Return air duct sizing

Return duct and grille chart

Return duct sized at 0.05 in/100 ft; grille free area at 400 fpm face velocity, grossed up for a typical bar grille.

Sizes are round equivalents — convert to a rectangular return with the ductulator, which applies the Huebscher equivalent diameter.

The method

return CFM      = supply CFM                     (a balanced system)
return duct ⌀   = equal-friction sizing at 0.05 in/100 ft
grille free area = CFM ÷ 400 fpm                 (sq ft)
grille gross     = free area ÷ 0.75              (bar grille)

Total return capacity must match the blower, not the sum of what looks reasonable room by room. Start from the system airflow — at 400 CFM per ton, a 3-ton system moves 1,200 CFM, so the return path has to carry all of it.

FAQ

How do I size a return air duct?

Size it for the same airflow as the supply, but design it at a lower friction rate — 0.05 in. w.c. per 100 ft instead of the 0.10 used for supply runs. That usually lands one standard size larger than the supply duct carrying the same CFM, which is exactly the point: the return side has fewer, longer paths and far less tolerance for pressure loss.

What velocity should return air run at?

Keep return trunks near 600–700 fpm in a home and branches lower. The grille itself is the stricter limit: face velocity above roughly 400 fpm on a bar grille becomes audible in a quiet room, which is why grilles are sized by free area rather than by duct size.

How big should the return grille be?

Divide the airflow by 400 fpm to get the free area in square feet, then divide by about 0.75 to account for the blades — a typical bar grille passes only three-quarters of its gross size. For 400 CFM that is 1 sq ft free, so roughly 192 sq in gross: a 20 × 10 or 16 × 12 grille.

Does every room need its own return?

Not necessarily, but every room with a door does need a path back. A central return works when doors are undercut or transfer grilles are fitted; without a path, closing the door pressurises the room, cuts its supply airflow and starves the return.

What happens if the return is undersized?

Total system static pressure rises, blower airflow drops below the design CFM, and the coil runs colder than intended — which costs capacity, hurts dehumidification and can freeze the evaporator. Undersized returns are one of the most common causes of a system that measures fine on paper and underperforms in the house.