All furnaces come wired at a default speed and yes they can be changed. The
default speed is based on an assumed external static pressure of .5" WC. If
your system pressure is lower, then you could be moving an excess amount of
air. The only way this could not be lowered is if the unit is shipped with
the heating at the lowest fan speed. If this is the case, an after market
accessory, such as a variable speed drive, could improve the unit's
performance. For more info on drives e-mail me for info at cowlesandco at
hotmail dot com