To preserve historical integrity, engineers used existing structures, like fireplace chimneys, to place ductwork. The rough interiors of the chimneys were lined with Tyvek building material first to guard against tearing the duct installation.

The two buildings constituted Portland's art museum through most of the 1900s, but both were closed to the public in 1980 when construction began for a new building. Designed by I.M. Pei & Partners, the post-modern Charles Shipman Payson Building is attached to the Sweat Galleries and the McLellan House, and opened in 1983. The plan even then was to restore the historic buildings, forming a three-structure museum complex that would span three centuries of art and architecture. That goal was achieved last fall, with the completion of a two-year, $13.5 million restoration of the historic buildings.

Radiators are located under the windows and are configured with three rows of flat panels to fit in with the McLellan House's Federal-era architecture.

Meticulous Restoration

Every detail of its restoration was carefully researched, from the selection of wallpaper and carpeting to the type of nails used. The craftsman who worked on the architectural millwork even devised tools that could replicate intricate molding.

The restoration of the galleries that adjoin the McLellan House received the same careful attention. Margaret Jane Mussey Sweat, the widow of Col. Lorenzo de Medici Sweat, a former congressman, bequeathed the McLellan House, along with money to build the adjoining galleries, to the art society upon her death in 1908. Designed by architect John Calvin Stevens, the Beaux-Arts style L.D.M. Sweat Memorial Galleries opened in 1911. The McLellan House and the Sweat Galleries housed the Portland museum's collection until 1980.

HVAC Design Considerations

The McLellan House is considered a work of art in itself, and its age and brick-and-mortar construction necessitated special considerations for climate control. A major concern was the effect of condensation if insulation were to be installed to provide energy efficiency. In addition, if artwork were to be displayed, it would be necessary to maintain 40% to 50% rh in accordance with guidelines established by the American Association of Museums for accreditation. It was vital to understand the effects of installing insulation or maintaining those humidity levels, and calculate whether it would cause any deterioration to the building.

Maine's cold weather is part of the problem. The colder it is outside, the less moisture it takes to cause condensation. That meant the possibility of high humidity levels, which would raise the dewpoint within the walls. Insulating the walls would cause a higher dewpoint as well. The result could be condensation in the walls that could cause enough pressure as it froze and thawed to deteriorate the face of the brick and jeopardize the integrity of other parts of the structure

That was a consequence that had to be avoided, so a series of detailed analyses were performed in which engineers calculated the dewpoints at various temperature and humidity values. Based on their findings, no insulation was installed in the McLellan House, and maintaining an rh of 29% to 30% was determined to be optimum for the integrity of the building structure. This level would be appropriate for interactive computer displays and for visitors to see how a house of that era would be decorated. It would also facilitate the display of artwork in protective boxes that provide a microenvironment that meets the higher levels of temperature and humidity.

For the Sweat Galleries, however, the 40% to 50% rh for the display of art could be maintained without damage to the building itself. Because the building is nearly a hundred years younger and constructed in a different manner than the McLellan House, it could handle the effects of dewpoint better. The Sweat Galleries are essentially a building within a building, with air space between two wall sections, and dewpoint migration into the air space would not be as damaging as it would be in the McLellan House.

Similar Systems

Because the two historic buildings are part of a three-building complex, Harriman Associates analyzed the efficacy of providing heating and cooling by extending the systems that were in the newer Payson Building. A critical part of that decision was the need to make the connections without interruptions to the climate control in the Payson Building, since a disruption could have adverse effects on the artwork displayed there.

Designers of the Payson Building had planned for an expansion of the hot water and chilled water systems for use in a future restoration of the Sweat Galleries. The two Weil-McLain boilers and 250-ton Carrier Corp. chiller installed in the Payson wing boiler plant were large enough to handle the additional load introduced by the Sweat Galleries. In addition, hot water and chilled water lines were run to a point just outside the Sweat Galleries basement and were valved off for future connections. All that was needed was to connect new piping, modify pumps, and rebalance and adjust the hot water and chilled water flows.

In the McLellan House, two Neca mini boilers had been installed about three years ago. Since the current renovation called for an elevator where the existing boilers were located, they were relocated to a new mechanical room, and double-walled, insulated breeching was used to make the final connection.

The new radiators in both buildings are made by Panel Radiator Inc., and of a flat, radiant-tube design that is much less noticeable than the more traditional fin-style baseboard radiators. In the McLellan House, the radiators are located under the windows and are configured with three rows of the flat panels, painted to blend with the decor in each room. The flat perspective fits in well with the Federal-era architecture. The same configuration is used for the radiators in the community studio located in the basement of the Sweat Galleries, and it is just as compatible with the building's Beaux-Arts style architecture.

The AHUs in both buildings are manufactured by McQuay International, with a 9,000-cfm-capacity unit in the Sweat Galleries and a 7,000-cfm capacity unit in the McLellan House. Each has a Herrmidifier humidification unit mounted within a dedicated module of the AHU. Both units are of a "blow-through" configuration, with the heating coil module installed after the cooling coil module, allowing reheating during the dehumidification mode. The larger AHU in the Sweat Galleries has a heating capacity of 462 Mbtuh and a cooling capacity of 360 Mbtuh. The heating capacity of the McLellan House unit is 209 Mbtuh and its cooling capacity is 277 Mbtuh. The humidifier in the Sweat Galleries has a capacity of 80 lb/hr of steam, and in the McLellan House, 50 lb/hr. A stacked configuration AHU was chosen for the McLellan House to save footprint space.

The AHUs are also very quiet. They are double-walled with strategically located perforated interior metal sections that not only act as an insulation barrier but also provide substantial acoustical benefit, cutting sound to fan sections and discharge plenums.

"Some noise in the operation of any HVAC system is inevitable," said Kevin Eames, the facility director who has worked in the facilities department of the museum for the past 20 years. "But we wanted an environment that would provide the least disruption to a visitor's enjoyment of the buildings and the artwork. These units don't detract from that enjoyment. In fact, the noise they make is hardly noticeable in either building," added Eames, who was also the project manager for the museum throughout the restoration project. A special effort was made to see that velocities in the duct systems were kept lower, as an added effort to minimize air noise.

VAV boxes are part of the system in the McLellan House, automatically set to maintain the temperature levels critical to protecting the building. In the Sweat Galleries, however, more flexibility was needed. While it was designed to be a constant-volume unit, it is equipped with VSDs that can be used to manually adjust airflow as needed. This allows the museum's facilities director to fine-tune airflow for any condition change that may be required. Ducts in the Sweat Galleries have reheat coils that can be used to further adjust the temperature in each individual room as required.

Systems in both buildings are tied into a central DDC system, located in the Payson wing of the museum and manufactured by Carrier. "We already had a Carrier system in the Payson Building, so it made sense to have continuity among all three buildings. And since I've updated the systems originally installed in Payson to match the level now in McLellan and Sweat Galleries, all systems 'talk' the same language. That makes it much more efficient to manage the system and to do the programming changes that are required," said Eames.

The project team elected not to install any insulation in the McLellan House. This unusual choice eliminated the risk of in-wall condensation that could otherwise be a product of Maine's cold climate and the humidity needs of the artwork.

Installation Ingenuities

The lack of space between ceilings and the adjacent floor above in the McLellan House meant that the more traditional overhead installation of ducts could not be used, and each floor of the building required a different solution. On the first floor, the ductwork for supply and return air comes up directly from the basement where the AHUs are located. However, that could not be done for the second and third floors due to space constraints.

The second floor solution was found in the fireplaces that once heated the house. All three floors of the McLellan House have fireplaces which are connected to chimneys located on the east and west sides of the building. Engineers saw the unused fireplace chimneys as an ideal receptacle for the ductwork feed to the second floor, and believed that the fireplace openings would be an ideal way to bring conditioned air, almost unnoticed, from the basement to the second floor. However, the brick chimney's unevenness and jaggedness could have damaged the vinyl insulating material on the flexible metal ductwork.

The solution was to temporarily line the chimney with Tyvek building material first. Its slippery surface made it possible to snake the ductwork in place without damaging the insulation. Each fireplace was deep enough to be given a false back that would conceal the ductwork. A supply air grille, integrated into the back of the fireplace and barely noticeable, sends conditioned air into the rooms.

Conditioned air for three spaces - a first-floor welcoming area for the public with interactive displays, a second-floor administrative office, and a meeting room on the third floor - arrives through a large shaft that was strategically placed so it didn't interfere with the use of the building and made efficient use of space. A decorative diffuser panel located in a vaulted ceiling in a third-floor conference room provides an inconspicuous spot for return air. The existing diffuser panel was carefully removed and reinstalled by skilled craftsmen to allow the duct installation.

Interior walls in the McLellan House are constructed of vertical planks, 8- to 14-in. wide and about 2.5-in. thick. Without the space provided by the more traditional 2-by-4 type of construction, there was little room to add piping. Nevertheless, engineers were able to use space along the sides of the fireplaces, and in a few instances, they had channels cut into the 2.5-in. planks so that piping could be fit in.

The Sweat Galleries had been constructed when central heating systems were available, so finding space for ductwork and piping was less of a problem than it was for the McLellan House, but some unique challenges remained. The building had been designed with a gravity heating system. Large, steel plenums with multiple steam coil radiators distributed heat by convection up from the basement, through grates about midway up the walls of the five rooms and vaulted rotunda making up the galleries. These original plenums - essentially triangulated, hollow spaces behind the plaster walls on which art was displayed - provided space for the new ductwork and insulation.

However, the modifications required were still difficult because of the imperfections and unevenness in the interior cavity walls. The original plaster walls were removed and replaced by drywall over plywood to allow more flexibility in hanging artwork. Removing the plaster exposed uneven terra cotta blocking walls, so plywood was cemented to the terra cotta, and drywall installed over that. And by repositioning the return air grates near the ceiling and supply air near the floor, they became less noticeable and also left more wall space to hang paintings.

Humidity and temperature sensors are installed in the return air ducts in the galleries, and they continually send measurements to the DDC system. The system calculates the average of the readings and uses that to determine the levels of humidity, cooling, or heating that must be set for the proper climate control, avoiding rapid temperature and humidity changes that could damage the artwork on display.

Visitors to the museum praise both the artwork and the sensitive restoration of the McLellan House and the Sweat Galleries, but few are even aware of the HVAC system. And that's the way the way it should be. ES

The sixth-oldest design firm in the country, Harriman Associates (www.harriman.com) was founded in 1870 and works with educational, cultural, corporate, health care, and government organizations throughout New England. Fax either author at 207-782-3017.