Why does the fan on an HVAC unit kick on intermitently when the unit is turned off at the thermostat?

Answer:

With the thermostat in the off position, the answer would most likely be the fan control or fan switch. This would most likely occur during the summer when temperatures are higher. The function of the control in normal operation is to keep the fan going to extract the balance of the heat from the heat exchanger after the burner cycles off in a call for heat. If it is a Honeywell adjustable, you can hold the dial in place and adjust the pointer that is set to the "lower" temperature of the two pointers. You would increase the setpoint of the pointer a bit. If it is not adjustable, replacing the control would be the answer if you can't live with it.

Answer

It continues to run after shut off for a minute or so to clean the lines of heat/cool air. It also runs intermittantly to recirculate air to filter dust out and even out the temperature.

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