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By Ted Smalley Bowen and Deborah Snoonian, P.E.

An HVAC system that’s nearly invisible

The mechanical systems at Trinity were long overdue for an upgrade, and the addition of the undercroft meant an increase in heating and cooling loads. Concerns about energy efficiency led to the installation of a geothermal HVAC system that draws on six, 1,500-foot-deep geothermal wells that tap the constant temperature of the bedrock to efficiently condition Trinity’s interior [record, July 2003, page 156]. While this form of heating and cooling is popular with preservationists, in part because of its relatively unobtrusive equipment, it wasn’t initially part of the program for Trinity because the architects didn’t realize it was feasible at the site. As the design team struggled with where to place bulky mechanical equipment and ways to disguise it, Cosentini Associates suggested using the geothermal system. “We kind of backed into it,” admits Jean Carroon, AIA, a principal at Goody Clancy and head of the firm’s preservation and renovation practice.

The undercroft (left) adds much-needed flexible gathering spaces underneath Trinity. Scaffolding (right) was designed to protect the exterior granite and brownstone while visually blocking them as little as possible.

Although geothermal systems are increasingly popular in the Boston area, the Back Bay’s gravel landfill and variable surface water posed problems at the site. “There’s surface water down to 270 feet, so we had to seal the wells down to 280 feet until they hit bedrock,” Carroon says.

As of early November, there were no performance data yet for the HVAC system, which had only recently come online, according to Carroon. The initial cost of the ground-source heat pumps for Trinity was comparable to conventional HVAC systems, which would have required structural reinforcement of the church’s upper story, complex piping to reach a mechanical room, and labor-intensive camouflaging of the system, she noted.

At present, all the HVAC equipment, along with security systems, alarms, and water monitoring, are handled through a centralized direct digital control system. Such systems are standard for many types of institutional buildings, but “it’s an incredible leap forward for the church because it allows the director of facilities to monitor and change fan speeds, room temperature, and security from off-site or one central location on-site,” says Carroon. Lighting systems are also managed through a programmed master panel, allowing better control over, and oversight of, Trinity’s energy consumption.

Digging deep to add space

Expanding the basement to create the undercroft required careful choreography to preserve the structural integrity of the building while accommodating construction work and permitting regular church services. Before the expansion, the area underneath the church was a shallow storage cellar. The contractors had to excavate 4 feet down to create enough clearance for the undercroft, which, because of the hemmed-in site, had to be fitted entirely within the confines of Trinity’s existing foundation.

Shawmut dug more than 20 test pits around the foundation so that engineers could inspect the wood piles. Few of the roughly 4,500 piles under Trinity were found to be damaged, according to Carroon. The design team also discovered that redundant piles had been driven underneath the church’s chancel area, evidence that Richardson’s design evolved during construction.

Goody Clancy’s design for the undercroft provides modern amenities while mirroring elements of Trinity’s main floor. The architects replaced the thicket of brick piers that supported the cellar with widely spaced columns to create a more open, flowing interior space that can accommodate gatherings of many sizes. Some of those new columns were placed on the redundant wood beams, according to Carroon. By exposing the massive pyramid-shaped granite piers that support Trinity’s 130-foot central tower, they evoke the monumental aspects of the church above while visually anchoring the space. LeMessurier’s structural scheme also allowed the designers to float new steel grade beams over the four piers, instead of transferring more weight to them. The new steel grade beams and columns also carried the weight of the interior scaffolding used by painting conservationists in the central tower.

The details of the new space—wooden benches, columns, and small expanses of paneling—are spare but well-crafted. Other than the granite piers, the undercroft’s boldest features are a pair of intensely colorful, structural glass walls, which echo the stained-glass windows, murals, and decorative painting above. The 32-by-9-foot walls, designed by Alexander Beleschenko and Rafaella Sirtoli Schnell, each feature a pair of 8-foot-wide pivoting doors, permitting the space to be partitioned as needed. The walls are hung from the grade beams and closely fit the contours of the granite piers that the church’s tower rests on.

Access to the undercroft is gained via new stairwells from the main floor to the basement. These line up below the original narthex (vestibule) stairs to the balcony and have comparably proportioned but simplified elements, in keeping with preservation goals. Similarly, a new elevator was installed in a former storage closet to minimize its visual impact; and metal handrails of a handicapped entrance that are cut into the brownstone cloister colonnade are made of a different material, and are not attached to the stone.