Measuring System Airflow
From Grilles and Registers
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Session Overview
!  How a Balancing Hood Works
!  Using a Balancing Hood
!  Traversing Registers and Grilles
!  Creating a Register Correction Factor
!  Diagnosing Airflow Issues
Overview
Some Airflow Measurement Methods Just
Aren’t Practical for the HVAC Industry
Measuring System Airflow from Grilles and Registers
How the Balancing Hood Works
Skirt captures airflow
from register
Airflow directed evenly
over manifold in base
Manifold averages
airflow velocity
Hood multiplies average
velocity x sq. footage of
manifold (1.23 sq. ft.)
1.23 sq. ft.
Area x Velocity = CFM
Measuring System Airflow from Grilles and Registers
Basic User Instructions
The instructions for an air balancing hood are
pretty simple:
1.  Cover the grille or register completely
assuring the hood is capturing all the airflow
into or out of the register or grille.
2.  Allow the reading to stabilize for several
seconds.
3.  Press the Read button.
4.  Read and record the airflow of the grille or
register on the screen.
Measuring System Airflow from Grilles and Registers
Soffit Grilles
Block off the exposed area of the hood
Measuring System Airflow from Grilles and Registers
Oversized Grilles
25%
Max
You may block up to 25% of grille
when using a balancing hood.
If the velocity remains below 500 fpm
Measuring System Airflow from Grilles and Registers
Baseboard Grilles
Gasket
Make gaskets of 3” high density foam rubber
Measuring System Airflow from Grilles and Registers
Suspended Grilles
Prepare a cutout ½” smaller than the grille to be measured.
Attach to the hood to capture airflow.
Use weather stripping if needed.
Measuring System Airflow from Grilles and Registers
Airflow Traverses
!  Measuring airflow in duct or at the face of a grille or register requires
a procedure called an airflow traverse.
!  A traverse uses an anemometer to measure average air speed.
When measuring air speed, or velocity in the U.S. the unit of
measurement used is Feet Per Minute (fpm).
!  Average velocity through duct or grille is measured and calculated,
then multiplied by area of the duct to find the volume of air moving
through duct. This measurement is Cubic Feet Per Minute (cfm).
Measuring System Airflow from Grilles and Registers
Airflow Traverses
The airflow traverse formula is:
CFM = Area x Velocity
When doing a traverse manually:
1. Measure the area of the duct or the grille in inches.
2. Convert the area to square feet by dividing by 144.
3. Measure and record airflow in fpm at multiple points across the
airstream in a grid pattern, holding the probe at 90˚ to airflow.
4. Multiply the airflow area times the average velocity to find the cfm
passing through the duct.
Measuring System Airflow from Grilles and Registers
Example
!  A duct measures 12” x 12”.
!  Find the duct area by multiplying 12 x 12 or 144 sq. in.
!  Convert the duct area to square feet by dividing by 144.
So for our 12” x 12” duct: 12” x 12” = 144 sq. in.
144
÷
144 = 1 sq. ft.
Measuring System Airflow from Grilles and Registers
!  Next, measure the air velocity in the duct in a grid pattern.
!  Add together all of the velocity measurements and divide by the
number of readings taken. In this case 9 readings.
Add together all
of the velocity
measurements
Divide by the
number of
measurements
taken to find
average velocity
510
500
520
510
520
510
480
490
470
! The last step is to multiply
the duct area of one square
foot by the average fpm to
find duct airflow.
501 fpm x 1 sq. ft. = 501 cfm
4510
9
÷
501 fpm
Measuring System Airflow from Grilles and Registers
Residential Duct Traverses
When measuring airflow in a rectangular duct, drill test holes in one
side of the duct every 2” to 4” depending on size and consistency of the
airflow and stopping to measure with the probe every 2” to 4”.
12” x 8”
Rectangular Duct
Measuring System Airflow from Grilles and Registers
Residential Duct Traverses
When traversing a round duct drill, 2 test holes at 90º. Then take
and record a series of velocity readings in a grid across the airstream
holding the probe at 90º to airflow.
Measuring System Airflow from Grilles and Registers
Where to Traverse a Duct
!  Ideally a straight section of duct 10 times the diameter of the duct.
(5 may do the trick)
!  Measure downstream 80% of length of the duct to determine site
Measuring System Airflow from Grilles and Registers
Anemometers
Hot Wire Anemometer
The tip of the probe this device
contains a small wire or chip that heats
up warmer than the ambient air being
tested.
As cooler airflow removes heat from
the tip. The rate heat is removed is
interpreted by the anemometer in fpm
and displayed on the screen.
It can be inserted through a small hole
in the side of a duct where velocity
readings can be taken.
Measuring System Airflow from Grilles and Registers
Anemometers
Rotating Vane Anemometer
Named for the rotating vane contained
in the instrument.
Vane adapts its speed to speed of the
airstream and air speed is displayed
on the face of the instrument in fpm.
The disadvantage of a rotating vane is
that it cannot easily be placed into the
holes drilled into the side of a duct.
The larger the head, the greater the
area of the airstream it can measure.
Measuring System Airflow from Grilles and Registers
Finding Rectangular Grille Area In Square Feet
Cubic Feet Per Minute is expressed in feet, and ducts are measured
in inches, so we must convert square inches into square feet.
Example:
Find the area in square feet of a 10” x 12” Grille
Area = 10” x 12”
144”
Area =
Area =
120”
144”
.83 Sq. Ft.
Measuring System Airflow from Grilles and Registers
Supply Register Traverse Correction Factors
When measuring airflow through supply registers, a correction factor
is required to offset velocity increases as airflow passes through
the register.
Manufacturer’s published Ak or correction factors rarely match test
instrument or field test conditions. Free area factors are not correction
factors.
The best accuracy is achieved by creating your own correction
factor in the field following ASHRAE Standard 111.
Measuring System Airflow from Grilles and Registers
Supply Register Traverse Correction Factors
Measuring System Airflow from Grilles and Registers
Supply Register Traverse Correction Factors
CFM = Area x Average Velocity x Register Correction Factor
Example:
1.5 sq. ft. x 300 fpm x .65 Correction Factor = 293 cfm
To create a correction factor for a supply register in the field, two
identical “sister” registers on same system with similar airflows are
needed.
Register 1
Register 2, the sister register
Measuring System Airflow from Grilles and Registers
Supply Register Traverse Correction Factors
Register 1 is inaccessible to the balancing hood, but can be reached
to be traversed.
Register 2 is accessible to the hood and also can be traversed.
Register 1 – Access Blocked
Register 2, the sister register
Measuring System Airflow from Grilles and Registers
Supply Register Traverse Correction Factors
Step One: Begin to create the correction factor by measuring
supply register number 2, with the balancing hood. Read and
record register 2 hood reading.
Register 1
Register 2, the sister register
Measuring System Airflow from Grilles and Registers
Supply Register Traverse Correction Factors
Step Two: Measure supply register number 2 again, but by traversing
airflow. Calculate register cfm, apply no correction factor.
Register 1
Register 2, the sister register
Measuring System Airflow from Grilles and Registers
Supply Register Traverse Correction Factors
Step Three: Divide the hood reading from Register 2 by the
traverse reading from register 2 to create the correction factor.
Example:
Hood cfm 100
Traverse cfm 200
= .50 Correction Factor
Register 2, the sister register
Measuring System Airflow from Grilles and Registers
Supply Register Traverse Correction Factors
Step Four: Traverse Register 1 in the exact same manner that the
second register was traversed.
Register 1
Step Five: Multiply the cfm for the first register traverse by the
correction factor created from the second register to find the
actual airflow from the first register.
Example: If the traverse from Register 1 is 220 cfm:
220 cfm x .50 = 110 cfm actual airflow from this register
Measuring System Airflow from Grilles and Registers
Supply Register Traverse Methods
Residential Stamped faced Registers
To traverse a residential stamped face supply registers, hold the probe
perpendicular to airflow on each side of the register, then take 12 to
24 velocity readings.
Measuring System Airflow from Grilles and Registers
Supply Register Traverse Methods
Round Supply Registers
Round registers require an “X’ pattern over the face of the register.
Typically a minimum of 12 velocity readings are required.
If there is a sharp elbow entering the register, straddle the area of
the register with highest airflow. More readings may be required.
While testing around a round register, keep probe at 90º to airflow.
Measuring System Airflow from Grilles and Registers
Supply Register Traverse Methods
Step down registers often require up to 24 velocity readings. They
may have a correction factor of .20 to .40 because the sensing
device on the probe receives airflow directly as it comes off of the
bottom of the louver where air velocity is at its highest.
7
12
8
6
5
1
2
11
9
10
4
13
15 14
3
16
22
23
24
17 18
21
20
19
Measuring System Airflow from Grilles and Registers
Supply Register Traverse Methods
For perforated registers the probe is held one inch off the face
of the register perpendicular to the face. 12 to 24 velocity
readings are typical.
Measuring System Airflow from Grilles and Registers
Diagnosing Airflow Issues
Measure Total System Airflow
1.  Measure each supply register and
add together
2.  Compare to the required system
airflow.
3.  Measure each return grille and add
together
4.  Compare to the required system
airflow
Poor Quality Installations
Diagnosing Airflow Issues
Measure Individual Room Airflow
Room Name
Living Room
Home Office
Master Bedroom
Required
CFM
200
120
130
Actual
CFM
188
38
122
Which Room Would You
Suspect Has a Comfort Problem?
Imbalanced Airflow
Rob Falke
David Richardson
National Comfort Institute
©2015 NCI, Inc.