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.
CSE: When retrofitting an existing building, what challenges have you faced and how have you overcome them?
Jill Walsh: One of the biggest challenges with retrofitting a large tenant space in an existing office building is getting involved with the project early on, preferably during the lease-review phase. For our clients, we prefer to invest time before they sign the lease. This may include visiting several sites with the client. By being involved early, we can review what the landlord is providing for power, the conditions of existing HVAC equipment, and the options for supplemental cooling. Clients are then able to better project their costs, i.e., if the existing HVAC unit is 20 to 25 yr old, they may want to include the cost for a new unit in the project budget. If the power being provided is insufficient for that tenant and a new feeder must be installed from the basement, that cost can be negotiated during the lease phase. By understanding their options for supplemental cooling (condenser water, louvers to the outside, overtime charges) they can better understand if a particular space is appropriate for their needs. Early involvement can mitigate or eliminate many of the challenges we see when engineering a tenant’s space in an existing building.
Nazeri: Challenges in retrofitting existing buildings include:
- Unforeseen conditions; absent accurate as-built drawings, a detailed survey is often required.
- Structural support for new equipment must be carefully coordinated.
- Thermal performance of a building envelope should be enhanced where possible. This is more difficult in historical building facades.
- Historic building structures where many architectural elements are to remain may present interesting challenges. Oftentimes, alternate means of cooling or heating delivery must be investigated. Interestingly, these buildings also provide opportunities for adaptive-control application and operable windows in moderate climates. Hydronic or radiant heating are two possible heating alternatives. For cooling applications, we have used displacement ventilation, variable refrigerant flow (VRF) systems or concealed air-distribution devices, and reused and enhanced old distribution openings.
Michael Walsh: PEDCO recently designed a major 235,000-sq-ft renovation to a historic building in the Cincinnati area. To achieve the owner’s vision of an open and collaborative work environment for the office areas, the architect needed to raise the ceiling height. This caused the interstitial space above the ceiling to be reduced to less than 12 in. in some locations, making more traditional mechanical-cooling systems an engineering design challenge. The age and historic nature of the building presented a couple of unique challenges: how to address an older building skin with an inefficient thermal barrier with a mixture of replaced windows, and how to document 75 yr of system rework to minimize unexpected outages during construction. One of the first steps in the renovation process was to address the building envelope to understand potential issues with weather-related damage and excessive air leakage or infiltration, leaks, and condensation. With the lack of ceiling height in the office areas, PEDCO was challenged to design a HVAC system to meet the cooling demand along with maintaining the open vision. PEDCO designed the HVAC system with 4,400 lineal ft of active chilled beams to manage the large sensible cooling loads in the office yet achieve excellent energy performance and low acoustic signatures. The engineering design incorporated new vertical chases within the architectural layout to provide a phased approach to distribute air and hot and chilled water where needed and maintain an operational building during the construction phase.
Arnold: A common challenge of retrofit projects, in general, is designing within existing spatial constraints. In many retrofit projects, we need to work around low floor-to-floor heights and existing obstructions. These issues are not easily overcome within the budget constraints of most projects. However, careful coordination with the project architect can often allow creative ceiling designs while still maintaining adequate headroom for MEP services.
Sullivan: Building retrofits are always changing, especially in Washington, D.C., where the ceiling plenum heights are between 12 in. and 16 in. To work with this challenge, the design team must specify specialized equipment, such as low-profile VAV terminals low-profile LED lighting, and use very shallow ductwork where the aspect ratio can exceed 10:1.
CSE: Have you specified more alternative HVAC systems on office building projects recently? This may include displacement ventilation, UFAD, VRF systems, chilled beams, etc.
Michael Walsh: We have definitely seen a rise in the acceptance of alternative HVAC systems with clients. We have designed a 376,000-sq-ft new office building using a combination of UFAD and passive chilled beams as well as a major renovation of a 235,000-sq-ft office building using active chilled beams. We have also designed several new and renovation projects using VRF systems.
Nazeri: We have designed UFAD and displacement systems for more than 15 yr in the United States for a variety of building types and applications. Chilled-beam systems, VRF systems, radiant cooling and heating panels, and high-temperature chilled water systems are some of the systems we typically consider.
CSE: Describe a challenging building envelope project you recently designed in an office building.
Michael Walsh: When we designed a new 376,000-sq-ft office building for a global financial services corporation, the chairman challenged the design team to develop a high-performance building. The mechanical design process resulted in a combination of UFAD and a passive chilled-beam system. We worked closely with the architect, who understood the need for the building envelope to match the high performance of the HVAC system. Through modeling and constant coordination with the architect, the design team made educated decisions on the envelope. The results prove the success, as the building performs over 40% more efficient on a square-foot basis as compared with a sister building that was designed using a standard VAV-type system.