Measuring System Airflow From Grilles and Registers

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

1.23 sq. ft.

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.)

Area x Velocity = CFM

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.


Block off the exposed area of the hood Soffit Grilles


Oversized Grilles

You may block up to 25% of grille when using a balancing hood.

If the velocity remains below 500 fpm

25% Max


Baseboard Grilles

Make gaskets of 3” high density foam rubber

Gasket


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 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).

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.

Airflow Traverses


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.

510 500 520 510 520 510 480 490 470

4510 9 ÷

Add together all of the velocity measurements

501 fpm x 1 sq. ft. = 501 cfm

! The last step is to multiply the duct area of one square foot by the average fpm to find duct airflow.

501 fpm

Divide by the number of

measurements taken to find

average velocity


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


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.


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


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.

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.

Anemometers


Finding Rectangular Grille Area In Square Feet

Example:

Find the area in square feet of a 10” x 12” Grille

10” x 12” 144”

Area = 120” 144”

.83 Sq. Ft.

Cubic Feet Per Minute is expressed in feet, and ducts are measured in inches, so we must convert square inches into square feet.

Area =

Area =


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.

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 2, the sister register Register 1


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 2, the sister register Register 1 – Access Blocked



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.





Step Two: Measure supply register number 2 again, but by traversing airflow. Calculate register cfm, apply no correction factor.



Example:

Hood cfm 100 Traverse cfm 200

Step Three: Divide the hood reading from Register 2 by the traverse reading from register 2 to create the correction factor.

= .50 Correction Factor

Register 2, the sister register



Step Four: Traverse Register 1 in the exact same manner that the second register was traversed.

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


Register 1


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.



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.



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 11 9 10

22 21 23 20

24 19

5

2

4

3

6

1

13

18

15

16

14

17



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.


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

Measure Total System Airflow

Poor Quality Installations

Diagnosing Airflow Issues

Room Name Required Actual

CFM CFM Living Room 200 188 Home Office 120 38 Master Bedroom 130 122

Imbalanced Airflow

Diagnosing Airflow Issues

Measure Individual Room Airflow

Which Room Would You Suspect Has a Comfort Problem?

Rob Falke David Richardson National Comfort Institute ©2015 NCI, Inc.

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Measuring System Airflow From Grilles and Registers

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