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Buildings with a central heating plant and room air conditioning may see increased use of cooling with a central VRF system, and cooling energy could potentially increase. However, each building’s savings will depend on baseline energy use and occupant use of cooling. For high-rise buildings with steam or hot water heating systems and significant cooling needs that can be met with a central heat pump plant, a VRF distribution system with either an air or ground source is a plausible near or mid-term retrofit. However, the complexity of the system, relatively short lifespan of components, and extensive use of environmentally harmful refrigerants in a leak-prone installation should limit how and where this technology is promoted and recommended. For an air-source VRF system, the rooftop is typically a viable installation location because the heat pumps can get sufficient airflow and noise concerns are usually avoided. Available Technology There are several global manufacturers of central heat pump systems including Daikin (VRV), Fujitsu (“AirstageTM”), LG (“MULTI V”), Mitsubishi (“CITY MULTI®”), Panasonic (“ECOi”), and others. Current offerings are manufacturer specific, so a single manufacturer would provide the central plant, piping distribution design, and room units, as well as the integrated control system. Central units typically start at around 72 MBH heating capacity and can be installed in banks to achieve nearly any capacity, space permitting. Central heat pump plants can use air as the heat source/sink or a water loop to exchange energy with the ground or cooling tower. The physical size of the heat pumps, either air source or water source, are similar by capacity, but water source heat pumps do not require large volumes of air and can thus be installed in tighter areas. However, because of the amount of refrigerant they contain, locating the heat pumps indoors may be difficult while complying with ASHRAE Standard 3447, which is integrated into many local codes and the International Mechanical Code (IMC)48. This regulation sets limits on the amount of possible refrigerant that could leak into a confined space, which would displace air and present a safety concern for occupants. The size and noise of VRF outdoor units makes them well-suited to be placed outdoors on rooftops, though some installations place them in distributed mechanical rooms throughout the building.49 Third-party control systems can be overlaid on the VRF system for billing and other building system integration purposes, though the actual control mechanisms and detailed algorithms are typically customized software by each manufacturer and do not allow complete integration.50 When: Opportunities for Phased Retrofit The nature of the central heat pump system is a complete departure from a steam or hot water system, and the new system cannot reuse any existing components. There is little opportunity for phasing this retrofit over time and replacing components in certain areas of the building before others. The only real way to stage the project in phases is to replace a hot water or steam riser, which typically travels vertically up through all floors of a building, serving one apartment on each floor. All apartments on the riser line would need to be replaced at the same time with a central plant and distribution system that serves only those apartments. Each riser line can be completed separately. Following this strategy removes most opportunities for a completely central plant that can serve all apartments, instead creating one central plant for each riser line, though central heat pumps can be collocated as they are installed. The best timing for a retrofit is when the existing central plant and existing distribution system will need replacement in the next 2-5 years, which allows time for design and planning of the new system while the existing system is still operational. Barriers to Consider As with all heat pumps, the cost of electricity relative to fuel should be fully analyzed. Large multifamily buildings with steam heating systems tend to have inherent inefficiencies due to steam balancing and overheating, so there is a better chance to overcome the higher cost of electricity. Hot water systems tend to have less waste and better control, so the economics of converting to VRF will likely be unattractive. Compared to the high cost of installation, there is nearly no chance for a financial payback in energy savings alone. Steven Winter Associates,Inc. 29PDF Image | HEAT PUMP RETROFIT STRATEGIES FOR MULTIFAMILY BUILDINGS
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