One 16x20 return grille on a 4-ton system: how to size return air before you set the box

Start with the airflow the equipment needs. Contractor David Richardson's example, quoted by HVAC School, pairs a 2-ton unit with 800 CFM, the familiar 400 CFM per ton. At that ratio a 4-ton system moves about 1,600 CFM, and every cubic foot has to come back through the returns.

Grilles are sized by face velocity. HVAC School puts the ACCA Manual D target at 400 feet per minute. When the grille maker's performance data is not at hand, Richardson's rule is 2 CFM per square inch of filter grille, which holds velocity under that target. A 16x20 grille is 320 square inches, or about 640 CFM. Moving 1,600 CFM takes roughly 800 square inches, such as two 20x20 filter grilles. Grille style matters as well: HVAC School's Matt Bruner found a 30x12 stamped-face grille passes 551 CFM at 400 FPM, while a commercial grille with a higher free-area (AK) factor passes 916 CFM.

An undersized return shows up as static pressure. Energy Vanguard founder Allison Bailes writes that manufacturers typically design residential heat pumps and AC equipment to operate around 0.5 inches of water column of total external static pressure, and installed duct systems usually exceed that. When Bruner measured a system he had just rebuilt, much of the excess was on the return side, and the reading dropped sharply as soon as he took the grille off. As static climbs, airflow falls.

To check an install, measure total external static with a manometer at the supply and return plenums and compare it with the equipment's rating. Then add up the free area of every return against the design CFM. Filters belong in that math too: Bailes sizes filter area for a face velocity of 200 FPM or less, which lets a MERV-13 filter run with little pressure drop.