Tips to design multifamily and mixed-use buildings: HVAC
Multifamily dwellings and mixed-use buildings are becoming more prevalent. Some best practices and tips are offered for HVAC and mechanical engineering systems in these residential buildings.
- Brian Berg, PE, LEED AP, CEM, Associate Principal, Glumac, Irvine, Calif.
- David Crutchfield, PE, Principal, RMF Engineering, Charleston, S.C.
- Kieran Healy, PE, Mechanical Engineer, CCJM, Chicago
- Lui Tai, PE, Technical Services Director, JENSEN HUGHES, Toronto
- Robert J. Voth, Executive Vice President, Bala Consulting Engineers, King of Prussia, Pa.
CSE: What unique HVAC requirements do multifamily dwellings and mixed-use buildings have that you wouldn’t encounter in other buildings?
Healy: Once of the biggest differences in multifamily residential buildings with owned units (condominiums) versus other types of rental and commercial buildings is ensuring that all systems for each unit are locally powered and accessible and under the control of the unit owner. Due to the relatively small size of these units, the HVAC equipment serving these units are fairly small and generally not too efficient, and there’s no coordination between units to allow diversity and central system economies of scale.
Berg: We tend to end up with multiple system types to suit the occupancy. I’m working on a mixed-use building right now with a condenser-water system serving multiple types of mechanical units. We have water-source heat pumps for the residential floors and podium retail, water-cooled VRF for hotel floors, vertical self-contained water-cooled variable air volume for office floors, and radiant in the lobby areas that gets hot water and chilled water produced by water-cooled VRF systems.
CSE: Have you specified distinctive HVAC systems on any such facilities? What unusual or infrequently specified products or systems did you use to meet challenging HVAC needs?
Berg: In high-rise projects where we’ve brought outside air in from the sidewall, we’ve used some pressure-independent, constant-airflow regulators to maintain outside airflow to the HVAC equipment. This guards against outdoor wind currents and indoor stack effect that may impact the air balance of the outside-air systems.
CSE: Have you specified VRF systems, chilled beams, or other types of HVAC systems into one of your multifamily dwellings and mixed-use buildings? If so, describe its challenges and solutions.
Berg: We’ve used VRF on some mixed-use buildings, typically for hotel occupancies, but not for residential. The preference of splitting up utilities between residential tenants usually steers us toward water-source heat pumps or chilled-water fan coil units to more easily submeter energy usage via British thermal unit meters.
Voth: We see a significant move toward VRF in many of our multifamily projects, especially when LEED certification is an objective. We typically do not use chilled beams in any projects that include operable windows, as the beams will sweat.
Healy: One of the biggest challenges in implementing a large-scale VRF system for a multifamily complex is managing refrigerant loss that may occur from the small pipe sizes and magnitude of fittings required to distribute refrigerant throughout a large footprint. While investigating existing installations, our firm has come upon VRF installations that were losing more than 100% refrigerant charge each year. Consider installing refrigerant-monitoring systems in shafts or other areas where fittings may be concealed, installing valves at branch controllers to isolate zones, and using continuous-refrigerant line sets between branch controllers and terminal units to reduce the likelihood of leak points being buried within walls or ceilings. Refrigerant piping will inevitably leak over time; make sure the design solution will allow a property manager to find and seal those leaks easily.
CSE: What types of DOAS are owners and facility managers requesting to keep their facility air fresh?
Voth: As a standard of practice, we use a DOAS with heat recovery including latent heat recovery on all of our projects. We have been powering the relief fans on emergency power to eliminate the need for fire/smoke dampers throughout the building risers. We document subduct connections in lieu of fire smoke dampers when the relief fan is on emergency power.
Healy: For multifamily dwellings, facility managers have often requested variable-volume DOAS with energy-recovery plates. This allows them to reduce airflow when not needed and recover energy between code-required toilet exhaust and corridor make-up airstreams. For mixed-used facilities, systems with DCV are often requested for commercial areas to account for occupant loads using carbon dioxide sensors. Facility managers are able to account for commercial vacancies and varying occupancy throughout the day and week.
CSE: When designing multifamily dwellings and mixed-use buildings with pools or spa facilities, what unique HVAC, humidity, and air-balancing issues must you address?
Healy: Sound design practices can easily address the unique conditioning and humidity requirements of an indoor pool or spa. The biggest complaint from residents within a multifamily complex often isn’t the pool-room temperature, but the smell of chlorine and other pool chemicals that leave the area. Pressurization of vestibules/corridors leading to the pool area and proper sealing of structures will reduce odors internally, but care must be taken to select and locate exhaust systems such that strong odors are not easily blown back into the building from prevailing wind effects.