furnace fan limit switch control switch (C) Daniel FriedmanFurnace Blower Fan Stops too Early
Diagnose problems with heating furnace that won't stay on long enough
     


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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.

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

Unsafe return air input at furnace (C) D FriedmanQuestion: 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: 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 ou 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 our REFERENCES 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.

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.

 

Continue reading at FAN LIMIT SWITCH or select a topic from the More Reading links shown below.

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

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