Chilled beam design provides energy-efficient building

Learning objectives:

• Understand the conditions that could make an active chilled beam system a viable specification for the mechanical system renovation of an older building.
• Explain under what circumstances humidity can be a major risk for such a system.
• Identify how HVAC systems can work together to overcome the humidity risk associated with an active chilled beam system.

It’s rare for an engineer to design the renovation of a building in which he or she once studied, but that was the opportunity as the project manager and lead mechanical engineer for the recent renovation of Marston Hall at Iowa State University (ISU) in Ames.

Marston Hall had been through several piecemeal renovations over the years, creating an incoherent mix of small offices, student spaces, and classrooms. Some spaces had been modernized, and some spaces remained original. The mechanical systems, some of which were original to the building, were well beyond their useful life and not meeting the needs of the occupants.

Returning to Marston Hall some 20 years later, it remained largely in the same condition. The renovation project, however, would provide updated classrooms, an auditorium, student interaction spaces, offices, and a welcome center. Most of the design team had connections to Iowa State, and the project represented a huge opportunity to positively impact the university for decades to come.

The project started in 2010 with a building master plan study, which led to two goals for the $18.9 million (construction cost), 2-year renovation that began in 2014:

• Transform all spaces into a modern, comfortable and energy-efficient learning environment.
• Preserve the building’s history and architectural integrity.

Meeting these often-competing goals would prove challenging.

Marston Hall’s existing structure—a combination of flat masonry arches, steel, concrete, and stone—required different considerations than a newer building’s renovation. Seventy-five percent of the original interior structure would have to be removed, including much of the existing hollow clay tile floor and interior load-bearing masonry. (At one point during the construction the roof structure was supported by the exterior walls and 17 interior shoring towers.) New steel frames with steel composite decking were inserted to create large, flexible classrooms, and the north and south wings of the building were completely restructured—significantly increasing the size of spaces that could be accommodated.

System selection

Marston Hall’s new HVAC system—which would be served by the campus 44°F chilled water and 90 psi steam supply—needed to meet three main criteria: fit within the physical limitations of the existing building, provide individual room-by-room zoning for temperature control, and achieve ISU’s energy efficiency and U.S. Green Building Council LEED Gold certification.

Two envelope improvements served as a first step in achieving the energy-efficiency goals: new insulation that increased the masonry building’s exterior wall R-value from 2.7 to 15, and a prior replacement of the building’s operable windows with modern (but still operable) double-pane units.

Other load-reducing features included carbon dioxide control of ventilation to high-density spaces, vacancy sensors controlling space lighting, light dimming with daylighting, and LED lighting throughout the building at just 0.86 W/sq ft, 27% below the requirements of ASHRAE Standard 90.1-2010: Energy Standard for Buildings Except Low-Rise Residential Buildings.

The design team discussed several possible mechanical system combinations, then narrowed down the list to four HVAC systems that were analyzed and compared for suitability:

• Central air handling unit (AHU) with variable air volume (VAV) distribution.
• Fan coil units with dedicated outside air system (DOAS).
• Variable refrigerant flow fan coils with DOAS
• Chilled beam/radiant heat with DOAS.

While each system had its advantages (see comparison charts), the chosen design—chilled beam/radiant heat with DOAS—was selected as the most cost-effective combination of systems that would provide excellent energy efficiency and user comfort—in addition to the best solution for fitting within the building. (See related article, “Solutions for tight spaces in HVAC design.”)

Two other cooling systems are used in spaces that are not conducive to chilled beams:

• Fan coil units are used in corridors and entry vestibules, where substantial amounts of outside air can enter the building, causing quick fluctuations in temperature and humidity.
• An underfloor air displacement system for cooling and ventilation is employed in the large auditorium, which has higher ceilings than practical for using chilled beams. The underfloor supply system also increases ventilation effectiveness in this higher-density space. (Outside air from the DOAS unit is ducted separately into the space and controlled by carbon dioxide sensing. As such, this air handler is not a mixed air system and can be set back or shut down if desired when the space temperature set point is satisfied.)

By using active chilled beam and perimeter radiant hydronic heat systems throughout most of the building, the system provides a much more energy-efficient means of transferring cooling and heating energy compared to a traditional ducted air system. As a point of interest, the campus steam and chilled water supplies are partly produced through cogeneration at the campus power plant.

The DOAS unit enabled the engineering team to minimize ventilation—one of the largest loads that must be accommodated due to extreme winter and summer conditions experienced by facilities in Climate Zone 5A, where ISU is located. This unit allowed the amount of ventilation air provided to the building to be the minimum required (based on occupancy and square footage) by ASHRAE Standard 62.1-2010: Ventilation for Acceptable Indoor Air Quality.

Whereas a traditional ducted system in Marston Hall may require 15,000 to 20,000 cfm of ventilation air, the DOAS unit allowed us to provide just 10,000 cfm. (Engineers also ensured the DOAS supply duct leakage was minimized by specifying Class A duct sealing and supplemental joint taping at all duct joints—including all VAV box joints and seams—to prevent ventilation air from escaping before reaching the chilled beams.) With the amount of ventilation air minimized, the peak heating and cooling loads associated with the ventilation air were then further reduced by energy recovery and sensible reheat wheels within the DOAS AHU.

Chilled beam operation

This project demonstrates that using a chilled beam system for cooling in an older building in a humid climate can be successful if handled properly from design, operation, and occupant education standpoints. For Marston Hall, these challenges and solutions included:

Providing needed activation airflow. An active chilled beam system requires a supply air source to induce airflow through the beam and create the cooling effect. However, the activation air required by the beams is at times more than the minimum ventilation (required by code) that is being supplied by the DOAS. To activate the beams in these situations without increasing the ventilation load, fan-powered VAV boxes were added to serve these spaces to recirculate additional air. (The boxes are located outside classrooms and offices to accommodate acoustic considerations.)

Working in parallel with the DOAS unit, these booster fans serve as a key component in minimizing the ventilation air, equipment and ductwork size, and energy use. In the winter, the fans turn off and the chilled beams are simply a conduit for the ventilation air to enter the space.

Controlling humidity. Humidity is a risk with an active chilled beam system; if the space gets too humid, condensation will form on the cold coils of the chilled beams, causing dripping. This was a huge challenge on this project, a century-old masonry building in the Midwest with operable windows—conditions that would usually preclude the use of chilled beams. But the university understood the operational risk and was on board with the design. To overcome this risk, early in design an analysis was conducted of the peak space latent moisture loads by source: external vapor permeation, external vapor infiltration, and occupants. By far, moisture from occupants was shown to be the dominant source.

Knowing Marston Hall would have classrooms potentially full of 80 people, the design team determined how dry the air needed to be to absorb the moisture given off to the space by those people through normal breathing and sweating out a test. Based on the latent load calculated in the spaces, it was shown supply air with a moisture content of no more than 44 grains of water per pound of air was needed.

To combat the risk of condensation on the chilled beams, the following strategies were employed:

• A passive dehumidification wheel was included on the DOAS to provide ventilation air to the building at the following conditions: 60°F dry bulb/51.2°F wet bulb/44 grains/lb of moisture/44°F dewpoint. This dry ventilation air absorbs space latent loads and ensures the space dewpoint remains below 55°F, which is below the chilled beam loop setpoint temperature of 57°F. (By comparison, typical 55°F saturated supply air—as may be used in a traditional VAV system—has a moisture content of 60 to 65 grains/lb with a dewpoint of 54°F to 55°F.) As noted earlier, the DOAS supply ductwork and VAV box seams were tightly sealed to ensure the ventilation air reached the chilled beams.
• Dewpoint sensors were installed in each space to lock out the chilled beam cooling when a space’s dewpoint reaches 57°F.
• Occupants were educated on the mechanical system operation, and to understand the potential issues caused by leaving office space windows open at the wrong times. In addition, the chilled beam and other mechanical systems are comprised of equipment that is familiar to the ISU facilities staff, and only require simple regular maintenance.

Drastically improved EUI, LEED Gold Certification, Key M&V Findings

While LEED energy modeling analysis predicted the building would consume approximately 53 kBtu/sq ft/year—a 33.5% reduction over the code allowable 86 kBtu/sq ft/year—the first year of data indicated that the building used approximately 75 kBtu/sq ft/year. While this was still a 30% improvement over the pre-renovation EUI of 105 KBtu, it did not meet the team’s expectations. The university and design team executed the LEED measurement and verification (M&V) analysis credit and determined a key cause of the higher-than-anticipated energy consumption. A steam preheat coil control valve serving the DOAS unit had been commanded open 100% at all times. This inflated both chilled water and steam energy use unnecessarily, and corrections to the control valve operation were made in spring 2018. Data gathered since the steam valve correction was made indicates that the building EUI has been reduced to 58 KBtu, within 10% of the original LEED modeling.

The improvement has been  achieved even with the building now fully occupied, properly ventilated per ASHRAE 62.1-2010, and being used more than it was before the renovation.

Marston Hall was completed on time for occupancy during the fall semester of 2016, after nearly 2 years of renovation. The building received LEED-NC Gold certification in 2017, with more than 25% of its LEED credits earned for energy efficiency.

HVAC systems considered for Marston Hall

Chilled beam/radiant heat with dedicated outside air system (DOAS)

Variable refrigerant flow (VRF) fan coils with DOAS

Fan coil units with DOAS

Central air handling unit (AHU) with variable air volume (VAV) distribution

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