Getting it right in mixed-use buildings: HVAC
Mixed-use buildings—often a combination of retail and residential—are unique structures with varying needs. HVAC systems should be carefully considered.
Michael Albanese, PE, LEED GA, Senior Associate, Kohrs Lonnemann Heil Engineers, Fort Thomas, Ky.
David Callan, PE, CEM, HBDP, LEED AP, QCxP, Vice President, McGuire Engineers, Chicago
Donna Miller, PE, PEng, LEED AP, Vice President, Engineering, WD Partners, Dublin, Ohio
Gary Poole, PE, Principal, Bury Inc., Houston
Andrew H. Smith, PE, CEM, LEED AP, Principal, Jordan & Skala Engineers, Dallas
CSE: What unique HVAC requirements do mixed-use projects have that you wouldn't encounter in other engineering disciplines?
Poole: Unique in the MEP/FP world is usually the singular application of proven systems for a unique environment. When designing HVAC systems, the key is determining the impact placed onto the facility's infrastructure by the varying conditions that the facility will experience.
Smith: HVAC design is unique in mixed-use projects due to the various unknowns. Mixed-use projects are very fluid and the mechanical engineer must have the experience and foresight to anticipate the appropriate HVAC systems that may be installed during future tenant improvement work. Proper up-front space planning and coordination with the architect and developer are critical to a successful mixed-use project flexible enough to accommodate ever-changing tenant criteria.
Miller: Some of the unique challenges we encounter with mixed-use projects are the diversity of code requirements and HVAC systems required for mixed-use occupancy. Additionally, the need for independent temperature control, greater quantities of terminal equipment, and thorough reviews of outside-air ventilation strategies require that these projects receive more attention from the HVAC engineer. Where these facilities have restaurants, we focus our engineering on reducing the odor and noise into adjacent occupancies that can result from the restaurant kitchens.
Callan: The real challenge is not the movement of Btus about the facility—we understand that fairly well. For engineers, the main challenge is understanding who will own, operate, and eventually have to replace all of these components in a mixed-use building. If you consider the eventual failure of your design, in addition to all the great criteria we design for today, you'll saddle the owner with a lower total cost of ownership.
Albanese: Spatial requirements in high-density locations is a driving factor in the engineering of HVAC systems. Typically, other engineering disciplines do not face the coordination and spatial challenges of the larger HVAC systems. A high level of coordination is required between the engineering, structural, and architectural teams.
CSE: What changes in fans, variable frequency drives (VFDs), and other related equipment have you experienced?
Smith: We are seeing more use of VFDs over the past several years due to affordability and desire for efficient monitoring, as well as the control of fan and pump systems.
Poole: The use of fan array, electronically commutated motors (ECM), and VFDs have greatly reduced the energy use of many of the HVAC motor loads.
Callan: The mechanical side of things tends to work predictably. As alluded to earlier, it is the electrical and electronic aspects of these systems that provide entertainment. We recently experienced a series of unexplained new-component failures on a project that stymied everyone, only to learn that the software within a single VFD on the system caused cascading failure. This isn't your father's centrifugal pump anymore.
Miller: The single-zone variable air volume (VAV), two-speed fan via VFD, and general exhaust fan direct-drive pseudo VFDs are becoming more common in our experience. Building owners have been requesting these systems in their facilities in part because they have noticed the benefit of energy savings and reduced costs as a result of having these systems.
CSE: What indoor air quality (IAQ) or indoor environmental quality (IEQ) challenges have you recently overcome? Describe the project, and how you solved the problem.
Miller: When working with a very small mixed-use building, the need for continuous ventilation and the use of split-system HVAC can result in unacceptable indoor conditions of cold humid air, or moderate temperature-humid air. These conditions were resolved by adding reheat, a new terminal unit, and revising airflows to some of the spaces to reduce outside air percentage at each unit.
Callan: IAQ and IEQ are fairly well-documented. Some of the biggest IEQ and IAQ issues stem from sources not often found in commercial buildings like large commercial kitchens and swimming pools, for instance. Many of the IEQ issues will stem from inadequate ventilation, poor pressurization control, or general lack of humidity control. In addition, the usual culprits, like cooling tower and air-intake proximity, are at play. Occasionally, we will run into special process uses where pollution containment is critical. In buildings that can house nearly any function, IEQ can be a challenge. For buildings with a residential or hospitality component, oftentimes developers will not invest in adequate HVAC for the common areas and corridors. Besides being comfortable, these areas of the building should be properly dehumidified. Modern building envelopes are tighter and do not breathe. Residential HVAC equipment is designed to primarily heat and cool, not dehumidify. The paradigm has shifted and designers and developers need to do a better job.
CSE: Have you specified more alternative HVAC systems on mixed-use building projects recently? This may include displacement ventilation, underfloor air distribution, variable refrigerant flow (VRF) systems, chilled beams, etc.
Poole: VRF systems are a rising force in the HVAC world. There is still a lot more to learn in the next few years about the practical application of these systems.
Callan: We find it more acceptable to developers and owners to use alternative HVAC products and systems. Since every "new" technology has been applied in the field for at least 15 to 25 yr, the risk is acceptable. Many of our alternative HVAC applications are driven by end-user preferences or first-cost reduction.
Smith: VRF systems are finding their way into residential mixed-use projects, but mainly within the owner's scope of work where the energy savings will have a direct impact on operating costs. In our experience, developers are not willing to pay the premium for such systems in speculative tenant spaces with separate utility metering.
CSE: Describe a challenging building envelope project you recently specified on a mixed-use project.
Albanese: Imperfections in the vapor barrier can materialize several months after occupancy as the ambient temperature and humidity fluctuates with the seasons. The HVAC system is engineered with a thermal envelope and vapor barrier in mind. The vapor barrier is critical in controlling the latent energy load the mechanical system will need to address. Projects that are renovating historic buildings or that do not address the necessary envelope requirements between tenants provide the largest challenge. A recent mixed-use project renovated the first floor while the second-, third-, and fourth-floor apartments were occupied. Construction challenges resulted in the storefront remaining open for an extended period of time. This resulted in a compromised building envelope creating a pathway for humidity to bypass the vapor barrier and infiltrate the apartment building. The apartment building became uncomfortable as mechanical systems could not address this increased load. Ensuring an extended vapor barrier was installed between tenants and sealing the first-floor storefront quickly remedied the problem.