A public-housing retrofit project replaced PTAC units.
Melling Engineering, Raleigh, NC (mellingengineering.com), an MEP firm that specializes in apartment buildings, apartment communities, and commercial space, joined several other firms in a renovation of Tabco Towers, a 22-story, 200-apartment HUD (Housing and Urban Development) building in Towson, MD. The building’s owner, Wishrock Investment Group, had planned HVAC and building upgrades at the high-rise for some time. After Wishrock management consulted with New Ecology Inc. (newecology.org) a non-profit sustainability consulting firm with offices in Boston; Providence, RI; Hartford, CT; and Baltimore, they decided to pursue 2011 Enterprise Green Communities (EGC) certification. “EGC certification is based on a number of factors, including a building’s energy performance,” said Marty Davey, director of portfolio services at New Ecology. “Building to this certification opens the door to significant funding in Maryland.”
In addition to certification and efficiency, Wishrock management was looking for a longer-term solution that would provide greater resident comfort, system control, and reduced maintenance costs. Because the wall penetrations for the current packaged terminal air conditioners (PTAC) units had caused maintenance issues in recent years, sealing up the building and controlling condensate drainage were also goals.
While New Ecology staff gathered building-performance data, created an energy model of the building, and helped define the overall green project goals, Melling Engineering was brought onboard.
Choosing a system
Once a performance baseline was established, the first challenge was selecting a replacement HVAC system. More-efficient PTACs were considered, but the performance levels needed weren’t available. As a high-efficiency, long-term solution, water-source heat pumps would work well, but without the need to individually monitor energy use across all the apartments, it was hard to justify the extensive core drilling necessary to install a system.
The use of mini-split heat pumps as a one-for-one replacement for the existing PTACs was also discussed. Efficiency levels were on par with what was needed, but with the tall, slender building footprint, finding a place to install all the condensers was difficult and it required too many wall penetrations.
Melling engineers began to consider a central variable refrigerant flow (VRF) system. Since the retrofit needed to take place while the building was completely occupied, VRF was chosen.
With a variety of systems considered for the 200 tons of capacity needed, the Airstage heat-recovery system from Fujitsu General America, Fairfield, NJ (fujitsugeneral.com), was selected because of its overall efficiency. In addition, the company’s RBUs (refrigerant branch units) fit in the small spaces allotted.
The Tabco building’s shape meant that most of the rooms had external walls and windows. This creates the potential for widely varying loads, and increases the likelihood of different zones simultaneously calling for heat and cooling. Being able to recycle heat, or extract heat from one space and reject it into another zone on the same system, was a big advantage, especially during the shoulder seasons.
The final plan used 22 VRF condensers, most 10-tons each. The units were paired into 20-ton systems, with one system serving two floors. All but two systems were placed on the roof. Apartments on the first and second floor are served by a ground-mounted system, while common areas in the basement and first floors use a four-ton Fujitsu JII single-phase VRF system, also located at grade.
Work began in February 2016. One-bedroom apartments were outfitted with an 18,000 Btu slim-duct unit that was enclosed in a new soffit with an access panel. Studio apartments are conditioned by a wall-mount evaporator. Depending on the size of the apartment, these range from 9,000 to 14,000 Btu. The PTAC units remained in service for the duration of the project. Without interrupting either the heating or cooling in any apartments, tenants were only displaced for several hours.
On each floor, there are two, four-branch RBUs and two single-branch RBUs. Unlike a mini-split system, the use of the RBUs with heat-recovery condensing units not only allows the system to supply simultaneous heating and cooling, but also to share refrigerant between two evaporators. If one room is calling for heat, and another on the same floor is in cooling mode, the cooling unit can effectively reject heat into the room calling for heat.
Line-sets from the roof were run through an old common exhaust vent. In addition, the vent was used to run new power lines to the roof. After the Airstage condensers were craned to the roof, they were placed on top of a custom-made framework on both sides of the elevator tower. A single, 48,000 Btu Fujitsu Halcyon unit was also used to help condition the upper portion of the elevator shaft. Work on the mechanical system culminated in October.
“At New Ecology, we perform comprehensive audits with payback analysis, to help owners make decisions,” said Davey. “In simple numbers, modeling showed that the VRF retrofit at Tabco Towers would yield a 15-year payback before any subsidy or grant money was accounted for. After receiving funding based on the criteria in Enterprise Green Communities Certification, the real return on investment might be about five or six years. That said, we perform robust post-project analyses for every project we’re involved with, tracking building performance for years to make sure it stays on track and to deepen our understanding.”
The hope is that Tabco Towers can serve as a template for similar projects across the country, especially as redevelopment requirements set higher energy-efficiency standards.