For a heating and cooling system to operate at their design energy efficiency, the equipment must not exceed the TESP (Total External Static Pressure) the manufacturer has listed on the Engineering Data Plate located inside the heating equipment.

If you open the bottom attachment, you'll notice on the lower left hand side the TESP is .05 inches. Anything higher than .05 static will cause the energy efficiency of the equipment to drop like a rock. A variable speed motor is slightly higher at .08.