Forced warm air heater troubleshooting: blower fan stops too soon:

This article describes what to check if the warm air heating blower fan seems to stop sooner than it should. Some conditions that cause unexpected furnace fan operation may be dangerous, risking overheating of the furnace heat exchanger which in turn risks cracks and even carbon monoxide leaks. So it is worth checking out this problem promptly.

This article series describes how to diagnose & fix just about any problem with forced air heating & cooling systems.

How to Diagnose & Fix a Blower Fan that Stops Before Thermostat is Satisfied

Question: our fan limit control shuts down the furnace before the thermostat temperature was satisfied

I tested the fan/limit switch by removing the fan connection on the switch.

The burners fired up, went through the cycle and cut off just as it should. I reattached the fan, the system kicked in and the fan came on.

The furnace ran for about 20 minutes, burners cycled on and off normally, but then the system shut down before satisfying the desired temperature.

What's wrong? Howard Curtis 1/4/12

Reply: which temperature are we checking, and where?

Normally a furnace blower fan will not turn on until the burner has heated air in the furnace plenum to the ON temperature on

the FAN LIMIT SWITCH.

The burner will stay on and the blower will continue to run on most forced warm air heating systems until the room thermostat is satisfied. At that point the burner turns off; the blower continues to run for a brief time to cool down the heat exchanger and supply plenum.

Watch out: locating a return air inlet close to a heating furnace, particularly a gas or oil fired heater may be unsafe - as shown in our photo above.

See details at UNSAFE DUCT OPENINGS.

Watch out: If your heating system is shutting down before the room thermostat has been satisfied, the system may be unsafe. In particular, if the supply air plenum gets too hot the LIMIT on the FAN LIMIT SWITCH may be turning off the burner for safety reasons.

We don't want to overheat the heat exchanger or supply plenum - doing so can crack the steel, resulting in potentially fatal carbon monoxide poisoning of building occupants.

Several defects can cause early burner shut-down such as:

• a dirty air filter reducing air flow through the heater
• a dirty blower fan assembly that reduces air flow through the heater
• improperly set controls
• shorted wiring
• other stuff I haven't thought-of

Howard make sure you are considering the right "temperatures" and fan connections? Recapping:

The set temperature at the room thermostat is the desired room temperature. As long as the room temp is below the set temperature, (with a slight technical exception around the heat anticipator circuit) the thermostat will continue to call for heat.

On a call for heat on most warm air heating systems, the furnace heater turns on, the supply air plenum air heats up, the then the blower assembly runs and will often continue to run until the call for heat is satisfied.

But in some cases, such as blocked airflow due to a dirty air filter, the supply plenum could reach an abnormally high temperature and the fan limit switch will then shut off the burner for safety. In such cases, the furnace heats up to the FAN OFF high limit before the room thermostat is satisfied.

Watch out: overheating the heat exchanger is dangerous. If this is going on your furnace should be checked by a professional.

Blower Fan Turns Off Before Reaching the High Limit Setting

Reader Question: My furnace turns off at 20C even thermostat is set to a High Limit of 35C.

My furnace turns off at 20C even thermostat is set to max 35C. New thermostat was installed I have looked and monitor the fan limit switch.

The silver dial is moving between ON and OFF position and never get to the Limit position. Just wondering if there is something wrong with Fan Limit Switch and how do I reset the Fan limit switch.

Appreciate - Dungthieu@optusnet.com.au 5/27/2012

Reply:

Dungthieu

Watch out: The furnace should never reach the HIGH LIMIT in normal operating conditions - that is a safety off switch to prevent damage due to overheating.

So in normal operation the furnace plenum temperature will reach the OFF limit and turn off the burner. The blower assembly will continue (normally) until the plenum has cooled down. It sounds as if your furnace is working normally.

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.

At FAN LIMIT SWITCH INSTALLATION & WIRING you will see a link to an example installation instruction sheet for the Honeywell L4064 fan limit swithc and

in further details and copies of installation instuctions are in a ourReferences or Citations section below.

Reader 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: 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.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-12-16 by Dj

my furnace will fire up and blower kicks on same time it will reach high limit the everything shuts off the blower does not kepp running to cool down. fyi new blower limit switch

On 2020-12-03 - by (mod) -

Joe

Sorry I can't be smarter but guessing from remote is always a bit of a pig-in-a-poke.

Better to be on the safe side.

Keep me posted - what you do or find works will help other readers.

On 2020-12-03 by Joe U

Thanks for the tips, I'm going to have a pro look at it as it sounds like it's at least worth doing so for precautionary reasons. Appreciate your help!

On 2020-12-03 - by (mod) -

That still could be quite correct, Joe.

Consider that the temperature is falling down through the FAN-ON setting so once it's below the FAN ON it makes sense that the blower would stop.

But I agree that the control could be damaged - we could ask why not keep running until the temp falls down to FAN OFF.

I'm a little worried that someone has been fooling with the settings and may not have known how critical it is to avoid bending the internal spring in the limit control. If you just try moving the pointers away from factory settings (and your settings don't sound like factory) without holding the dial at the same time, you bend the bimetallic spring and the fan switch is damaged and out of calibration.

The worry is that the high end or FAN LIMIT may not shut the system off when it should, risking a cracked heat exchanger and fatal carbon monoxide poisoning.

I might be inclined to replace the limit switch.

On 2020-12-03 by Joe U

Maybe to clarify this the sequence of events I'm seeing
1. Thermostat calls for heat
2. Burners ignitte
3. Fan limit wheel turns and once it reaches the FAN ON the fan turns on
4. X number of minutes later the thermostat stops calling for heat
5. Fan limit wheel turns counter clockwise and once it reaches FAN ON the fan turns off.

On 2020-12-03 by Anonymous

Right so with mine the fan is turning on and turning off at the FAN ON setting. So I'm wondering if this indicates a defective fan limiter.
Thanks!

On 2020-12-03 - by (mod) -

No, Joe but I can see why you'd ask as those fan on-off labels can be confusing.

1- FAN OFF: the low temperature at which the furnace blower fan will STOP to prevent blowing cool air onto room occupants.

Typical factory setting: 90°F

2- FAN ON: the temperature at which the blower fan will turn ON to begin to deliver heat to the building's occupied spaces.

Typical factory setting: 100°F

3- FAN LIMIT: the high temperature at which the high limit switch will open (turn OFF) to stop the burner to prevent overheating of and damage to the heat exchanger. Such damage would make the furnace unsafe.

Typical factory setting: 200 °F

By keeping the fan off until the supply air plenum is warm, 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.

Details about setting the temperature controls on a fan limit switch are

at FAN LIMIT CONTROL SETTINGS https://inspectapedia.com/heat/Fan_Limit_Switch_L4064B_Settings.php

Also see the general explanation of this control at FAN LIMIT SWITCH https://inspectapedia.com/heat/Fan_Limit_Switch_Guide.php

On 2020-12-03 by Joe U

I have a 1970s Lennox and when the burners fire once the limit switch reaches the "FAN ON" settings (120F) the fan turns on as expected. But once the burners turn off and the heat exchanger cools the fan turns off at the same FAN ON setting rather than going past it and turning off at the FAN OFF setting at 100F. Defective limit switch?

On 2020-02-21 - by (mod) -

Jeff cleaning the flame sensor (is this a gas or oil fired unit? ) was a reasonable step but you could still have a bad sensor (cad cell eye for oil or thermocouple for gas) or even a bad electrical connection.

The fact that you have to power-off the system and power it back on to get it to try to start does suggest that a safety device is shutting down the system.

The article above describes why a fan limit controller might be shutting off a furnace before the call for heat is satisified.

And of course other defects can cause a furnace to shut down such as gas spillage, defective burner adjustment, poor exhaust draft, etc.

Sometimes you can get a clue of improper operation by watching the burner flame, damper, and listening to the burner - but I'm flying too blind here (not even knowing fuel) to speculate further.

Watch out: shutting off by a safety control means that the system is unsafe and needs service and repair.

On 2020-02-21 by Jeff

I have a 15 yr old Goodman furnace that runs 15-20 minutes and shuts down before set point is met. It will not fire back up without powering down and back up. I cleaned the flame sensor, pulled out the filter and it still does it. I did replace the vent motor assembly about 2 weeks ago and everything was fine until last night. Whats my next step? No error codes on the board or anything.

Reader 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:

1. if the fan control switch at the FAN LIMIT Swithc AND at the Thermostat are both in AUTO and
2. the thermostat calls for heat, and
3. the furnace burner turns on and
4. heats the heat exchanger,
   
   then


5. 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.

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

Or see FURNACE FAN STOPS EARLY FAQs - Q&A about diagnosing and fixing a blower fan that seems to stop before it should, posted originally at this page

Or see these

Recommended Articles

• AIR HANDLER / BLOWER UNITS - home
  • BLOWER FAN CONTINUOUS OPERATION
  • BLOWER FAN CYCLES ON INTERMITTENTLY
  • BLOWER FAN OPERATION & TESTING - home
  • BLOWER FAN WONT START
  • FURNACE FAN CYCLES AFTER HEAT
  • FURNACE FAN CYCLES DURING HEAT
  • FURNACE FAN STOPS EARLY
  • FURNACE FAN WONT START
  • FURNACE FAN WONT STOP
• FAN, AIR HANDLER BLOWER UNIT
• FAN LIMIT SWITCH
• FAN LIMIT SWITCH TROUBLESHOOTING
• FAN MOTOR START CAPACITORS
• FAN ON AUTO MAN THERMOSTAT SWITCH
• FAN WONT STOP - THERMOSTAT SWITCH

Suggested citation for this web page

FURNACE FAN STOPS EARLY at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING FURNACES

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