Duct Design and Ventilation
This domain covers duct sizing methods, pressure loss in fittings and flexible duct, and the outdoor air calculations that set ventilation rates in commercial and residential work.
Core rules
- Common sizing methods: equal friction, static regain and velocity reduction. A design friction rate near 0.1 in. w.c. per 100 ft is typical, adjusted for the actual available static and total equivalent length.
- Keep velocities within noise limits: roughly 1,000 to 1,200 fpm in supply trunks, 600 to 900 fpm in branches, and lower near occupied terminals.
- Fittings and flexible duct dominate losses. Flex duct must be fully extended, supported and not sharply bent; its rated friction is much higher than smooth galvanized.
- Seal all joints with mastic or listed tape. Leakage into an attic is lost capacity, not just wasted air.
- ASHRAE 62.1 applies to most commercial buildings using the ventilation rate procedure: Vbz = Rp x people + Ra x floor area, with zone and system effectiveness factors. ASHRAE 62.2 governs dwellings.
Worked example
An office zone of 900 ft2 with 8 occupants, using Rp = 5 cfm/person and Ra = 0.06 cfm/ft2, gives Vbz = (5 x 8) + (0.06 x 900) = 40 + 54 = 94 CFM. With a zone air distribution effectiveness of 1.0, the zone outdoor airflow is 94 CFM. If the layout gives Ez = 0.8, the required zone outdoor airflow rises to 94 / 0.8 = 118 CFM, so the design must deliver more air for the same people and area.
Traps that cost marks
- Using cfm/person alone and ignoring the area component.
- Mixing up 62.1 and 62.2 or applying residential rates to commercial zones.
- Ignoring system ventilation efficiency in multiple-zone recirculating systems.
- Compressing or sagging flex duct, which multiplies its pressure drop.
- Providing exhaust without makeup air, driving the building negative and risking backdrafting.