Chilled Beams and Duct Leaks in HVAC systems
For those interested in energy-efficient buildings, chilled beam technology is rapidly gaining the attention here in the U.S. that it has long enjoyed elsewhere around the globe. As effective as these systems may be, however, their successful use is contingent upon a balancing act of various factors, the primary one being leak-free ductwork.
Chilled beam systems have been successfully used for decades throughout Australia and central Europe. It is chilled beam technology that is behind the success of 30 The Bond, the first building in Australia to achieve a five-star Australian Building Greenhouse Rating (ABGR) — equivalent to Leadership in Energy and Environmental Design (LEED) Gold. It is also being used successfully at London’s Heathrow Airport; the Barclaycard building in Northampton, England; and the Airbus U.K. building, among others.
Here in the U.S., chilled beam technology is less prominent but is growing in popularity. Its largest deployment in the U.S. can be found at the Constitutional Center in Washington, District of Columbia. Doubts regarding the technology’s ability to handle ultra-humid environments were squelched after years of effective deployment at the National Geospatial-Intelligence Agency (NGA) facility in Springfield, Virginia.
The primary attraction of chilled beam HVAC is its efficiency potential. Since chilled beam systems treat the air in and around the location where it is being used, there is no need for delivery ducts, no temperature dissipation, and no over-cooling — all of which adds up to reduced energy use.
Also, without the need for large fans to circulate the treated air, chilled beam systems require even less energy to operate. The amount of energy saved through minimizing fan power far exceeds any increased energy associated with pumping water though the system. And since the temperature of cooled water is higher than the temperature of cooled air, chilled beam systems can deliver the same cooling ability at a lower cost.
A BALANCING ACT
Despite their relatively simple design and reduced energy requirements, however, chilled beam systems have their drawbacks. The most prevalent of which is their sensitivity to humidity. If the temperature of the chilled water circulating in the system coils dips below the room’s dew-point temperature, moisture forms outside the coils and creates what is descriptively known as indoor rain. Maintaining the required environmental balance in order to prevent condensation has proven to be particularly difficult in much of the U.S. where humidity is high.
“It really is a balancing act,” said Rod Lord, managing director of SEED Engineering, Sydney, Australia. “The temperature of the space, the number of occupants, air humidity, and the amount of outside air being delivered to the system all play a role in the proper functioning of a chilled beam system. Changing one of these calculations can make the difference between a highly effective HVAC system that provides a comfortable, high air quality environment or one that creates an untenable mess.”
To help control this critical concern, most chilled beam systems rely on the introduction of drier air to the central air-handling unit via ductwork. This process lowers the dew-point temperature and allows the removal of heat and moisture gains found in the surrounding environment. Without the delivery of sufficiently drier air to the surrounding unit, a chilled beam system can have disastrous effects.
Such was the case during an elementary school renovation project in northern Maryland. Engineers working with Prince George’s County Public Schools Department installed a state-of-the-art chilled beam system into the school for energy-efficient heating and cooling. Despite detailed planning, design, and analysis, the engineers found that excessive condensation plagued the newly installed chilled beam system.
“The engineers suspected poor duct sealing was the root cause of the problem,” said Neal Walsh, Aeroseal LLC. “Leaks in the ductwork prevented adequate amounts of treated air from reaching the unit. In fact, our testing showed that there was substantial duct leakage evident throughout the new trunks and branch lines, as well as within the flex lines connecting the branches to the chilled beams.”
Without the proper amount of fresh air reaching the chilled beams, the system was unable to provide sufficient amounts of treated air to the surrounding space. Excessive humidity caused condensation around the beams, and evidence of water dripping from the ceiling halted the project until the problem could be resolved.
“Once we confirmed that duct leakage was the main cause, fixing the problem was a straight forward operation,” said Walsh. “We used an aerosol duct sealing process that quickly located and sealed the leaks. The computer-controlled sealing system monitored results as they happened. Following setup, it was only a matter of hours before each section of ductwork was verified to be virtually leak-free. Once the sealing was completed and minor adjustments made, the chilled beam system worked like a charm.”
A similar project-halting situation unfolded for the owners of a luxury high-rise in the heart of north Sydney, Australia’s business district. Their 23-story building’s variable air volume (VAV)/chilled beam hybrid system never provided the energy efficiency it was designed to deliver. Tenants of the building were never happy with the a/c, and energy costs were well beyond initial design levels. Any adjustments to the system pushed it beyond safe dew-point levels where condensation threatened to create a disastrous effect.
After conducting an analysis of the building, Lord and his team at SEED concluded duct leakage was at the root of the problem. Leaks throughout the 20 vertical shafts and hundreds of square feet of horizontal ductwork, serving 23 individual air handling units, prevented the required amount of air from reaching the chilled beam units.
“Chilled beam systems are particularly sensitive to external factors, and since leaks in the duct system were preventing sufficient air from reaching the beams, they were unable to provide effective cooling, and, worse, there was condensation dripping from the various units,” said Lord. “The system would never function properly unless those leaks could be sealed.”
Given the complexity and inaccessibility of the building’s duct system, SEED recommended the leaks be aerosealed shut. Since aerosealing works from inside the ducts, accessing the entire duct system was easily accomplished without the demolition and reconstruction typically required with manual sealing. And by partitioning off the massive duct system into 50 individual sections and sealing each sector one at a time, the sealing team was able to complete the entire project over a period of three weekends — a total of nine days.
“We eliminated virtually all the leakage for about 10 percent of the cost estimated for manual sealing,” said Geoff Kelly, managing director, Clean-Air Australia. “Most importantly, by eliminating the leaks, the chilled beam system was functioning effectively, and the engineers were able to turn down a/c power while increasing the comfort of the building.”
There are reasons why chilled beam systems have been so widely used across Australia, the U.K., and central Europe for the past 20 years or so. The promise of quiet, energy-efficient heating and cooling makes chilled beam systems an appealing option for building owners looking to reduce operational costs and lower their environmental footprint.
But as the use of chilled beam systems expand and their popularity takes hold here in the U.S., the importance of leak-free ducting becomes more critical than ever before. As with any HVAC system, ensuring efficiency and high performance with chilled beams requires a whole-system approach to design, installation, and maintenance. Leaks in this strategy can leave a contractor all wet.
John Dixon is a freelance writer headquartered in Portland, Oregon. Many of his articles have appeared in industry publications and other media outlets focused on HVAC technology and sustainability.
Publication date: 1/22/2018