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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.
Blower Fan Won't Stop: What to Check if your Furnace Fan Runs Continuously - (the fan won't shut off on its own)
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.
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.
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. We discuss this switch here.
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
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.
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.
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.
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