Since air balancing is a recurring theme in HotMail articles, I receive many calls and emails from those unfamiliar with the air balancing profession and its test instruments. Many inquire with basic questions about balancing hoods and how they’re used to measure airflow and quantify the performance of an HVAC system. This article provides answers to those questions that may be beneficial to novices as well as some of the air balancing masters.

Let’s start at the beginning. Airflow measurement began over 250 years ago. In those days airflow was measured using a liquid filled water manometer and a pitot tube. By using a pitot tube, air balancers measure velocity pressure and convert it to velocity. The average velocity was then multiplied by the area of the duct to determine CFM.

As registers were added to air moving systems another layer was added to the testing as a correction factor, and was applied to the register velocity reading to determine register CFM.

Using this method, the airflow from a register could be measured in about half an hour. In the 1970’s, an engineer named Ernie Shortridge created the first air balancing hood. We owe a debt of gratitude to Mr. Shortridge every time a hood is placed on a register and it measures airflow in about 15 seconds.

Balancing hoods have come a long way since those early days and have become the go-to test instrument when measuring airflow at grilles and registers. As easy as it is to measure airflow with a balancing hood, guessing airflow is still widely used in the industry today.

Practical Applications of a Balancing Hood

Air balancing hoods can allow you to quickly and effectively measure airflow out of a supply register or into a return grille. By developing the skills and judgment needed to use an air balancing hood you will be able to:

• Effectively and accurately measure grille and register airflow

• Compare actual grille airflow to required grille airflow, then you can adjust airflow to the required amount and solve room comfort problems

• Be able to calculate the delivered BTUs into the building by measuring delivered airflow and temperatures at the grilles and applying BTU formulas

• Prescribe solutions for poorly performing systems.

How a Balancing Hood Works

When an air balancing hood measures airflow, it gathers a series of readings across a manifold found in the base of the hood. The hood then averages the velocity of the captured airflow and multiplies the average velocity by the area of the manifold to calculate CFM. The answer appears on the display of the hood.

Here’s how it works:

1. The skirt of the hood captures airflow from the register

2. Airflow is directed evenly over the manifold in the base of the hood

3. The manifold averages the velocity of the airflow passing over it

4. The hood multiplies the average velocity of the airflow times the square foot area of the manifold, for example the base of a TSI model 8371 measures 1.23 sq. ft.

5. The hood applies the formula Area x Velocity = CFM.

Here’s an Example

A supply register discharges its airflow into the skirt of the hood. The air stream passes over the manifold at an average velocity of 100 feet per minute. The hood multiplies the average velocity by the area (in square feet) of the manifold:

100 FPM x 1.23 sq. ft. = 123 CFM

Basic User Instructions

The basic user instructions for most air balancing hoods are quite simple:

Step One: Cover the grille or register completely assuring the hood is capturing all the airflow into or out of the register or grille

Step Two: Allow the airflow to stabilize for several seconds

Step Three: Press the “Read” button

Step Four: Read and record the airflow of the grille or register on the screen.

Measuring Variable Speed Fan Airflow

Most variable speed fans maintain airflow as they respond to pressure changes in the system such as a filter that loads up over time. Unfortunately when balancing high volume registers and grilles, the fan may sense the back pressure or restriction to airflow caused by the hood being placed over the grille as volume is lowered and the fan will respond by maintaining airflow.

By watching the CFM on the face of the hood, it looks like airflow increases once the hood is placed over the high volume resister. If you don’t understand that the fan is merely maintaining airflow, this appears terribly confusing.

To avoid confusing readings and chasing airflow around the system, and to preserve your sanity, the solution is to simply leave the hood in place on the register or grille for 10 to 15 seconds and allow the airflow to stabilize.

After the airflow has stabilized, take and record the airflow measurement as normal. Then move on to the next grille.

The same principle applies when adjusting a damper, you must wait 10 or 15 seconds until the fan senses the change in pressure and adjusts the airflow to compensate for the damper adjustment.

Most commercial balancing hoods have a throat area of 1.2 sq. ft. or greater. There’s little back-pressure caused by the hood when measuring registers below 200 CFM. In this case you will find little, if any variable speed fan changes in airflow during testing.

Sounds too simple, doesn’t it? The time delay allows the fan to do its job and respond to the change in static pressure. Delaying taking the reading until the fan has done its job is the airflow measurement solution.

Air Balancing Hood User Tips

Often the CFM reading on the screen of an air balancing hood will jump up and down typically 2-3%. This is normal and is to be expected, airflow rolls and tumbles through the hood and the volume is constantly changing slightly.

Know the CFM limits of the balancing hood you are using. The most common hoods read airflow over 30 CFM, but below 30 CFM a “0” will appear on the screen. The maximum airflow many hoods read is 2000 CFM. Take time to read the user’s manual to know the limits of your hood and how it should be used.

You will find the back edge of the hood skirt cannot be seen when testing. Before taking the reading, rock the hood away from you to assure a tight seal is achieved and all airflow is captured into the hood.

Using a Pressure Sensing Balancing Hood

When using a pressure sensing hood, a back pressure compensation feature is built into the program of the hood. Two readings are required to measure airflow on each register. Take the initial measurement with the flaps open. Then close the flaps and take a second measurement. The second measurement is the actual airflow that is recorded.

Whichever kind of hood you use, understand there’s much more to accurately reading airflow than just “covering the hole and mashing the button.”

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company and membership organization. If you're an HVAC contractor, technician or energy rater interested in a free Air Balancing Hood Test Procedure, contact Doc at robf@ncihvac.com or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.