U.S. District Courthouse for the Northern District of Iowa
New construction; U.S. District Courthouse for the Northern District of Iowa; KJWW Engineering Consultants
Engineering firm: KJWW Engineering Consultants
2013 MEP Giants rank: 29
Project: U.S. District Courthouse for the Northern District of Iowa
Address: Cedar Rapids, Iowa, United States
Building type: Office
Project type: New construction
Engineering services: Code Compliance, Electrical/Power, HVAC
Project timeline: April 2009 to July 2012
Engineering services budget: N/A
MEP budget: $1.48 million
The new 287,600-sq-ft LEED Gold federal courthouse in Cedar Rapids presented many challenges due to the often-competing security and sustainability requirements of the General Services Administration (GSA). At the same time, KJWW needed to make sure the systems designed to address these challenges did not undermine the architect’s “Big ideas of the building,” which included a 59,675-sq-ft glass curtain wall facing north and a spacious atrium in the center which splits the building into two distinct sides. The GSA mandated that the building be extremely energy efficient, using no more than 55 kBtu/sq ft each year. The GSA’s security criteria, meanwhile, included such items as enhanced steel reinforcements and systems with greater-than-normal energy use, such as security-based lighting and continuously-running dedicated ventilation for holding cells and mail room.
The most overlap between security, HVAC design, and sustainability occurred in the air distribution design. The GSA’s Facilities Standards for the Public Buildings Service (PBS P100) requires that no air handling unit (AHU) can be larger than 25,000 cfm and no unit can serve more than one floor. With 287,600 sq ft over eight floors, split in two sides (courtroom side and tenant side), there had to be an AHU for each floor on each side (16 total AHUs). In addition, code required each AHU to bring in outside ventilation air, but the GSA’s anti-terrorism standards do not allow for any air intake below the 4th floor, and the architect did not want any air intake or relief on any exterior walls. Traditional mixed air AHUs normally would have air economizers, which bring in air from the outside to condition the building when outside air conditions are right. For the courthouse, this would have required each floor’s AHU to be ducted to the roof. With all the required AHUs, such a system would have a far greater need for mechanical room space, something that was not cost effective where floor space was at a premium and being built at $300 to $400 per sq ft.
To meet the GSA’s energy efficiency goals and security requirements and not compromise key architectural features, KJWW developed two key solutions which were supported through energy modeling evaluations by The Weidt Group:
- Elimination of the outside air capability for all AHUs and installation of a completely decoupled dedicated outside air system. Two units on the ground floor have the sole purpose of providing outside air for the building. They draw in air from the roof, condition it, and send it through the building for ventilation air. They also take all the air that they put into the building, bring it back to the units, and exhaust the air at grade level. The outside air coming in and the exhaust air going out exchange energy through enthalpy wheels (another energy-saving measure).
- The use of water economizers instead of air economizers for the AHUs. Water economizers create chilled water by routing the chiller system condenser water outdoors to be cooled by the cold ambient air, and then use that cold water to cool the indoor system chilled water via a heat exchanger. The system creates chilled water in the winter without having to run mechanical cooling. Water economizers have their own challenges, however. Ice can form on the cooling tower in the colder months. During transitional months when days are warm but nights are still cold, the mechanical chillers, needed during the day, often don’t start up easily when there is still cold water coming to them from the water economizer that may have operated overnight. The engineers developed a control sequence for going from one system to the other during these periods to ensure the system ran reliably.
Commissioning played a critical role in verifying the systems operated as designed. Although the water economizers added a degree of operation complexity, KJWW and the GSA determined that the water economizers would cost far less on a lifecycle basis than air economizers, which would have required larger air handling rooms throughout the building. In addition, the chases and chase space required by air economizers for intake and relief would have been impractical for the building, where floor space was at a premium. Another key energy-saving feature was the use of electric “point-of-use” water heaters instead of a central hot water system, since there is not much need for hot water in the building.
While gas is less expensive for heating water, installing small point-of-use electric heaters where the water is needed—at each sink or bathroom group—eliminated the need for central storage of hot water, distribution and recirculation piping, saving all the cost involved and eliminating “parasitic” heat loss. As for the glass curtain wall, members of the core design team traveled to Washington, DC, where the GSA’s HVAC Peer Review panel accepted the engineers’ solution that flushing air along the wall would prevent condensation.