Ventilation retrofit improves energy, IAQ
Duct-sealing technology helps high-rise reap energy savings, improve indoor air quality, and enhance comfort level for tenants.
In 2008, the property owner of Carlyle Towers, a 48-year-old multi-family apartment building in Caldwell, N.J., was concerned about rising energy costs and searching for practical solutions to improve the energy efficiency of the 100-unit building.
Like many buildings constructed in the United States over the past 60 years, Carlyle Towers was designed with mechanical ventilation systems for exhausting stale air from bathrooms and kitchens. Intrinsic with its design, the exhaust system was inadequate for proper ventilation, resulting in high energy costs for heating and cooling the building. In addition, comfort levels throughout the building were uneven, with some units receiving too much heat while others felt cool. Due to uneven airflow, the building’s upper floor apartments were receiving too much ventilation while bottom floor units were under-ventilated.
Carlyle Towers was positively ventilated with roof fans. As is often the case, engineers designed the system to provide fan flow that is much higher than the International Mechanical Code requires. From the fans, several long vertical riser shafts were connected to additional ductwork leading to the individual bathrooms and kitchens within each apartment.
Also typical in older buildings such as this, the ductwork was not sealed externally. While the shafts retained their overall structural integrity, there were cracks and gaps throughout the entire ductwork structure, particularly around the S and drive connections in the sheet metal shaft and at the connection of the return grill and shaft. As a consequence, pre-testing of the ventilation system showed an average leakage rate of about 30% to 40%—two to three times more than rates outlined by SMACNA specifications. According to industry insiders, leakage rates of 30% are typical in these types of buildings; rates of 50% to 95% or more are not uncommon.
These high leakage rates can lead to enormous energy waste. Leaks in the shafts cause a reduction in pressure within the duct system, requiring more energy to move the air. No matter how powerful the exhaust fans may be, they often remain insufficient to adequately vent the targets farthest from the fan. Like drawing liquid from a straw with holes in it, no amount of sucking will do the job.
According to a 2003 report published by Lawrence Berkeley National Laboratory titled Thermal Distribution Systems in Commercial Buildings, even a low leakage rate can lead to a substantial increase in fan power consumption—and energy usage.
Indoor air quality
High leakage rates also affect indoor air quality (IAQ). With a leaky ventilation system, stale and odor-filled air in the affected areas remains ever present. Moisture from bathrooms and cooking continues to circulate, causing mold and other health-related concerns. Without an adequate supply of fresh air, the buildup of indoor pollutants is inevitable.
ASHRAE Standard 62, Ventilation for Acceptable Indoor Air Quality, was specifically developed to address IAQ concerns, and while building codes in most states reference at least some of this standard as part of their own minimum ventilation requirements, IAQ and the illnesses associated with it remain a significant problem.
Though the size and scope of the problems associated with central ventilation systems are well known, fixing these problems has been difficult. Short of tearing down walls to access and manually seal each of the individual leaks throughout the duct system, there has been no adequate fix.
In summer 2008, New York-based energy consultant Steven Winter Assocs. Inc. (SWA), armed with new duct sealing technology from Lawrence Berkeley National Laboratory and self-regulating dampers from American Aldes, engaged in the Carlyle Towers project, the first comprehensive ventilation retrofit for improving both energy and IAQ performance.
“I was aware of the Aeroseal technology and its use in residential and commercial heating and cooling duct sealing projects,” said Marc Zuluaga, VP at SWA. “So when we were called in to solve the ventilation-related issues at the Carlyle Towers, I thought this would be a perfect time to try it for this application as well.”
Aeroseal is an aerosol-based sealant that works from the inside of the duct system. The process begins by blocking the wall registers so that air can escape only through the leaks in the ductwork. The duct system is slightly pressurized to 0.1 inWC, increasing to a maximum of 3 inWC during the sealing process. The sealant is than heated and blown into the ventilation shaft through an existing access point or through a temporary entranceway cut into the system. The dry, 7 to 10 micron-sized adhesive particles remain suspended in air as they travel throughout the ductwork until they reach a leak. Here they begin to accumulate around the leak, bonding to other sealant particles until the entire hole is permanently filled.
Unlike some sealants, such as those used for weatherization or material bonding, the Aeroseal sealant is a vinyl acetate polymer with a rapid cure rate of approximately 2 hours. The sealant exhibits minimal volatile organic compounds (VOC) off-gassing and its nontoxic properties afford it no OSHA maximum exposure limitations. Aeroseal has been used to seal leaks in school buildings, health clinics, and hospital buildings. As is often the case, the Carlyle Tower duct-sealing project was successfully and safely conducted while the building was still occupied.
Aeroseal was first used to seal leaks throughout the Carlyle Tower’s 25 individual ventilation shafts. Each time, the process began and ended with a measurement of duct leakage. Before the duct sealing, average shaft leakage rates were well above 200 cfm. Post-sealing tests showed leakage rates reduced to 15 cfm or less.
“Aeroseal sealed on average 90% of the duct leakage,” said Zuluaga, “and that, along with the installation of constant airflow regulators (CAR) dampers at each register, had a significant impact on the ability to properly balance ventilation and reducing the use of both gas and electricity.”
The American Aldes CAR dampers regulate airflow over a wide range of pressure conditions. The dampers automatically adjust to compensate for changes in the pressure field due to wind and stack effect. This adjustment assures that the ventilation system remains balanced despite changing seasons and other environmental factors.
It took the engineers approximately 4 weeks to seal the ductwork and install the dampers. With the leaks sealed and the pressure within the ventilation shafts balanced, SWA was able to recommend replacing the 25 300 W roof exhaust fans with much smaller 140 W fans. While the net overall ventilation for the building was much lower than before the retrofit, the system was now delivering the right amount at every location. The electricity savings from fan optimization alone resulted in an annual utility savings of $7,000.
In addition, by eliminating the heating and cooling air that was previously escaping through the air duct leaks, the building owners realized a dramatic decrease in gas use for space heating. Utility records examined over a 3-year period after the retrofit completion show an approximate 30% reduction in gas usage, from an average of 57,500 therms (EC)/year to 41,000 therms (EC)/year.
“Soon after the retrofit was completed, we heard from one tenant who thanked the building manager for fixing his heat,” said David Legow, president of Legow Management Co., owner of the property. “While we didn’t touch the heating system per se, by sealing the shaft leaks and automating the damper adjustments, the furnace was noticeably more efficient and effective.”
The results from the retrofit have gained industry attention. Lessons learned from the project were incorporated into the U.S. Green Building Council New York Chapter Green Code Task Force recommendations to “ensure ventilation airflow in residences.” In addition, the National Center for Healthy Homes and a recent study funded by HUD both reference the project in guidelines for improving IAQ.
Information provided by Aeroseal, Centerville, Ohio.