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Measuring static pressure resistance Photo by Testo
<p>Measuring static pressure resistance. Photo by Testo.</p>

So, Not Air Balancing Yet?

Air balancing has been the required quality assurance program for commercial HVAC commercial projects for nearly a half a century. Let&rsquo;s take a look at how the tools or air balancing are being applied and utilized today &nbsp; &nbsp;

In the residential and commercial sectors of the HVAC industry air balancing is becoming part of everyday shop talk. This profession within our trade has come of age and is now included in nearly every emerging standard requiring quality assurance.

Let’s take a look at how the tools or air balancing are being applied and utilized today

Air balancing has been the required quality assurance program for commercial HVAC commercial projects for nearly a half a century. It preceded blower doors and home performance testing by decades. It appears its day has finally come.

Demanded by Consumers
As consumer demands drive us all towards increased comfort, even room to room temperatures are no longer optional for savvy consumers. Through duct design we have implied even temperatures throughout a building, but delivering that promise without testing is a dream. Adjusting and balancing a system is the only way to effectively deliver the even temperatures and efficiency we have promised our customers for decades.

How to Use an Air Balance Hood
Air Balancing as a Verification Tool
Effective Use of Air Balancing Test Instruments

The energy efficiency world is calling for air balancing. Interestingly enough, as far as I have been able to identify, there has never been a study commissioned to identify the energy savings from a balanced HVAC System.

Lacking experience with the profession, many in the energy efficiency field are still unsure if air balancing hoods are even accurate, primarily because they lack the knowledge and experience to use them and how to apply the significant standards that govern their use. Although uninformed critics still loom in dark corners, it appears air balancing is here to stay.

Test Instruments are Not Optional
“Air balancing is an exclusive game. If you don’t have the tools, you just can’t play”… Sam Sugarman

Unfortunately, this statement is true. Unless you have not only basic tools, but the right quality, type and grade of tools, you’ll never be an effective air balancer. In addition to owning the right test instrument, you must survive the learning curve required before you become proficient at the use of each tool.

Many come to air balancing training believing all they need to do is to learn to cover a register with a balancing hood and they’re all set. As you’re beginning to see, this is far from reality.

Test Results Agree
As a matter of fact, after a week or two in the field you will begin to see that very often, the validity of one test value will be confirmed by a related air balancing test.

For example, if the total external static pressure of a .50” w.c. fan is 1.22” w.c., you will soon learn to expect airflow somewhere near the 280 CFM per ton range from an unimproved system.

Learn to Trust Your Instruments
While you’re first learning to effectively test with your balancing instruments, you will pass through a challenging learning curve. Initially you may trust the airflow of the system more than you’re instruments. 

Field Scenario I
A new construction contractor has built new homes for many years. On his first day of field testing he approaches one of his 2.5 ton systems with a single return in the hall. He places the hood over the return grill expecting 1000 CFM to appear on the face of the hood. Suddenly 585 CFM is read by the hood. The contractor shakes the hood to see if it’s working and mumbles, “What’s wrong with this hood? I’m sure my system is delivering 1000 CFM!”

You may run into these same conflicts as you learn to trust your instruments more than you trust the airflow of the system that you just built. The cost required of you to learn through study and experience how to use each test instrument, requires a dedication to practice and constant evaluation.

With time, you will begin to understand what happened when you get a bogus reading.   

False Readings Happen
As you learn to perform each of the air balancing tests, you will need to master several very specific test procedures. Until the time that you have fully developed your testing skills, you will discover some test results may not agree with other related tests taken on the system. When this happens, it’s time to review carefully how you took the readings and perhaps check the settings on your test instrument.

Field Scenario 2
You perform an airflow traverse on a long straight return duct entering a furnace. It’s a 2.5 ton unit, static pressure is just below fan rated pressure and the fan speed setting is correct. The fan is in good condition and the amp draw is about 80% of full load amps. You check the fan tables and plot fan airflow and expect about 1030 CFM. You traverse the duct and find 732 CFM, Something’s up.

You recheck the system and controls and find everything else is as it should be. Then you review the traverse procedure and note that probe must remain at 90 degrees to airflow. You’re not sure you followed that step, so you begin the traverse again, holding your thumb securely on the shaft of the probe as you measure velocity. You find the average velocity appears to be around 30% higher. The final calculation shows 1041 CFM. It looks like you just learned an important step in traversing airflow.

All Test Instruments are Not Created Equal
Over the years NCI has offered a broad scope of test instruments to those we train. Through our tech support we also deal daily with the problems of students who have purchased test instruments that just don’t hold up to the demands of air balancing.

There are unfortunately a number of instrument manufacturer’s that offer tools that just do not meet the quality standards or have the accuracy to function as you will come to depend on in the field. When you acquire a test instrument, take time to compare it to the specifications listed above and be sure to get the right tool for the job.

There are low price and low flow balancing hoods that do not have the ability to provide accurate information in the field. Please don’t buy that type of hood. You will regret your purchase. You’ll find manometers that have ranges which will prohibit you from getting the accuracy you’ll need at low residential pressures.

Some anemometers on the market will not produce the consistency that will enable you to rely on the readings you take with them. The list of don’ts for balancing test instruments goes on and on. Do your best to acquire top quality instruments that are made specifically for balancing.

Remember back to when you were new to the air conditioning industry and you had to struggle to figure out what the heck you were doing from one job to another? If you miss that challenge, air balancing can return you to new learning and advancement in your career.

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 or technician interested in a Fee Air Balancing Hood Test Procedure, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at for free information, articles and downloads

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