Air distribution systems based on the forced-air principle of delivery utilize a system of ducts to deliver the heated or cooled air to the various rooms and spaces within the structure. These air ducts are generally rectangular or round pipes made from a variety of different materials. When these ducts are accurately sized and the duct system correctly designed, the air will be delivered to the rooms and spaces with a minimum of resistance, the result being a more efficient operation with reduced operating costs. The purpose of this chapter is to suggest methods for sizing ducts and designing an efficient duct system.
Methods for sizing fans are described in Chapter 7 of Volume 3 (V“ entilation and Exhaust Fans). Chapter 4 of V olume 1 (Sizing Residential Heating and Air-Conditioning Systems)”and Chapter 8 of Volume 3 (Air -Conditioning)”provide information and methods for sizing the heating and cooling units.
Codes and Standards
Always consult local codes and standards first before designing and installing a duct system. Any aspect of a duct system that does not comply with these codes and standards will have to be changed, and these changes could be expensive.
Information about duct sizing, installation methods, air distribution, and air duct design methods is contained in the latest edition of the ASHRAE Handbook of Fundamentals and publications of the Air Conditioning Contractors of America (ACCA). Detailed information about ducts and duct fittings is available from the Commodity Standards Division of the U.S. Department of Commerce.
Types of Duct Systems
The two duct systems most commonly used in forced-warm-air heating are (1) the perimeter duct system and (2) the extended plenum duct system. Both are available in several design modifications and are described in the sections that follow.
Details about the piping arrangements used with gravity warm- air furnaces are included in the section describing these furnaces in Chapter 10 of Volume 1 (Furnace Fundamentals).