The demands of mixed-use facilities: HVAC
Timothy Chatterton, PE, Project Manager, RMF Engineering, Selbyville, Del.
Kari Engen, PE, CxA, LEED AP, Senior Mechanical Engineer, WD Partners, Dublin, Ohio
Taner Tekin, PE, LEED AP, Project Manager, exp, Maitland, Fla.
John Torre, PE, LEED AP, Principal in Charge of Electrical Engineering Services, OLA Consulting Engineers, Hawthorne, N.Y.
Scott Vollmoeller, PE, LEED AP BD+C, Associate DBIA, Managing Principal, Glumac, Seattle
CSE: Have you specified distinctive HVAC systems on any mixed-use projects? What unusual or infrequently specified products or systems did you use to meet challenging HVAC needs?
Engen: Often, retail projects with small square footage require the use of packaged equipment. But on occasion, due to space constraints limiting duct distribution, VRF systems are an appropriate alternative and provide a comfort solution to this challenge.
Tekin: Mixed-use projects usually have different types of HVAC systems including but not limited to chillers, cooling towers, fluid coolers, dedicated outdoor-air system (DOAS) units, water-source heat pumps (WSHPs), chilled-water fan-coil units, and fans. Especially for hotel and residential buildings, the architect usually locates pools atop the building, which makes it challenging for us to locate any grease-exhaust fans on the roof. In a recent project, we have used precipitators in lieu of grease-exhaust fans to overcome the hurdle of eliminating smoke and odors for a better guest experience.
CSE: Have you specified VRF systems into a mixed-use building? If so, describe its challenges and solutions.
Vollmoeller: Yes, we have specified VRF systems for higher-end, mixed-use buildings. The challenges include space for the heat-recovery module, routing refrigerant lines across corridors, and concerns for tenant safety in the event of a catastrophic failure. The solutions for each challenge include upfront coordination for additional space requirements, ensuring the refrigerant piping routed across the corridor includes no joints during the traverse, and providing local refrigerant-detection alarms in each living unit.
Tekin: Yes, but rarely. VRF systems cannot meet outdoor ventilation-air code requirements easily, therefore another separate outdoor ventilation-air system must be designed and installed that ducts air into each space. Once the ducted air system is in place for ventilation, it only costs a little bit more for it to also be the cooling source. VRF systems are more expensive than conventional HVAC systems to begin with, so adding a separate ventilation system just increases costs too much to be competitive. Additionally, since the refrigerant piping is in the space, a separate large ventilation system is often required by code (ANSI/ASHRAE 15-2013: Safety Standard for Refrigeration Systems and Designation and Classification of Refrigerants), to make sure the occupants are safe if there is a leak in the system. Some VRF system manufacturers are now offering small units that handle up to 400 cfm of outdoor air. But again, this air needs to be ducted into individual spaces, which impacts cost.
Chatterton: The biggest challenge to specifying VRF systems in mixed-use buildings is in providing the required outside air. The solution that we have found to be most effective is to implement a DOAS, which provides neutral air to the space(s). This will also keep the size of the ductwork to a minimum.
Engen: For applications with larger square footage, the cost of VRF systems in mixed-use buildings is fairly competitive, but if they are less than 10,000 sq ft, cost of such systems per square foot is high. Outside air conditioning systems can be a challenge if no roof or mechanical room space is available. Options for small, outside air conditioning systems are limited and can be costly.
CSE: What unique HVAC requirements do such projects have that you wouldn’t encounter in other projects?
Tekin: Most mixed-use complexes have a substantial amount of retail spaces on the first level or first two levels of every building. We typically perform our cooling and heating load calculations during the design phases, without any hard data from the owner or the developer in regards to the program for retail spaces, so we have to make some assumptions to determine the ratio of restaurant and kitchen spaces to shops and stores.
Engen: Designing filtration for VRF systems is unusual. Due to the relative newness of VRF systems in the United States, dimensions of unit-connection flanges are not suitable for filter frames. Filter boxes within ductwork or filters at return-air devices need to be included in the designs, which occasionally create space conflicts.
Vollmoeller: Some additional challenges associated with these types of projects are adequate interstitial space if and when dwelling units do not stack and adequate head height in the first garage level to collect and drain storm and sanitary sewer piping. Coordinating sufficient space for required utility metering (gas and electric) is another challenge.
Chatterton: When using a VRF system, the ASHRAE 15 requirements must be met as well as any requirements that the AHJ may have. On one of our more recent hotel projects, we were required by the AHJ to provide refrigerant sensors in each bedroom space, even though we met the ASHRAE 15 requirements. Another requirement that can often be overlooked is that each terminal unit of a VRF system will require a condensate drain pipe, which must be directed to the exterior. A condensate pump may be required if the terminal unit is not provided with one by the manufacturer.
CSE: When retrofitting existing facilities, what challenges have you faced and how have you overcome them?
Tekin: We usually work on new construction projects; however, sometimes we do come across some renovation projects. I would say the most common challenge for retrofit projects is obtaining information on existing conditions and verifying capacities of existing systems, such as chilled-water capacity, air-handling capacity, and electrical power. Identifying the locations of the existing systems without interrupting the operation of the existing facility is another obstacle.
Engen: A common problem is existing infrastructure was never designed for the cooling demands of modern spaces. In large, mixed-use buildings with existing HVAC infrastructure, existing systems were designed for high occupant density and low miscellaneous heat-contributing systems. Newer retail, office, and medical office spaces require much higher HVAC cooling capacity per square foot, which sometimes requires new infrastructure or decentralization of equipment to meet space-cooling or outdoor air-ventilation conditioning needs.
Chatterton: One challenge with existing facilities is minimal ceiling space. The lack of ceiling space is what will most often push the HVAC system toward a VRF system. This is due to the smaller duct sizes for ventilation air and the small refrigerant lines. VRF systems are also capable of providing each space with temperature control and typically exceed the 2015 IECC energy efficiency ratio/seasonal energy efficiency ratio requirements.