How to Size a Return Air Grille

Start with the airflow assigned to the return, then select a grille whose manufacturer pressure drop and noise data meet the project limits. Duct diameter, nominal grille size and open area are different inputs.

Face area, free area and core area

Gross face area A_face

The area used for a face-velocity calculation. State whether dimensions describe the face, opening or neck; a decorative border is not airflow area.

Free area A_free

The unobstructed area between blades or perforations. Use A_free = f × A_face only when the documented free-area ratio f uses that same face-area basis.

Catalog core area A_core

The manufacturer's area basis for its performance table. It may differ from both gross face and free area; do not substitute a guessed percentage.

Velocity = airflow ÷ area. CFM ÷ ft² gives FPM; m³/s ÷ m² gives m/s. For a rectangular gross area, multiply dimensions in inches and divide by 144 to obtain ft². A velocity limit only makes sense with its stated area basis.

Worked area example: 600 CFM

This arithmetic illustration assumes a 500 FPM free-area velocity and f = 0.60. These are example assumptions, not a universal grille recommendation or a product specification.

1. Free area: 600 ÷ 500 = 1.20 ft².
2. Gross area: 1.20 ÷ 0.60 = 2.00 ft² = 288 in². A 12 × 24 in. rectangle has this gross area; actual catalog dimensions must be checked.
3. Gross face velocity: 600 ÷ 2.00 = 300 FPM.

If 500 FPM means gross face velocity instead, the gross area is 600 ÷ 500 = 1.20 ft². Dividing by 0.60 again would change the question. In SI, 600 CFM ≈ 1,019 m³/h = 0.283 m³/s; 1.20 ft² ≈ 0.1115 m², giving about 2.54 m/s free-area velocity.

Check a real manufacturer performance row

The Titus 355R/355F performance sheet (2017 file, p. H41 / PDF page 1) lists a 14 × 14 in. nominal duct size, nominal duct area 1.36 ft² and core area 1.22 ft². The table uses core velocity; nominal duct area is not gross face area or free area.

610 ÷ 1.22 = 500 FPM checks the area basis. For a 600 CFM project, use the nearby 610 CFM point as an initial comparison, then obtain the selected product's exact duty-point data. Catalog pressures are negative return pressures; use their magnitude in a loss budget. NC assumes 10 dB room absorption and is not a guaranteed installed-room noise level.

Include the filter and the full return path

A filter's resistance needs its own manufacturer curve at the assigned airflow and maintenance condition. A grille free-area percentage cannot predict filter pressure loss. For a filter grille, check whether the published test includes the filter; adding its loss again would double count it.

Illustrative project limits: if the grille allowance is 0.080 in. w.g. and the noise limit is NC 20, the nearby 610 CFM catalog point passes those two screening limits; the 732 CFM point fails both. If an independently specified filter contributes an assumed 0.100 in. w.g., the combined grille-plus-filter loss is 0.171 in. w.g. before ducts and transitions. These limits and filter loss are teaching inputs, not Titus ratings or design recommendations.

Split airflow across multiple returns

Two returns assigned 300 CFM each sum to 600 CFM. Size each grille at its assigned flow and check each route, including room transfer paths when doors are closed. Parallel branches do not add their pressure losses together; check the complete controlling return and supply circuit. Equal airflow is an assumption until balancing confirms it.

Record the selection

1. Write the design airflow per grille and the project pressure/noise limits.
2. Record product model, nominal dimensions, mounting arrangement and catalog area basis.
3. Check pressure and noise at the design flow; include the filter only once.
4. Verify the blower budget, ducts, transitions and transfer paths; measure and balance after installation.

Choose a candidate return duct Check the static pressure budget

Calculate duct airflow and velocity · ACCA Manual D system design

Manufacturer row checked 2026-10-08. Arithmetic examples are illustrative; product availability and current performance data must be confirmed for the selected model.