When considering the performance of your HVAC system, pay close attention to indicators of inefficiency. Do you have:
- Indoor air quality (IAQ) issues?
- Occupant discomfort and complaints?
- Visible cracks and leaks in the ductwork?
- Inefficient fans or chillers that are working too hard?
The common reaction to these problems is to get new equipment. However, upgrades won’t remedy the issues if your ductwork is leaking.
Perform an inspection of the duct system and utilize new technologies to get the most out of your HVAC system.
Impact on Energy Efficiency
Duct leakage in commercial buildings is estimated to be anywhere from 15-40% of cubic feet per minute (CFM). A leaky system uses 25-35% more fan power than a tight system, according to the Lawrence Berkeley National Laboratory report Rationale for Measuring Duct Leakage in Large Commercial Buildings.
“In terms of energy consumption in buildings, HVAC is now the biggest piece of the pie at 39%,” says Frank Forrest, efficient solutions products manager at manufacturer Carlisle HVAC. “When you consider such significant loss on such a heavily consuming piece, it really points out the importance of having an efficient system.”
A common solution is to put new chillers on the building or different drivers on the supply fan to speed it up, Forrest says. The duct system typically gets left out but should be evaluated annually.
“Look at the ductwork first, find out how much leakage you have, and work from there. You may find that new equipment is unnecessary,” Forrest suggests. “Once the duct system is cleaned and sealed, you can more easily consider other components. It’s a trickle-down effect.”
Most don’t even realize the significance of duct leakage or that they should be paying attention to it, agrees Steven G. Liescheidt, president of engineering and consulting firm SPPECSS Consulting, LLC.
“The arguments was that duct systems don’t need sealing because the leakage rate is not that great,” he explains. “It may be time to rethink this old paradigm in light of several ASHRAE requirements.”
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New Technology and Solutions
Cleaning or sealing your ductwork can be much easier on your wallet than upgrades.
“Some in the industry have suggested that duct leakage above 15% indicates sealing will be cost-effective,” Liescheidt explains. “There are very good products on the market that make the first cost more attractive.”
A development in sealing duct leakage in commercial buildings involves sealing leaks from the inside out. “This aerosol-based sealing technology works by pressurizing a duct system with a fog of sealant particles,” writes ASHRAE member Mark P. Modera, director of the UC-Davis Western Cooling Efficiency Center. “By temporarily blocking the diffusers, the sealant-laden air is forced to the leaks.”
Carlisle HVAC has developed robotics to inspect, clean, and seal ducts from the inside. Its robot visually detects leaks by recording video on an SD card, which also allows you to verify problems and results as opposed to taking a contractor’s word for it. It cleans with a brush and vacuum and seals using a rubber-like substance and spray nozzle.
There are currently 20 projects using the robot, Forrest says. One contractor estimates the charge to inspect, clean, and seal the ductwork of a 100,000-square-foot building at $1.50 per square foot.
The cost can be worth it though. Tighter ducts can have an attractive payback, Liescheidt says.
When design engineers calculate CFM, their equation typically accounts for duct leakage as a given. This doesn’t have to be the case, Forrest says, adding that ductwork should be like plumbing. Zero leaks are doable, he explains, but 2.5% leakage is very feasible.
“The Lawrence Berkeley lab determined that if you get your leakage down from 10% to 2.5%, it would have a 30-33% impact on your energy bill,” says Forrest. “The savings are very significant.”
Chris Curtland ([email protected]) is assistant editor of BUILDINGS.
