July 1, 2005
In our last installment, we talked about the value of air balancing hoods in measuring HVAC system performance. However, air balancing hoods alone cannot

In our last installment, we talked about the value of air balancing hoods in measuring HVAC system performance. However, air balancing hoods alone cannot verify the actual performance of an HVAC system. While airflow to each grille is probably the single most important test you can perform on a system, many other tests are included in today’s air balancing.

Static pressure testing is critical to identify the causes of poor system airflow. Anemometers offer the only valid method of measuring airflow when an air balancing hood can’t fit on a register or grille. Thermometers are the key to documenting duct losses, and perform a vital role in evaluating refrigeration performance once airflow has been verified using a time-proven live test.

Psychrometers tie all these tests together by allowing us to effectively measure system BTU delivery. Accurate wet bulb traverse readings across a system allow us to gather extremely accurate system temperatures. Those temperatures can be converted to enthalpy to effectively calculate system total Btu delivery that includes the duct system’s gains and losses. Each of these tests work together to verify and document operating HVAC system performance.

Third Party Efforts Intensify

Numerous test methods are being presented in response to increasing “third party verification” efforts. These efforts are being launched to keep the HVAC energy efficiency businesses alive after the 13 SEER minimum has eliminated the return on investment energy savings promised by higher efficiency equipment.

The idea of proving system performance through the refrigerant cycle alone is inadequate. Although these tests can provide valuable information about the refrigerant part of the system, duct losses can still leave the system operating at 60% of capacity. It’s like proving a cooling system in a car works great, but the tires are flat and the transmission won’t engage. Likewise, knowing the airflow over a coil is essential, but leads to the assumption that the duct system is just fine, which is rarely true.

“Tight duct” programs have been the downfall of many utility programs over the years, as sealing undersized ducts often increases system static pressure to the point where airflow becomes so low that Btu delivery plummets. Tight ducts are only a sliver of the energy efficiency pie. Recent testing has proven many tight duct systems deliver only 55% to 65% of the rated equipment capacity under tough weather conditions. Difficult weather conditions are when we need maximum system capacity the most.

All of these typical energy efficiency methods (equipment efficiency, refrigeration testing, and tight ducts) have been relied upon for years, yet present only a piece of the picture. Alone each of these methods of verifying system efficiency is inadequate and delivers a false representation of energy saving to consumers and those who sponsor such programs.

A Tough Job, A Tired Tech, A Proud Result

A few weeks ago, I had a recently certified air balancing tech call in. It was hot, he was tired, and he was trying to get performance maximized on a cooling system at the end of the day. He had tested airflow and static pressure and thought he was done. I asked for his temperature readings, his enthalpy change thought the system, his fan RPM, refrigeration pressures, and some engineering data from the manufacturer’s equipment specifications. Then I needed to know total BTU delivery and his installed Cooling System Efficiency Ratio (CSER™).

Finally he sighed deeply and said, “Doc, there have to be some shortcuts to verify the performance of an HVAC system!” I replied, “Sorry, we have to look at the whole system. There are no effective shortcuts.” So he worked late and called me in the morning and announced he had achieved 97% cooling capacity from the system. It wasn’t easy, but he was ultimately very pleased with his work.

Verifying HVAC system performance with balancing hoods and related air balance testing to prove total installed system performance is the future of our industry. With the upcoming changes in minimum equipment efficiency, the next frontier is testing the duct system and quantifying installed system performance.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring and improving HVAC system performance. If you're an HVAC contractor or technician interested in a no cost one page test report and procedure on how to measure duct loss, contact Doc at [email protected] or call him at 800-633-7058.