furnace fan starts without thermostat

I have a forced air furnace. The air circulation fan has started coming on whenever it pleases. Even with the thermostat shut off. There is only two position for fan and they are "on" and "auto".

Is there a sensor on the furnace itself which allows the fan to turn on?

Any help is appreciated.

Many furnaces have a thermostat for the fan control. The fan typically comes on when it first calls for heat, then stays on for some time afterwards to cool the heat exchanger off after the furnace stops. It could be a time-delay relay instead of a thermostat that keeps it on for a fixed time after the burner turns off; the time-delay could be mechanical or controlled by the circuit board controller. Somewhere inside the furnace, maybe on the inside of one of the panels will be a diagram showing the controls.

jkbdavis said:

Is there a sensor on the furnace itself which allows the fan to turn on?

Click to expand...

The thermostat is likely wired to a controller that is responsible for signaling ignition. That controller might be jacked up.

I had a prob a couple months ago:
There is a flame sensor which can shut the furnace off if the furnace hasn't properly flamed on a few seconds after the power switch opens the valve for gas. Sometimes, when the flame sensor gets oxidized, it gives a false alarm; it can't see the flame, so the furnace could cycle: thermostat triggers gas, flame sensor shuts gas. thermostat sees it's still cold, triggers gas. Flame sensor shuts gas. So on and so on.

My fix was to sand clean the flame sensor. But sometimes unexpected stops like this can jack up the electronics that control the furnace. Try powering down the furnace and then restarting and see if yr problem goes away...

Thanks folks,

I'll try your ideas. Sounds like that is probably the problem.

Thanks again,