By Ryan Edwards, P.E., Ai Sexton, EIT, and Vimal Nair, P.E., LEED AP, M.SAME
Built as a barracks in 1891, today, Building 615 at Joint Base San Antonio, Texas, is home to the 323rd Army Band, “Fort Sam’s Own.” Over the years, the facility has housed a dining hall, gym, surgery training center, and classrooms. Measuring 26,800-ft2 over two stories, the structure was converted into a band hall in 2008, but soon needed upgrades to meet current needs and building codes.
A priority upgrade was to replace the outdated ventilation system. Due to the building’s historic nature, no roof vents or exterior wall penetrations could be added to bring in fresh air. Tight ceiling spaces were not originally designed for air conditioning and could not be expanded. Military restrictions on air systems limited design options as well.
To address these issues, an innovative approach was implemented that focused on handling the envelope and ventilation loads separately while preconditioning outside air with energy recovery wheels and controlling indoor temperatures with dual-technology occupancy sensors. This energy-saving system will give the facility a more sustainable life over the long term.
MASTERING KEY CHALLENGES
Building 615 was not designed for air conditioning 130 years ago. A new ventilation system needed to serve upgrades to offices and practices rooms, as well as a performance hall. Other than dual-temperature air handler units installed in 2013, the existing HVAC had exceeded its useful life. And the control system, although fairly new, relied on a manual switchover, causing poor temperature control and discomfort during temperate periods.
The building and its east and west wings were originally heated and cooled using a two-pipe, manual switchover system served by an air-cooled chiller and a gas-fired boiler, with zero energy recovery. An underground chilled water supply and return piping connected the chiller to the dual-temperature system. A single distribution pump delivered the chilled water and hot water to all air handler units. This setup violated the requirement for redundancy stipulated in UFC 3-410-01.
Additionally, outside air was provided to the indoor air handlers from repurposed antique window openings and unsightly roof vents—an approach no longer recommended by ASHRAE 90.1 or industry standards for spaces with high outside air loads. Some air conditioning equipment also was in open space, posing a hazard.
HARMONIZING WITH HISTORY
The project aimed to respect as many historic elements as possible and to get approval from the Texas Historical Commission. For the ventilation makeover, that meant using techniques such as adapting antique attic vents to avoid new openings in the slate roof; minimizing the number of exterior wall penetrations; reclaiming antique window openings that were used for outside air on previous renovations; limiting equipment near the distinctive building exterior; and preserving unique elements such as a stained-glass inlay over a duct intake.
The entire HVAC was replaced with a four-pipe system, including new fan coil units and dedicated outside air systems with energy recovery. The dedicated outside air systems, coupled with fan coil units, handle the envelope and ventilation loads separately. Fan coil units provide heating and cooling, and the dedicated outside air systems condition outside air. Both use variable air volume boxes to precisely meet the demands while minimizing energy consumption.
The dedicated outside air system and fan coil units are fed by a four-pipe chilled water/heating hot water loop system. The new air-cooled chiller is served by two dedicated variable-speed primary pumps that are installed in parallel and operated in a lead/lag fashion based on run time to maintain full redundancy. A new modulating condensing boiler has two redundant circulator pumps that deliver a constant flow to the boiler and modulate to maintain a water temperature of 140°-F. Additionally, a set of two variable-speed hot water pumps are tapped into the closed circulating pump loop and modulate flow to the air handling system based on downstream static pressure. All pumps are installed and operated similarly to the chilled water system.
The dedicated outside air system recovers energy using enthalpy wheels, which allows for the removal of heat and moisture from outside air—an essential feature for muggy San Antonio summers.
Cooling the Performance Hall. Even though it can hold up to 200 people, a new 3,100-ft2 performance hall is only used by a few people much of the time. Occupancy detection cannot rely on carbon dioxide concentration because UFC 3-410-01 Section 4-2.4.1 requires calculating ventilation for occupied spaces based on the ventilation rate procedure specified in ASHRAE Standard 62.1. A system continuously cooled at 55°-F would require a high volume of outside air and a heavy cooling load, and would waste energy by overcooling the space when it is largely empty.
To address this, three zones in the performance hall are monitored by a dual-technology occupancy sensor to detect movement and thermal projection. The dedicated outside air system unit modulates the amount of outside air brought in based on detected occupancy levels. This creates a healthy environment when the room is fully occupied, saves energy when occupancy is sparse, and improves overall comfort.
Meeting Needs of Musicians. Mitigating the sound entering and exiting practice rooms was an important consideration, as was carefully controlling the humidity level for instrument storage. To provide precise humidity control between 47 and 49 percent, each instrument storage room is equipped with a dedicated dehumidifier that carries a relative humidity setpoint of 50 percent and a dedicated humidifier with a relative humidity setpoint of 47 percent. Temperature control is provided with a fan coil unit.
To lessen sound transmission, the mechanical design team worked with the architects to separate the HVAC equipment and practice rooms. All practice room walls are rated STC-55, and each duct penetrating these walls applied an STC-55-rated double-wall silencer to a minimum of 6-ft from both sides of the wall. The size of those duct segments was increased to accommodate the pressure drop resulting from the silencer.
Reducing HVAC Energy Use. Building 615 consumes a considerable level of energy because of the outside air required to ventilate the space and the location’s inherent high humidity levels. Energy recovery using preconditioning with enthalpy wheels was incorporated to make the HVAC system sustainable. The rotary air-to-air enthalpy wheel is made of a desiccant material and can transfer both sensible and latent energy with an effectiveness exceeding 70 percent. A compact configuration also provides a solution to space limitations.
Outside air is precooled by energy recovery wheels with the heat transferred from exhausting air. The system can precool the total ventilation air from 99.3°-F to 86.7°-F, which contributes to reducing the primary cooling coils by 8.54-TR. The chilled water valves modulate the dedicated outside air system units to maintain 55°-F, leaving coil temperature in cooling. While this approach deviated from UFC 3-410-01 Section 3-2, it complies with ASHRAE 90.1 Section 126.96.36.199 “Ventilation Air Heating Control” and is consistent with the most current industry practices.
SCORING THE RESULTS
Multiple partners contributed to giving Building 615 new life. The ventilation overhaul succeeded in reducing the size of the chiller required from 100-TR to 72-TR and received 30 percent cooling energy savings and 5 percent heating energy savings. Minimizing energy waste while meeting the ventilation requirements and maintaining comfort were also effectively achieved, as was better controlling humidity without the need for a manual changeover.
With the renovation complete, Fort Sam’s Own can prepare for performances in comfort for the foreseeable future.
Ryan Edwards, P.E., is Mechanical Engineer, Ai Sexton, EIT, is Mechanical Engineer, and Vimal Nair, P.E., LEED AP, M.SAME, is Assistant Division Manager for Facilities, Freese and Nichols Inc. They can be reached at firstname.lastname@example.org; email@example.com; and firstname.lastname@example.org.
[This article first published in the March-April 2021 issue of The Military Engineer.]