Effective Date
Text
Transfer Grille Sizing Table
- Goal is to prevent pressurization of individual bedrooms when door is closed
- Maximum pressurization of 3 PA (0.012 WIC) is specified
- Transfer grille size is based on supply flow to room
- At master bedroom suite (or any other multi-room suite), supply flows must be totalled, and transfer grille sized based on that total
Given:
Door width: 32 inch
Door undercut: 0.5 inch
Transfer grille width: 10, 12 or 14 inches
Net free area: 0.75 fraction of total grille area
Find:
How high does the return air transfer grille have to be to meet the 3 Pa criteria?
(CFM) |
Net free area (in2) |
Area required (in2) | Transfer grille height required for listed width in inches |
Jump Duct (in) | ||
| 10 (in) | 12 (in) | 14 (in) | ||||
| 50 | 27.0 | 11.0 | 1.5 | 1.2 | 1.0 | 3.7 |
| 75 | 40.5 | 24.5 | 3.3 | 2.7 | 2.3 | 5.6 |
| 100 | 54.0 | 38.0 | 5.1 | 4.2 | 3.6 | 7.0 |
| 125 | 67.4 | 51.4 | 6.9 | 5.7 | 4.9 | 8.1 |
| 150 | 80.9 | 64.9 | 8.7 | 7.2 | 6.2 | 9.1 |
| 175 | 94.4 | 78.4 | 10.5 | 8.7 | 7.5 | 10.0 |
| 200 | 107.9 | 91.9 | 12.3 | 10.2 | 8.8 | 10.8 |
| 225 | 121.4 | 105.4 | 14.1 | 11.7 | 10.0 | 11.6 |
| 250 | 134.9 | 118.9 | 15.9 | 13.2 | 11.3 | 12.3 |
| 275 | 14.4 | 132.4 | 17.7 | 14.7 | 12.6 | 13.0 |
| 300 | 161.9 | 145.9 | 19.4 | 16.2 | 13.9 | 13.6 |
| 325 | 175.4 | 159.4 | 21.2 | 17.7 | 15.2 | 14.2 |
| 350 | 188.9 | 172.9 | 23.0 | 19.2 | 16.5 | 14.8 |
Based on Q = 1.07 X A ∆P0.5 for square edged orifice flow
where: Q = room supply flow (CFM)
1.07 = constant including unit conversions
A = free area (in2)
∆P = pressure difference between room and central area (Pa)
Typical Free Area Measurements of Return Air Grilles
- Ameri-Flow Return Air Grilles
- Data from Grainger Catalog (2001)
Width | Nominal Height | Width | Overall size Height | Free Area (in2) |
| 10 | 6 | 11.5 | 7.5 | 51 |
| 12 | 6 | 13.5 | 7.5 | 61 |
| 12 | 12 | 13.5 | 13.5 | 121 |
| 14 | 6 | 15.5 | 7.5 | 71 |
| 14 | 14 | 15.5 | 15.5 | 163 |
| 20 | 20 | 21.5 | 21.5 | 336 |
| 30 | 6 | 31.5 | 7.5 | 153 |