Duct Design, Sizing, and Static Pressure
The Duct Design, Sizing, and Static Pressure domain tests whether you can select duct size from airflow and available pressure, account for fitting losses, and recognize when a design will be noisy or starved. Expect friction-rate calculations, equivalent-length tables, and questions about velocity limits.
Core design rules
Common sizing methods are equal friction, velocity reduction, and static regain. Residential forced-air duct systems are often designed near 0.10 in. wc per 100 ft of effective length, but the correct value comes from the blower's available static pressure divided by total effective length (TEL). TEL is the measured centerline length plus equivalent lengths for elbows, tees, wyes, transitions, and takeoffs. Keep aspect ratio at or below about 4:1; flat, wide ducts cost more friction and space. Limit velocity by application to control noise and pressure drop.
Static pressure
Available static pressure (ASP) equals the blower's rated external static pressure minus allowances for components not included in that rating, such as a coil, filter, or heater. Friction rate (in. wc/100 ft) = ASP × 100 ÷ TEL. Use the manufacturer's fan table and duct friction chart to pick a size, then verify velocity.
Worked example
A blower is rated 0.50 in. wc ESP. The coil and filter allowances total 0.15 in. wc, leaving ASP = 0.35 in. wc. TEL is 250 ft. Friction rate = 0.35 × 100 ÷ 250 = 0.14 in. wc/100 ft. At 600 cfm, read the duct chart at 0.14 and select the size whose velocity is acceptable. If the chart gives about 10 in. round, check that the velocity is not above the limit for the space.
Traps
Do not design from 0.10 in. wc/100 ft without checking available static. Do not use total pressure instead of static pressure. Do not omit fitting equivalent lengths or deduct a coil drop that the rating already includes. Do not size only for CFM and ignore noise.