Planning multifamily dwellings: HVAC and plumbing
Multifamily dwellings are fascinating projects with unique HVAC needs. In essence, they could be considered giant people storage facilities for different stages of life. College dormitories, for example, contain young adults in their late teens and early twenties. Assisted living facilities hold people in their later years. However, engineers know these structures are more than just simply boxes to keep human beings in. They’re as complex and varied as the people inside.
Kris Cotharn, PE, LEED AP
Wayne Griswold, PE, CFPS
Principal Fire Protection Engineer
Randall V. Moss, P.E., LEED AP
Joseph Russo, PE, LEED AP BD+C
Rafi W. Wartan, PE, LEED AP BD+C, CQM-C
CSE: What unique cooling systems have you specified into such projects? Describe a difficult climate in which you designed an HVAC system for a multifamily dwelling.
Russo: Located on the Williamstown, Massachusetts, Williams College’s rural New England campus, the CDE Residence Hall is served by a ground source heat pump system producing chilled water and hot water to accommodate four-pipe valance units in each student’s room. The valance units were chosen as they provide advantages over typical fan coil units; valance units are quiet, efficient and allow for easy maintenance and installation. Coupling the ground source heat pumps with the valance units did provide challenges during design. With the maximum heating hot water temperature from the heat pump system limited to 120 F and the passive nature of the valance units, the decision was made to incorporate dc-brushless fans into valance units to meet the heating load in the rooms. To maintain the energy efficiency of the system, the fan assist is controlled by the BMS and is only enabled upon further drop of room space temperature. The fan assist mode is locked out when the valance units are in cooling mode.
CSE: What unusual or infrequently specified products or systems did you use to meet challenging heating or cooling needs?
Russo: For a recently completed dormitory project at Fairfield University in Fairfield, Connecticut, we specified a desiccant rooftop dedicated outside air unit to provide ventilation throughout the dormitory. The building was connected to the campus central dual pipe temperature system, which served the fan coil units and desiccant rooftop units.
The challenge posed by the dedicated outside air units with the dual temperature system was how to properly dehumidify the ventilation air to maintain the dew point in the building during the shoulder seasons — when either hot water or chilled water is not available. To overcome this challenge, we specified a desiccant dedicated outside air unit that allows us the flexibility to incorporate dual temperature coils, direct exchange cooling coils, electric reheat and reactivation air from the desiccant wheel. The specification of components allows the unit to maintain the dew point when chilled water is not available from the campus central system by using the direct exchange coils and reactivation air from the desiccant wheel. This is particularly helpful during seasonably warm days.
CSE: How have you worked with HVAC system or equipment design to increase a building’s energy efficiency?
Cotharn: Heat pumps and variable refrigerant flow systems have been successful in reducing energy consumption for these types of buildings. Both systems allow simultaneous heating and cooling; based on the functionality of different spaces and building exposure, heat generated from a space that needs cooling is moved to another space that needs heating. These systems are different from traditional chiller and boiler systems that separate the function of heating and cooling of the building.
Russo: The dedicated outside air system unit serving Williams College CDE Residence Hall was selected because of the total energy recovery wheel effectiveness of up to 84.8% in the winter and 85.2% in the summer. This helps to ensure that the unit is as efficient as possible, which, as it is the primary source of fresh air and ventilation to the building, is particularly important. Furthermore, incorporating a highly effective energy recovery wheel, resulted in a reduction in both cooling and heating capacities as well as electrical consumption. Being a net zero building, this reduction was important in reducing the overall photovoltaic array.
CSE: What best practices should be followed to ensure an efficient HVAC system is designed for a multifamily project?
Russo: For multifamily projects, centralized HVAC systems typically outperform localized units when comparing energy efficiency. Providing centralized heating and cooling plants and centralized air handling units and ventilation systems will provide greater energy efficiency in lieu of smaller localized systems. Designing centralized systems is viewed as best practice for multifamily dwellings; however, with higher upfront installation costs, these systems are usually cost prohibitive and not often pursued. In light of the challenge posed by upfront installation costs, if smaller decentralized systems are specified, it is best practice to ensure that the systems are Energy Star certified products.