duct design, and air distribution
Domain 4 tests duct sizing and air distribution: available static pressure, friction rate, velocity, fittings, and room delivery.
Design rules
Use Manual D or an equivalent method. Available static pressure equals the blower's external static pressure minus the pressure drops of the coil, filter, grilles, and other devices. Size ducts to the calculated friction rate, commonly around 0.1 in. w.c. per 100 ft for supply trunks, and check velocity limits. Fittings add equivalent length; flex duct must be fully extended and supported, not compressed. Return air must be sized for the same airflow as supply, and all joints should be sealed.
Worked example
A supply trunk carries 1,200 cfm. A 20 in. × 10 in. duct has 200 in² of area. Convert to square feet: 200 ÷ 144 = 1.389 ft². Velocity = 1,200 ÷ 1.389 = 864 fpm. If the friction chart or velocity limit calls for lower velocity, increase to 24 in. × 10 in. (240 in² = 1.667 ft²), giving 720 fpm. Then check the remaining static pressure budget for the longest run and add balancing dampers.
Air distribution
Supply diffusers must throw air into the occupied zone without dumping or short-circuiting to the return. Check throw, spread, and pressure balance. Bedrooms and rooms with high loads need individual balancing, not just a single central return.
Traps that cost marks
Do not ignore filter and coil pressure drop, undersize returns, use compressed flex duct, omit equivalent length for fittings, or balance by guesswork.