Designing, enhancing office buildings: HVAC
Office buildings might seem like relatively simple structures, but engineers with experience in the field know differently. The HVAC system is key to the comfort of building occupants.
Christopher Arnold, PE, Vice President, Wick Fisher White, Philadelphia
Saied Nazeri, PE, CPD, LEED BD+C, Senior Vice President, WSP | Parsons Brinckerhoff, San Francisco
Reardon D. Sullivan, PE, LEED AP, Principal, WFT Engineering Inc., Rockville, Md.
Jill Walsh, PE, LEED AP, Principal in Charge of Mechanical Engineering, OLA Consulting Engineers, Hawthorne, N.Y.
Michael Walsh, Project Manager, PEDCO E&A Services Inc., Cincinnati
CSE: What unique HVAC requirements do office buildings have that you wouldn't encounter in other buildings?
Arnold: Office building designs are much more driven by the perceptions of their occupants. In the design of laboratories and mission critical spaces, our systems are designed to support equipment or a specific process. We base these designs on very specific criteria. Although office environments also have specific, code-mandated criteria, we also must take the human element into consideration. All people do not perceive air temperature and movement in the same way. The best design for office space incorporates as much individual control as is possible within existing system constraints and budget.
Michael Walsh: Quite a few clients have been looking at dense packing options for their open-office space. We have also designed a few "training wings" within existing office buildings, which are also dense pack spaces. These types of spaces with a high-occupancy load create the unique HVAC challenge of meeting the outdoor air requirements for maximum occupancy. Sensors such as CO2 monitors can help control the amount of outdoor air being provided to the space, which helps improve the energy efficiency when the space is less than fully loaded. However, we have observed several cases where existing HVAC equipment designed under older versions of the codes or for a less densely occupied space cannot properly meet outdoor air requirements. In these instances, we have had to design a supplemental outdoor air unit and associated controls to make up the difference in what the existing unit can supply. The only other option is to oversupply air to the space and reheat it, which is less energy-efficient and often would not meet energy codes.
Jill Walsh: One HVAC challenge unique to office buildings is the misunderstanding that the base building systems will serve all the tenants' needs. In the New York City commercial interiors market, 100-yr-old office buildings are being used for various occupancies within the same tenant space. For instance, many fashion designers use the same floor for a showroom, a rig room (where the fashions are "rigged" for viewing and presentation), support offices, presentation rooms, and even laundry rooms. The hours of use, light levels, number of occupants, and expected room temperatures vary drastically for some tenants. Although a showroom is not in constant use, it needs to be very cool, quiet, and aesthetically pleasing when occupied. This presents a challenge to the engineer when the base building system is an older, constant-volume HVAC system. Technologies to consider include: supplemental cooling, converting to a VAV system, and sometimes even installing new and independent HVAC equipment. All of the above require money and coordination. If a client believes the base building system is a complete solution to their needs, they may not have the proper budget for the space. Educating clients on what is required early in the design phase results in a better project and typically a happier client.
Sullivan: The office building market in Washington, D.C., is unlike any place in the world. Being the capital of the free world, there are comfort, flexibility, and efficiency expectations that exceed other markets. One of the most challenging requirements has to do with the limited ceiling plenum height in Washington, D.C., buildings. By code, the height of the building is limited so to maximize the rentable floor area; the floor-to-floor height is minimized and the ceiling premium height is reduced. With above-ceiling heights that are typically between 12 and 16 in., there are only specific types of equipment that can be specified. The MEP designs also need to consider structural elements, custom access, and serviceability.
Nazeri: Office building tenants may have varying office hours or may need to work off-hours and must have access to a 24/7 cooling source for their data server or communication server rooms. HVAC systems in these buildings have large capacities, but they must also be flexible to operate under low load conditions to serve small, after-hours loads. Floor-by-floor or distributed-air systems are generally better suited to partial-capacity operation than centralized systems. Most often, 24/7 loads are dealt with by allocating certain components of the cooling plant to run 24/7 or by a totally dedicated cooling system. HVAC systems are designed to provide a thermally comfortable work environment under varying load conditions. These systems must be reliable and flexible. A certain level of redundancy must be built-in to minimize loss of business due to downtime.
CSE: What changes in fans, variable frequency drives (VFDs), and other related equipment have you experienced?
Sullivan: The world of variable-speed drives has grown significantly within the past few years. The use of active and passive filters has enabled the drives to become smaller, more efficient, and cost-effective. The electronics have become common and easily serviced by technicians.
Nazeri: We are seeing more device-integrated micro-VFDs.
Arnold: As the costs of variable-speed drives have come down over the years, their use as a means of reducing energy use has become common for both water and air distribution.
Michael Walsh: The reduction in VFD first-cost has made this technology much easier to integrate into a project with a tight budget. Partnered with premium-efficiency motors in HVAC equipment, VFDs allow significant energy savings. Similarly, electrically commutated motors (ECMs) provide energy-efficient variable-speed controls via an internal processor rather than a separate VFD. Advancements in this technology are producing larger ECM motors for use in HVAC equipment, which allows the same control as VFDs at a lower initial cost.