When measuring external static pressure I know that it is the measurement of the friction of the air against the ducting. It is directly proportional to velocity and referred to as friction loss and measured in inches of water column.
Is there an actual loss of air? In other words, does less air come out of the supply registers than is taken in from the return side due to friction loss? If a well sealed supply duct were measure for air flow, would there be a difference between the measured return air and the measured supply outlets that would be attributable to friction loss?