# RR-0005: Transfer Grille Sizing

Effective Date
Abstract

This report provides transfer grille sizing information excerpted from “BA-0006: Discussion of the Use of Transfer Grilles to Facilitate Air Flow in Central Return Systems.” The goal is to prevent pressurization of individual bedrooms when door is closed.

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?

 Room supplyair flow(CFM) Net free arearequired (in2) Area requiredafter doorundercut(in2) Transfer grille height requiredfor listed width in inches Jump DuctDiameterRequired(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 NominalHeight Width Overall sizeHeight 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