June 12, 2000

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.

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 supply
air flow

(CFM)


 

Net free area
required

 (in2)

 

Area required
after door
undercut

(in2)

Transfer grille height required
for listed width in inches

 

Jump Duct
Diameter
Required

(in)

10
(in)
12
(in)
14
(in)
5027.011.01.51.21.03.7
7540.524.53.32.72.35.6
10054.038.05.14.23.67.0
12567.451.46.95.74.98.1
15080.964.98.77.26.29.1
17594.478.410.58.77.510.0
200107.991.912.310.28.810.8
225121.4105.414.111.710.011.6
250134.9118.915.913.211.312.3
27514.4132.417.714.712.613.0
300161.9145.919.416.213.913.6
325175.4159.421.217.715.214.2
350188.9172.923.019.216.514.8

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)
10611.57.551
12613.57.561
121213.513.5121
14615.57.571
141415.515.5163
202021.521.5336
30631.57.5153