Duct System Sizing

Without question, the most common cause of premature equipment failure and/or customer discomfort complaints is an improperly designed duct system. When designing a new system, there is no substitute for a complete and accurate load and duct size calculation. Manuals published by Air Conditioning Contractors of America (Manual "J” - catalog no. 1L001 for residential load calculation, Manual "N"- catalog no. 1L007 for commercial loads, Manual "D" - catalog no. 1L004 for duct sizing, etc.) are considered the most universally accepted by architects and permit or code inspectors, although sever- al "short-form" calculation methods are available from equipment manufacturers, as are many excellent load calculation programs for PC computers. The A.C.C.A. manuals and several manufacturers’ versions of PC compatible programs are available at your local R.E. Michel branch.

Many times, however, a technician is called upon to correct or modify an existing duct system. It may be because air con- ditioning equipment is being installed where none was before, to accommodate a building addition, or to correct a comfort problem caused by faulty original design or previous modifications. In these instances it is essential that air volume, veloc- ity and equipment capacity be carefully matched and balanced. An air duct calculator such as Airserco #5125 (catalog no. 2F236) and an accurate air volume gauge such as Dwyer VT172 (catalog no. 4F117) or TIF VA105 (catalog no. 2F422) are indispensible tools for this purpose.

The chart below is a quick reference for evaluating the airflow capabilities of a duct system. Use it as a general guide to quickly determine whether more detailed evaluation of the system is merited.

Note: Sizes in this chart are based on a friction drop of .10" per 100 lineal feet of duct, and .5" E.S.P., accurate for most residential applications.