Duct Pressure Testing: An Air Balancer's Perspective

Nov. 1, 2006
Testing duct leakage by pressurizing ducts can be traced back to SMACNA air duct leakage testing standards published in the 1960s. Commercial duct leakage

Testing duct leakage by pressurizing ducts can be traced back to SMACNA air duct leakage testing standards published in the 1960s. Commercial duct leakage pressure testing prospered for decades as a method of verifying one of many factors of HVAC system performance. Unfortunately today, watered down residential duct pressure testing is being accepted as a stand-alone assurance that a residential HVAC system has achieved maximum efficiency by government and utility programs across the country.

The theory is that if a duct system is tight, all the BTUs generated by perfectly performing equipment will be delivered into the building envelope. So the current practice exists of verifying the tight duct system to assure citizens are getting maximum energy efficiency. But in reality, the odds are slim that all the other factors that determine an efficient system are automatically functioning as they should.

Let’s go back 50 years and take a look at the complete scope of testing that was, and still is, used to document installed HVAC system performance. We’ll contrast the old way that has stood the test of time to what is being accepted today.

Commercial Duct Pressurization Testing
During the construction process, when the duct is accessible before the grilles and equipment is installed, each intentional opening in the duct system is sealed air tight with plastic and tape. The duct system is then placed under high pressure, and the airflow through the fan is measured. The fan airflow is interpreted as duct leakage. If the level of leakage is out of spec, the leaky duct is repaired and then the test is repeated. The process continues until acceptable results are achieved.

Commercial testing utilizes a five to 10 horsepower fan to perform leakage testing, and in order to be effective up to 7-in. of pressure is required. This fan capacity is 70 times the pressure than residential duct testing utilizes today. The higher pressure is essential to adequately pressurize a duct system to achieve accurate results.

Higher pressure is critical to assure the pressure at the fan is equal to the pressure throughout the entire duct system. Only then can the pressure at the fan be interpreted to represent accurate airflow leakage throughout the system.

One of Many Tests
Commercially, duct pressurization testing is the first of many tests required to verify the actual performance of the installed system. It’s understood that duct pressurization is a “dead” test, because at this point, the equipment including the fan, heating, and refrigeration systems are not yet operable. So alone, this test portrays only one small piece of the system performance picture.

Once the system has been completed, then the real testing begins. Airflow is the primary performance that must be completed before all others can begin. Since air is the fluid that carries the heating or cooling throughout the building, its flow, temperatures, velocities, losses, gains, and enthalpy must be verified at many points throughout the system so it can be effectively diagnosed and performance rated.

Airflow is the life-blood of a system upon which all the other components and circuits depend. Air is as essential to the system as power is to the fan. Without verification of airflow throughout the entire system, it’s impossible to effectively test the heating and refrigeration performance.

Static Pressure
Static pressure is like blood pressure to a physician and is the initial go-to test used to get an overall view of the system. This is the next test that should be added to residential duct pressurization diagnostics. By learning to accurately measure total external static pressure, fan airflow can be accurately plotted on the manufacturer’s fan performance tables.

A recent NCI graduate called in to report a 5-ton system that had only six 6-in. supply ducts. The system had passed the mandatory duct pressurization test in flying colors, and had a “verified” refrigeration change, but had a static pressure three times the rated fan capacity. His rating proved this “verified” system was operating at less than 40% of equipment rated capacity. Not uncommon.

We find nationally an average total external static pressure exceeds .76-in. for fans rated at .50-in. or less. Not a pretty picture.

Pressure Drops
The cause of high static pressure is identified by measuring the pressure drop over many of the components in the system. The pressure drop over any one component can be divided into the total external static pressure of the system to determine the percent of system pressure drop being caused by that one component. Pressure drop measurement is excellent diagnostics.

Fan Airflow
Good system performance testing verifies total fan airflow determined by static pressure or by performing an airflow traverse at an adequate point in the ductwork near the fan. A return air drop, or a combination of trunk duct airflow readings, is normally adequate. This test also identifies airflow losses at the air handling equipment. This is a really juicy number, and varies from manufacturer to manufacturer, but that’s another article.

Fan Performance Test
To further verify fan performance, and also to check out equipment operation, measure fan and motor RPM, and fan motor Amp draw. Compare these numbers to the rated performance of the equipment as further performance verification factors.

Room by Room Airflow
Just because the “airflow over the indoor coil” may be adequate, it’s absurd to assume this airflow is adequately distributed into and throughout the building. The primary purpose of our systems is to deliver comfort, and comfort cannot be assured unless room-by-room airflow is measured and verified to agree with your room-by-room ACCA manual J calculations. If the air doesn’t get to each room, there’s no way the BTUs will ever make it there.

Temperature Testing
There are times when the air makes it to a room, but the BTUs are lost along the way. Many commercial air balance reports that simply match the engineer’s airflow numbers fail to verify BTU delivery by checking the delta-T between the room air and the supply air. CFM times the delta-T, times the appropriate BTU factor, equals delivered BTU.

Temperature testing can be used to verify room-by-room performance, system performance, and even equipment performance once the airflow of each is known. Accuracy is critical and requires an understanding of test procedures that are still uncommon throughout the industry.

Once airflow is known, it’s through temperature that we can verify the performance of the refrigeration and combustion circuits of the systems we measure. Once again, airflow is the only fluid that circulates through the entire building. Attempting system performance verification by measuring one of the minor circuits of the system will yield inferior results, and we have found this partial system testing often creates a negative energy impact on the system.

Duct Temperature Losses
When ducts are located in unconditioned spaces, the temperature losses through duct insulation can be extreme. Attics soar to over 150F in many parts of the country — R-4, R-6 or even R-8 insulation just can’t do the job under these conditions. And remember, the days air conditioning and heating are needed indoors the most are when these temperatures are the most extreme.

By measuring the differences in temperature from one point in the duct to the other, temperature losses or gains can be identified and isolated. Once the sources of the losses and gains are known, these system deficiencies can be addressed.

Total System Performance Verification
The bad news is that in order to verify total system performance, more than 200 data points should be gathered and recorded throughout a system. The good news is that with the use of new test procedures and HVAC diagnostic instruments, coupled with online verification capabilities, contractors can now complete this process in as little as 90 minutes. This is often less time than it takes to perform a good quality residential duct pressurization test.

There’s good in any test method that will make you a better contractor and move you and your systems toward improved efficiency.

As a technician or contractor, you will always be the one who decides on how you’ll verify a system is functioning at peak capacity. It’s your job to find what your customer wants and what will place you above your competitors.

Rob “Doc” Falke serves the industry as president of National Comfort Institute, a training company specializing in measuring, rating, improving, and verifying HVAC system performance. If you're an HVAC contractor or technician interested in a CO Poisoning Levels and Symptoms Table, contact Doc at [email protected] or call him at 800/633-7058. Go to NCI’s website at www.nationalcomfortinstitute.com for free information, technical articles and downloads.

About the Author

Rob 'Doc' Falke | President

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 building pressure measurement procedure, contact Doc at [email protected]  or call him at 800-633-7058. Go to NCI’s website at NationalComfortInstitute.com for free information, articles and downloads.