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airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 8164
Experience:  Furnace, A/C and heat pump specialist.
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My blower fan keeps running after the temperature is reached

Customer Question

My blower fan keeps running after the temperature is reached in the room. I changed the fan limit switch but still have the same problem. Is there another fan control?
Submitted: 5 years ago.
Category: HVAC
Expert:  airheatman replied 5 years ago.
Can you give me the model on the furnace please
Customer: replied 5 years ago.
Lenox G20Q5 / 6E- 150 - e
Expert:  airheatman replied 5 years ago.
Can you do some hands on trouble shooting? If so while I locate a furnace manual, disconnect the wires from the thermostat and see if furnace blower goes off or stays on.We first need to determine whether thermostat or something in furnace is keeping blower on
Customer: replied 5 years ago.
The blower stops running
Expert:  airheatman replied 5 years ago.
Ok it appears the problem is in the thermostat.You do have it set to FAN/AUTO don't you? Have you replaced batteries ?(if digital)
Customer: replied 5 years ago.
It is an old round thermostat 2 wire hook up
Expert:  airheatman replied 5 years ago.

But when you disconnect the wires, the blower stops correct? If you reconnect thermostat does blower come back on with thermostat off?. This sounds strange because blower should come on by furnace control

Customer: replied 5 years ago.
When I reconnect the thermostat the blower does not come on. If I turn the the thermostate up the blower comes on with the furnace. Once the thermostate reaches temperature,the furnace shuts down but the blower keeps running and starts to blow cold air.
Expert:  airheatman replied 5 years ago.
How long will blower run? If longer that 2 minutes, you need a new control board.On the Lennox furnace the fan relay that is apparently sticking, is integrated into the control board. Any other limits would be for shutting down in case of blockage,flame rollout or overtemperature,not for telling the blower to run
airheatman and 2 other HVAC Specialists are ready to help you
Customer: replied 5 years ago.
Is the control board located under the burner, in the blower section of the furnace
Expert:  airheatman replied 5 years ago.

The control board will be in blower section I believe attached to housing of squirrel cage.This is not the ignitor control, but main furnace control.Will look similar to this

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