I recently purchased a house with a Heil DC90 furnace. I just realized that the furnace fan never turns off. The heating is working properly and it keeps the temperature in the house just fine. However I am concerned that the fan is always on. The fan control switch on the thermostat is in the auto position.
It seams like it is in a lower speed when the furnace is not actually producing heat, and only goes into a higher mode when the heating/cooling is running. However I am concerned that with it being on 24/7 it will burn through the motor.
Is this a wiring problem with the thermostat, or is this something internal to the unit?

Some folks program them to run like that,, what is brand and model number of the tstat,, would you happen to have the manual for it?