Duct leakage isn’t everything. It used to be worse — really. That shouldn’t lull HVAC contractors into a false sense of complacency when it comes to fighting the problem, but context can help professionals direct their attention to where it is likely needed the most.
Code changes and more knowledgeable homebuilders have driven the new residential market toward much tighter duct systems, said Wes Davis, ACCA’s technical services director. The other side of that coin: the older the home, the better chance of leaky ducts due to sporadic adoption of duct sealing practices and enforcement of code requirements.
Staying with the big picture, the good news is that duct leakage truly isn’t everything when it comes to lost performance or even leakage. The bad news is that this doesn’t diminish the importance of avoiding duct leakage; it simply points to other factors to consider, too.
Mark Terzigni is SMACNA’s executive director of market sectors and construction technology. In 2019, he created a presentation slide deck about duct leakage and duct testing as the association’s director of engineering, technical services.
In that presentation, he emphasized that ‘duct leakage’ is not synonymous with ‘system leakage.’ System leakage comprises not just duct leakage but also equipment leakage and accessory leakage.
He continued that a VAV box is a good example. In addition to a damper, it may have coils, filters, electric reheat, and other options, he explained.
“Each of those items impact leakage rates,” Terzigni said.
He also clarified the nature of the enemy in three bullet points:
- Leakage is a function of pressure, and it is a function “of the size of the hole”;
- Leakage is not a function of the volume of air; and
- Leakage is not a function of conditioned floor space.
The entire presentation may be useful for contractors who conduct testing on a regular basis, as he discussed active versus static tests, testing condition and operating conditions, and questions like, “Why not use a % to fan flow?”
Terzigni added some tips regarding not the how but the when of duct testing. Test some of the system early on in the construction process, he advised for new builds. This, he said, can establish expectations across the team for testing and can also save money and time by identifying some potential issues early on when they are easier to fix.
Director of technical services ACCA
Codes and Testing
As ACCA’s Davis noted, the duct testing landscape is a little different thanks to IECC 2021. The latest IECC version “requires testing of duct leakage for ducts located within the thermal envelope, and not just leak testing for ducts outside the conditioned space,” he reminded.
The rationale of the code change is to avoid a scenario where the occupant pushes the thermostat further to achieve desired comfort and make up for conditioned air that is not making it to the intended space.
ACCA “found this rationale difficult to support,” David said, given the subjective nature of thermostat management. However, he pointed out that if ducts are constructed, designed, and sized properly (via Manual D, for example) in the first place, “there would be little leakage to make a difference.”
Back on the code side of things and the latest revision’s intention, Davis said that ACCA plans to propose an alternative process in lieu of leak testing: air balancing per the ACCA 5 QI Installation Specification.
Air balancing ensures the correct amount of air arrives at its intended destination, aligning with the stated motive behind the IECC’s testing change.
“Too much or too little air can create the problems that the new code requirement attempted to correct,” Davis explained. “Without air balancing, the new code requirement will resolve nothing.”
Beyond Duct Sealing
A line from Terzigni’s presentation put testing in further perspective.
“Testing ductwork does not reduce leakage. Sealing ductwork reduces leakage.”
Even then, the contractor can feel more confident but cannot relax entirely.
“I would say that once duct leakage is sealed, the biggest threat to the compressor is short cycling,” said Michael Robertson, senior account manager who works with compressor and motor business for LG Electronics U.S.A.
Robertson sees many times where the real problem is leaky ducts but a decision is made to treat the symptoms by installing higher tonnage equipment, leading to even greater inefficiencies.
Conducting an updated load calculation after duct sealing is the way to nip this and other associated problems in the bud. Robertson sees room for improvement on that front, though, describing load calculations as “the ‘kryptonite’ of the industry” since he sees them as often overlooked.
Davis echoes the need and the value of the task.
“Technicians need to check the system’s airflow during system start-up, during scheduled maintenance, and service calls,” he said.
“After all, clients deserve consistent competent service and forced air systems depend on the correct airflow. We believe it is a disservice to homeowners when technicians fail to measure airflow.”
Business of Doing Better
Skipping airflow evaluation earlier on can also be disservice to contractors, in Davis’ opinion.
“Tight ducts are only one piece of the puzzle,” Davis said, returning to a recurring theme.
“Contractors can help their bottom line and improve their customer’s comfort when they correct leaky undersized poorly installed duct systems. They also reduce callbacks and exposure to liability when they follow industry standards, code requirements, and use talented tradespeople to deliver the finished product.”
He considers that thorough duct evaluation to be crucial — before presenting options or a price for an installation.
The older the home, the more potential for leakage issues, as Davis noted earlier, but it is also worth remembering that similar materials do not necessarily degrade at the same pace, even on the same property. Ductwork subject to extreme temperature swings in attics or crawlspaces will degrade faster.
One other tip found its way into both Davis’ and Terzigni’s comments: Remember the returns.
“Leaks on return can introduce ‘raw’ air to the system. Supply leaks into conditioned space do deliver the ‘energy’ but not to the intended area,” Terzigni said.
Davis observed that efficiency and environmental climate aren’t the only victims in that case.
“Return duct leakage can introduce a wide variety of contaminants that can clog an evaporator coil or provide a food source for biological growths in the system,” he cautioned.
Construction and code improvements make it more likely than ever that a contractor will not encounter such a problem. Maintaining a watchful eye and thorough process can still reward both contractors and their customers, though, as long as technicians remember that duct sealing is a key step but not the end of the journey.