Volume and Pressure of Air

Volume and Pressure of Air
The amount of air available to provide comfort to the rooms is determined by the equipment
selected to meet the loads of the house. In energy efficient houses with lower loads, properly
selected equipment will have less capacity to supply air, and the performance of the system is
dependent upon making the best use of the available air. The volume of air the equipment is
rated to deliver is determined at a specific pressure. Every item, including the ductwork used in
the airway, adds to the pressure loss within the system.
2.1.2 Ductwork Materials and Configuration
The ductwork that is used to achieve the delivery of air from the equipment to the room can have
a great impact on comfort in the room. The capacity of a duct to carry air is affected by the
resistance within the duct. Typical materials used to make up ductwork are galvanized steel that
can be formed into round or rectangular shapes, foil-faced fiberglass duct board that can be
formed into angular shapes in the field, or an insulated flexible fabric round duct. Due to the
varying friction factors of the different materials, the type of material must be considered in the
design process. Increased friction or duct length adds to the total pressure loss. Galvanized steel
material ductwork has a lower friction rate than that of flexible duct due to the smoother inside
surface of the steel duct; therefore, the fittings chosen will have a greater impact on friction
losses in the system. Because air travels easiest in a straight line, a turn in the ductwork will
increase the effective length of the duct by more than just the physical length of the turn. For
example, a 90° bend will add as much as 35 additional feet of effective length to a duct run,
depending on the construction of the ducts. Effective length values for common fittings are listed
in Appendix 3 of ACCA Manual D (Rutkowski 2009).