Find the ceiling diffuser neck size or grille size for an airflow in CFM, L/s or m³/h, keeping the velocity within limits for the room's noise level.
Noise from a diffuser rises steeply with velocity, so outlets are sized to keep the neck (or grille face) velocity below a limit that suits the room. Throw, the distance the air travels before slowing to about 50 fpm (0.25 m/s), then decides the spacing and pattern; it comes from the manufacturer's catalogue.
| Space | Diffuser neck | Sidewall supply grille (face) | Return grille (face) |
|---|---|---|---|
| Bedrooms, hotel rooms, libraries (NC 25–30) | 400 fpm (2.0 m/s) | 400 fpm (2.0 m/s) | 300 fpm (1.5 m/s) |
| Offices, classrooms (NC 30–35) | 500 fpm (2.5 m/s) | 500 fpm (2.5 m/s) | 400 fpm (2.0 m/s) |
| Retail, restaurants, lobbies (NC 35–40) | 650 fpm (3.3 m/s) | 600 fpm (3.0 m/s) | 500 fpm (2.5 m/s) |
| Industrial, plant rooms (NC 45+) | 800 fpm (4.0 m/s) | 750 fpm (3.8 m/s) | 600 fpm (3.0 m/s) |
Airflow at the neck velocity shown, square neck (gross area).
| Neck (in) | Neck (mm) | Area (ft²) | CFM @ 400 fpm | CFM @ 500 fpm | CFM @ 650 fpm | L/s @ 2.5 m/s |
|---|---|---|---|---|---|---|
| 6 × 6 | 150 × 150 | 0.25 | 100 | 125 | 162 | 56 |
| 8 × 8 | 200 × 200 | 0.44 | 178 | 222 | 289 | 100 |
| 9 × 9 | 225 × 225 | 0.56 | 225 | 281 | 366 | 127 |
| 10 × 10 | 250 × 250 | 0.69 | 278 | 347 | 451 | 156 |
| 12 × 12 | 300 × 300 | 1.00 | 400 | 500 | 650 | 225 |
| 14 × 14 | 350 × 350 | 1.36 | 544 | 681 | 885 | 306 |
| 15 × 15 | 375 × 375 | 1.56 | 625 | 781 | 1,016 | 352 |
| 18 × 18 | 450 × 450 | 2.25 | 900 | 1,125 | 1,462 | 506 |
| 24 × 24 | 600 × 600 | 4.00 | 1,600 | 2,000 | 2,600 | 900 |
An office zone needs 1,200 CFM through 4 square ceiling diffusers, so 300 CFM each. At the 500 fpm office limit the neck area must be at least 300 ÷ 500 = 0.6 ft². A 10 × 10 in neck (0.69 ft²) gives 432 fpm, so 10 × 10 in (250 × 250 mm) is the choice; a 9 × 9 in neck would run at 533 fpm, slightly over the limit.
Find the zone airflow with the TR to CFM Calculator or Cooling Load Calculator, size the branch ducts with the Online Ductulator, and convert units with the Airflow Unit Converter.
Divide the airflow for the outlet by the allowed neck velocity to get the minimum neck area, then pick the next standard neck size. For 300 CFM at 500 fpm the neck must be at least 0.6 ft², so a 10 × 10 in neck is used. Check noise (NC) and throw in the manufacturer's data.
About 400 fpm (2 m/s) for quiet rooms, 500 fpm (2.5 m/s) for offices and 650 fpm (3.3 m/s) for retail spaces. Higher velocities give more throw but more noise.
At 500 fpm a 10 × 10 in neck (0.69 ft²) handles about 350 CFM (165 L/s); at 400 fpm about 280 CFM.
Typically 300–400 fpm (1.5–2 m/s) near occupants and up to 500 fpm (2.5 m/s) in ceiling or high-level locations.
The fraction of the grille face that is open for air to pass, after the blades and frame. Typical values are 0.6–0.8; the manufacturer lists the exact Ak or free area for each size.
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