Engineering flexible office buildings: HVAC
Office buildings need to have the ability to accommodate myriad types of businesses and activities to meet the needs of tenants. Here, engineers with experience on such buildings share their knowledge, lessons learned over the years for HVAC systems.
Cory J. Abramowicz, PE, HBDP, LEED AP, Associate, ESD (Environmental Systems Design), Chicago.
Matt Chandler, PE, LEED AP BD+C, BEAP, Senior Engineer, KJWW Engineering Consultants, St. Louis.
Andrew B. Horning, MS, LEED AP BD+C, Associate & Project Manager, Bala Consulting Engineers Inc., King of Prussia, Pa.
Julianne Laue, PE, LEED AP, BEAP, BEMP, Senior Energy Engineer, Mortenson Construction, Minneapolis.
Matthew Pastore, CxA, Director of Design-Build Services, GHT Ltd., Arlington, Va.
John Yoon, PE, LEED AP, Lead Electrical Engineer, McGuire Engineers Inc., Chicago.
Mike Walters, PE, LEED AP, Campus Energy Market Leader, MEP Associates LLC, St. Paul, Minn.
CSE: Have you specified distinctive HVAC systems on any office buildings? What unusual or infrequently specified products or systems did you use to meet challenging HVAC needs?
Laue: HVAC systems in office buildings have traditionally been variable air volume (VAV). We are seeing an increased demand to investigate alternative high-efficiency systems, such as variable refrigerant flow (VRF), UFAD, and chilled beams. Many times, these systems are necessary in remodels where floor-to-floor heights might restrict the ability to install ductwork. These systems used to be considered unusual, but are becoming more common in conversations surrounding HVAC design.
Horning: We have designed several active chilled-beam projects in low-, medium-, and high-rise settings. Additionally, we are currently engineering several VRF systems for conventional open/enclosed office scenarios, rather than typical VAV or fan-coil systems.
CSE: Have you specified VRF systems, chilled beams, or other types of HVAC systems into an office building? If so, describe its challenges and solutions.
Yoon: The concept of chilled beams may seem like a relatively new concept, but it isn't. At least, not in the Chicago metropolitan area. From about 1960 to 1975, the HVAC system of choice for perimeter areas in Chicago's commercial high-rise buildings was the induction unit-and most of those systems are still in operation today. Induction units are nothing more than active chilled beams. While those induction units allowed for a higher ceiling along the window line due to reduced ductwork requirements, the primary disadvantages were increased noise from air turbulence, potential condensation issues, and costs associated with modifying controls (adding valves) to subdivide existing control zones. Those same limitations, although reduced, are still a consideration with today's chilled-beam systems. However, there have been significant advances in technology that have made chilled beams much more effective as compared with those old induction unit systems. Today's dedicated outside air system (DOAS) units are typically more effective at controlling humidity in the primary airstream. The availability of DDC dew point/condensation sensors, which can be installed at the chilled beam, also makes it easier to squeeze every last bit of capacity out of the system on a design day.
Horning: Some advantages to chilled-beam systems are reduced energy consumption, reduced mechanical space on each floor, increased ceiling heights, improved thermal comfort and air quality, and low noise levels. Close attention to dehumidification is critical when designing a chilled-beam system.
Abramowicz: For the retrofit of an existing office building located in Wisconsin, we replaced the existing dual-duct system with a chilled-beam system to serve the perimeter and a VAV system to serve the interior. Chilled beams were alluring because we could maximize level-by-level ceiling heights. One of the original challenges was designing the chilled-beam system to be cost-effective. With a conventional overhead forced-air system to serve the perimeter, the floor-by-floor primary air ductwork is large, which can impede on the space ceiling heights. However, VAV boxes are also much cheaper on a unit basis than chilled beams. To make chilled beams cost-effective, we reused the existing ductwork risers, which would have been too small for a conventional system.
Laue: The biggest VRF challenges I've experienced involved locating the condensing units and owner/mechanical contractor familiarity. A good integrated design team can easily overcome these issues if they are addressed early on in the project. Chilled-beam challenges tend to be around first cost because you have more pipe.
CSE: What unique HVAC requirements do such projects have that you wouldn't encounter in other projects?
Abramowicz: Chilled beams became an optimal solution to achieve desired ceiling heights when we were faced with smaller-than-average floor-to-floor heights, yet the client wanted office build-out ceiling heights that were competitive with the local industry standard.
Horning: We have found that both active chilled-beam and VRF systems in the open-ceiling environment become more of an aesthetic feature, requiring a high degree of architectural coordination for the devices, ductwork, and piping.
CSE: When retrofitting existing office facilities, what challenges have you faced and how have you overcome them?
Yoon: A reoccurring challenge in existing office buildings is the industry trend toward higher ceilings in combination with open office layouts with high-density bench-type seating. This is a "double whammy," with less room for distribution ductwork and higher interior loads that the base building systems were never designed to accommodate. Where we have extreme cooling-capacity issues, we commonly specify distributed VRF systems for supplemental cooling. However, that doesn't necessarily address ventilation requirements associated with the increased population density. For smaller buildings where we can penetrate an exterior wall, energy-recovery ventilation is an unattractive yet quick fix. For larger buildings, it often involves modifying the base building system and adding demand-control ventilation.
Abramowicz: When designing a new chilled-beam perimeter system in an existing structure, some areas of the building may present distinct challenges. In one project we worked on, a new building envelope was included in the building design that cantilevered the additional floor footprint past the existing perimeter structural beam. Due to the location of the structural beam, the chilled beams were not able to be located adjacent to the perimeter glass. Each location where this occurred was reviewed and analyzed through computational fluid dynamics models.
CSE: When addressing indoor air quality issues, what best practices or tips do you have for other mechanical engineers? Describe air-change rates, particle concentrations, humidity, and other issues.
Chandler: When retrofitting existing office facilities, a common challenge is not having adequate plumbing in the area where new fixtures are desired. Domestic hot and cold water usually can be extended easily, possibly with the addition of local water heaters. Sanitary piping is much more challenging and can be addressed with a thorough site survey and design of new sanitary piping carefully routed through the floor below, or sawcutting the slab on grade to the appropriate tie-in location. In some instances, sewage pumps may be desired to reduce the construction costs and impact on building occupants.