchangers) Earlier today, it was short cycling and i caught it.

I have an electronic White Rodgers control (changed the batteries, but its also hardwired), s

So I turned it off for twenty minutes, flipped it back on and its been running fine since. I just can't figure out why it is totally random, sometimes late at night, early morning, or over a 100 outside, no difference.

Sometimes its fine for months, other times an hour. Is it time for a new blower, control board? Anyone have this problem with an older Rheem?

Also if i do replace the blower motor, can i install a multi speed, or a higher cfm? Thanks

(July 24, 2012) RICK POLK  said:

A/C starts fine but blower motor shuts off after about 1 minute

(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

Reply:

Tyler:

"So I turned it off for twenty minutes, flipped it back on and its been running fine since." might be a clue. If we turn an air conditioning system off and then back on too quickly, in many designs the compressor just can't get started against the high head pressure it confronts from when it was just pumping.

After you wait an hour or so the head pressure drops - pressures equalize throughout the system and the compressor has an easier time starting.

Don't flip quickly back between heat and cool or "off" and "on" for this reason.

If the problem is recurrent, however, then something else is wrong, perhaps with a control circuit.

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.

Question: fan shuts off after 30 seconds, furnace cycles on and off

(Feb 8, 2014) tom said:

after gas furnace shuts off about 30 seconds later furnace kicks back on and cycles like this untill thermostat heat temp is reached,runs like this all the time

Reply:

Tom,
See FURNACE FAN CYCLES DURING HEAT - - Furnace Blower Fan Cycles On/Off During the Heating Cycle

to read a diagnosis of this problem Tom, and let us know if you still have questions. Thanks. Daniel

Question: heat shut off by landlord

(June 9, 2014) 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.

Reply:

Xeanon your attorney will take a look at your lease to confirm that it's a standard form that entitles you to certain heating and cooling service. You may need to share that information with your landlord.

Do not mess with the controls to the heating system other than the wall thermostat as you risk making the system unsafe.

Question: furnace fan speed varies on its own

(July 7, 2014) Anonymous said:

A/C blows fast then slow by itself.

Reply:

Some AC systems use a variable speed fan under control of the unit.

If your unit is not one of those look for voltage variations, a bad fan run capacitor, or loose wiring.

Question: furnace fan runs when furnace is switched off

(July 22, 2014) Carl W Horneck said:

Why would my furnace fan turn on when my furnace switch is turned off?

Reply:

Carl, if the blower is used also for central air conditioning it can run on a call for cooling when heat is off.

Also at the end of a heating cycle if the fan limit switch senses that temperatures in the heat exchanger and plenum are too high the fan will cycle or keep running to cool down the unit to a safe level.

Question: Bryant Model 355BAV furnace crackling sound

(Sept 15, 2014) Woody said:

I have a Bryant furnace - Model 355BAV. When i turn up the air conditioner / heating there is a crackling sound before the furnace kicks in. The furnace is 2 years old. The sound has been occurring consistently over the last month.

Reply:

Woody I suspect we can't diagnose this sound by e-texting, but before calling the service technician, could the snap sound be occurring in an electrostatic air cleaner at the furnace? If so that may be normal. Let me know.

Question: furnace only works on MANUAL not on AUTO

(Sept 30, 2014) Paul said:

Why would by furnace work on manual but not auto?

Reply:

Check for shorted or broken thermostat wires or a thermostat not calling for heat.

Question: fan does not run on call for heat

(Oct 4, 2014) Will said:

I'm testing my furnace in anticipation of the snow season. The furnace is 8 years old. I can get the fan to kick on, but only on the fan option and not when I switch it over to heat. I have checked the wiring, changed the filters and still no luck.

I'm thinking it is possibly the fan relay switch or I need to replace the thermostat??? I installed a new thermostat, but have the same results. Looking at the control board, all it is showing is a solid green light. Your thoughts.

...

I got the pilot light to engage. It ran for about 30 seconds then shut off. I haven't gotten it to kick on since then.

Reply:

Will if the gas pilot won't stay on your system may need a new thermocouple.

Question: Older Bryant furnace fan runs constantly

(Oct 13, 2014) Trent said:

I am helping out an neighbor who has an older Bryant furnace. The fan has started running all of the time. Even after the burner has shut down. Could there be a problem with the fan limit switch.

Reply:

Trent before replacing the control, check for a manual fan ON switch setting at the thermostat and at the fan limit switch. Also check for control wires shorted together.

Question:

(Oct 18, 2014) tom said:

have a upflow gas furnace for a garage, tempstar (20+ yrs old) with a L4064e limit switch.
burners turn on as it should
fan comes on as it should
then less than 1 min the fan shuts off but the burners stay on.
Can you help me with what the problem is?

its has a no frills thermostat just heat with on and off with the dial, no auto/on fan switch
also fan limit switch is missing the auto/man push button. so I think its in the push man position.

Thanks

Question: factory settings for fan limit switch

(Oct 24, 2014) Alicia said:

What are the factory settings for the fan limit switch for an international furnace?

Question: Carrier furnace locks out every 3 minutes

(Nov 6, 2014) mike said:

My carrier furnace locks out every 3 minutes but the blower keeps blowing alternating warm and cold air blowing into the room. It's a carrier weathermaker. Is there a way to adjust this to turn off the blower during lockout, so I don't get cold air blowing between heat cycles.

Reply:

Mike

The air handler should stop after purging heat from the heat exchanger. It should NOT be stopped early as doing so leaves a hot heat exchanger that can crack the unit, making it unsafe - risking flue gas or CO leaks.

Check that your fan control switch is not in MAN or ON

Question: heater fan cycles on and off constantly

(Nov 7, 2014) Anonymous said:

My inside heater fan goes off and on constantly

Reply:

Anon see these diagnostic articles

• FURNACE FAN CYCLES AFTER HEAT


• FURNACE FAN CYCLES DURING HEAT

 

...

Continue reading at FAN LIMIT SWITCH - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see FAN LIMIT SWITCH FAQs-3 - more recent questions & answers about fan limit switch troubleshooting

Or see these

Recommended Articles

• FAN LIMIT SWITCH
• FAN LIMIT CONTROL SETTINGS
• FAN LIMIT SWITCH INSTALLATION & WIRING
• FAN LIMIT SWITCH TROUBLESHOOTING
• FAN RUNS ONLY ON FAN-ON / MAN
• FAN WONT STOP - LIMIT SWITCH
• FAN WONT STOP - THERMOSTAT SWITCH
• FURNACE BLOWS COLD AIR
• FURNACE FAN CYCLES AFTER HEAT
• FURNACE FAN CYCLES DURING HEAT
• FURNACE FAN STOPS EARLY
• FURNACE FAN WONT START
• FURNACE FAN WONT STOP
• WOOD FURNACE FAN LIMIT CONTROL
• NO HEAT - FURNACE

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