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Duct Pressure Testing: An Air Balancer's PerspectiveRob "Doc" Falke11/28/2006Testing duct leakage by pressurizing ducts can be traced back to SMACNA air duct leakage testing standards publishedin the 1960s. Commercial duct leakage pressure testing prospered for decades as a method of verifying one of manyfactors of HVAC system performance. Unfortunately today, watered down residential duct pressure testing is beingaccepted 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 bedelivered into the building envelope. So the current practice exists of verifying the tight duct system to assurecitizens are getting maximum energy efficiency. But in reality, the odds are slim that all the other factors thatdetermine an efficient system are automatically functioning as they should.

Lets go back 50 years and take a look at the complete scope of testing that was, and still is, used to document installedHVAC system performance. Well contrast the old way that has stood the test of time to what is being accepted today.

Commercial Duct Pressurization TestingDuring the construction process, when the duct is accessible before the grilles and equipment is installed, eachintentional opening in the duct system is sealed air tight with plastic and tape. The duct system is then placed under highpressure, and the airflow through the fan is measured. The fan airflow is interpreted as duct leakage. If the level ofleakage is out of spec, the leaky duct is repaired and then the test is repeated. The process continues until acceptableresults 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 higherpressure 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 TestsCommercially, duct pressurization testing is the first of many tests required to verify the actual performance of theinstalled system. Its understood that duct pressurization is a dead test, because at this point, the equipment includingthe fan, heating, and refrigeration systems are not yet operable. So alone, this test portrays only one small piece of thesystem performance picture.

Once the system has been completed, then the real testing begins. Airflow is the primary performance that must becompleted before all others can begin. Since air is the fluid that carries the heating or cooling throughout the building, itsflow, 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 thesystem as power is to the fan. Without verification of airflow throughout the entire system, its impossible to effectivelytest the heating and refrigeration performance.

Static PressureStatic 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 accuratelymeasure total external static pressure, fan airflow can be accurately plotted on the manufacturers fan performancetables.

A recent NCI graduate called in to report a 5-ton system that had only six 6-in. supply ducts. The system had passed themandatory duct pressurization test in flying colors, and had a verified refrigeration change, but had a static pressurethree times the rated fan capacity. His rating proved this verified system was operating at less than 40% of equipmentrated 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 prettypicture.

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

Fan AirflowGood system performance testing verifies total fan airflow determined by static pressure or by performing an airflowtraverse at an adequate point in the ductwork near the fan. A return air drop, or a combination of trunk duct airflowreadings, is normally adequate. This test also identifies airflow losses at the air handling equipment. This is a really juicynumber, and varies from manufacturer to manufacturer, but thats another article.

Fan Performance Test

To further verify fan performance, and also to check out equipment operation, measure fan and motor RPM, and fanmotor Amp draw. Compare these numbers to the rated performance of the equipment as further performance verificationfactors.

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Room by Room AirflowJust because the airflow over the indoor coil may be adequate, its absurd to assume this airflow is adequatelydistributed into and throughout the building. The primary purpose of our systems is to deliver comfort, and comfortcannot be assured unless room-by-room airflow is measured and verified to agree with your room-by-room ACCAmanual J calculations. If the air doesnt get to each room, theres no way the BTUs will ever make it there.

Temperature TestingThere 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 engineers airflow numbers fail to verify BTU delivery by checking the delta-T between theroom 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 equipmentperformance once the airflow of each is known. Accuracy is critical and requires an understanding of test proceduresthat are still uncommon throughout the industry.

Once airflow is known, its through temperature that we can verify the performance of the refrigeration and combustioncircuits 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 inferiorresults, and we have found this partial system testing often creates a negative energy impact on the system.

Duct Temperature LossesWhen ducts are located in unconditioned spaces, the temperature losses through duct insulation can be extreme. Atticssoar to over 150F in many parts of the country R-4, R-6 or even R-8 insulation just cant do the job under theseconditions. 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 beidentified and isolated. Once the sources of the losses and gains are known, these system deficiencies can beaddressed.

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

Theres good in any test method that will make you a better contractor and move you and your systems toward improvedefficiency.

As a technician or contractor, you will always be the one who decides on how youll verify a system is functioning at peakcapacity. Its 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 inmeasuring, 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 atrobf@nationalcomfortinstitute.comorcall him at 800/633-7058. Go to NCIs website atwww.nationalcomfortinstitute.com for free information, technicalarticles and downloads

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