Forced air heating or cooling air handler troubleshooting: what to chedk first if the air handler's 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.
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These 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.CHECK the FAN-AUTO setting - 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
at FAN WONT STOP - THERMOSTAT SWITCH
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
2. CHECK for a MANUAL - ON Switch - 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.
We discuss this switch right here. If you don't know where to find the limit control switch on your furnace or air handler,
see FAN LIMIT SWITCH.
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.)
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.
On the off chance that someone has inadvertently mis-connected or shorted fan control wires at the fan limit switch, also check
out FAN LIMIT SWITCH INSTALLATION & WIRING.
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
see THERMOSTAT WON'T TURN OFF.
Details about the manual FAN ON or FAN AUTO ON switch right at the furnace or air handler at the manual
at BLOWER FAN CONTINUOUS OPERATION.
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.
If the furnace does not even try to run, that is if it never even starts, then follow these steps:
If the furnace tries to start but cannot keep running then there is a different problem such as no fuel or an unsafe condition.
See DIAGNOSE & FIX HEATING PROBLEMS-FURNACE - home
A complete list of oil and gas fired furnace controls is at
AIR CONDITIONING & HEAT PUMP CONTROLS & SWITCHES full set of all controls
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Continue reading at FAN WONT STOP - THERMOSTAT SWITCH, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.
Or see FURNACE or AC BLOWER FAN WONT STOP FAQs - questions & answers about why the furnace blower fan won't stop running
Or see these
FURNACE or A/C BLOWER FAN WONT STOP at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.
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