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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
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. 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
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
On the off chance that someone has inadvertently mis-connected or shorted fan control wires at the fan limit switch, also check
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.
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
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.
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