Collapsing building © Daniel FriedmanFurnace or A/C Blower Fan Won't Stop
What to Check if the Blower Fan Keeps Running

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Forced air heating or cooling air handler troubleshooting: blower fan won't shut off:

This article describes what to check if the furnace or forced air blower fan does not stop when you expect it to do so.

These same diagnostics also aid in air conditioning blower assembly diagnosis when an A/C blower continues running. We explain what switches or controls may be set improperly or what else, such as a shorted wire, can cause continuous HVAC fan operation.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

What to Check if your Furnace Fan or Air Handler Blower Fan Runs Continuously

Fan ON AUTO switch settings explainedThese diagnostic steps are for an air handler, blower, or furnace fan that just won't quit running.

Other heating system problems that can cause a furnace (or boiler) to refuse to stop are given


First, confirm that the room thermostat is not calling for heating or cooling.

In the heating season, just set the thermostat(s) to the lowest setting and confirm that room temperature is above that point. In the cooling season do the opposite - set the thermostat to its highest setting and confirm that room temperature is below that point.

You have told the thermostat to turn off the HVAC equipment.

If the equipment was running give it three to four minutes to shut down.

Next: If the blower just keeps running no matter what, there are two places to check switch settings before calling your heating or air conditioning service company in response to a furnace fan or air conditioning fan that just keeps running without stopping:

1. At the room thermostat: the FAN-AUTO-OFF or FAN ON/OFF switch on the room thermostat should be checked.

The fan control should be set to AUTO. You can try turning the switch to OFF too.

Details of checking the thermostat's FAN-AUTO-OFF or FAN-ON switch position when the furnace (or air conditioning) fan won't stop running are


Other snafus such as a shorted thermostat wire can also cause the A/C or warm air furnace fan to run continuously.

Those problems are explained at THERMOSTAT WON'T TURN OFF.

2. At the furnace or air conditioner air handler or blower unit: the fan limit switch may include a switch that manually keeps the fan running.

furnace fan limit switch control switch (C) Daniel FriedmanWe discuss this switch right here. If you don't know where to find the limit control switch on your furnace or air handler,


Some models of Fan Limit Controls such as the Honeywell L4046B229& Universal Fan and Limit Controler, in particular the Honeywell L4064B,W, and R models have a manual fan switch that overrides the fan control to keep the fan running continuously. The white knob and white arrow in our photo point to this manual fan switch.

Take a look at the fan limit control switch. Before pulling the cover off of the switch, just see if the switch includes a control such as the white push-pull switch like the white device shown at the lower left in our photo.

Depending on the position of this switch (pushed-in or pulled out) the fan may be on manual override - causing the fan motor to run continuously.

Check the printing on the switch face to see if you should push the switch in or pull it out to leave the blower on "Auto". ("Man" or "Manual" would be forcing the fan to run.)

Which way to set the manual furnace or air conditioner blower fan switch:

Usually: "Push-in" position on this (Honeywell) fan limit switch is for continuous fan operation and "Pull out" position is for automatic fan operation.

There are some reasons (explained at BLOWER FAN CONTINUOUS OPERATION) to leave the blower fan on continuously for long periods, such as to use an air filtering system to improve indoor air quality.

And in some buildings we may run the blower continuously for more even or better conditioned (heated or cooled) air distribution.

What Else to Check if the Blower or Furnace Fan Won't Stop

Honeywell RTH2300 / RTH221 thermostat fan switch (C) Daniel Friedman On the off chance that someone has inadvertently mis-connected or shorted fan control wires at the fan limit switch, also check


On the chance that there is a shorted thermostat wire, a problem that can also cause the A/C or warm air furnace fan to run continuously


Details about the manual FAN ON or FAN AUTO ON switch right at the furnace or air handler at the manual


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.

Reader Question: Furnace fan keeps running with no heat coming from the furnace

2015/10/22 Anonymous said:

My fan keeps running with no heat on furnace. What can I do?

Reply: 6 steps to getting heat on when the fan runs but the furnace does not

If the furnace does not even try to run, that is if it never even starts, then follow these steps:

  1. Check the thermostat settings to be sure that you are calling for heat (set-temperature is above room temperature).

  2. Check that the thermostat FAN switch is set to AUTO not to ON.

  3. Check that the manual FAN ON switch at the fan limit switch in the furnace or air handler is not in manual ON position.

    See FAN LIMIT SWITCH for details.
  4. Check the furnace primary controller to see if the "reset" button on the control (or on the burner motor) has popped out - you can try pressing the switch in ONCE but don't keep pressing it as doing so is unsafe.

    For oil fired furnaces see RESET SWITCH - HEAT CONTROL or see RESET SWITCH, CAD CELL RELAY or on older furnaces see STACK RELAY SWITCH

    For electric motor reset switches that are present on some (not all) oil burners see ELECTRIC MOTOR OVERLOAD RESET SWITCH

    For gas fired furnaces see SPILL SWITCH, FLUE GAS DETECTOR; other defects such as a bad thermocouple can keep the gas burner from igniting or burning.

    For gas burner thermocouple problems
  5. Check for voltage at the oil burner primary control. This is a simple test to see that power is actually being delivered to the oil burner control. You can use a neon tester, a VOM, DMM, or a voltage detector stick.

    Watch out
    : if you are not expert in safe use of electrical test tools you could be electrocuted - killed. Call an expert. See VOLTS MEASUREMENT METHODS
  6. Check for a bad primary control: If there is voltage to the primary control, no reset buttons are popped, and the thermostat is calling for heat (check that right at the primary control thermostat wires) then I suspect a bad primary control unit itself.

    At that point your heating service tech will perform her own tests and then if needed repair wiring or replace the furnace control.

    Oil heat: see CAD CELL RELAY SWITCH
    Gas heat: see the gas furnace controls cited above at step 4.

If the furnace tries to start but cannot keep running then there is a different problem such as no fuel or an unsafe condition.


A complete list of oil and gas fired furnace controls is at FURNACE CONTROLS & SWITCHES


Continue reading at FAN WONT STOP - THERMOSTAT SWITCH, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see FURNACE FAN WONT STOP FAQs - questions & answers about why the furnace blower fan won't stop running

Or see BLOWER FAN OPERATION & TESTING if your heating or cooling system blower fan itself appears not to be working,

Or see HEAT WON'T TURN OFF for other reasons that a heating system won't stop running

Or see these

Articles on the Furnace Fan Limit Switch

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FURNACE FAN WONT STOP at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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