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Where: Use Today and Retrofit Applications Commercial-grade heat pump water heaters using fluorinated refrigerants are available in the U.S. from a few manufacturers.59 Residential-sized products are available domestically from many manufacturers. These systems come in two varieties: units that are located inside of the thermal envelope, and units that harvest heat from outside of the thermal envelope. Current offerings for internal units use fluorinated refrigerants and are widely available at home improvement stores. One model using R-744 (CO2) is available and can be installed to harvest heat from indoors or outdoors, even in cold climates. Residential products can be used in central plants for some multifamily buildings, with site-specific considerations.60 Alternative refrigerants such as R-744 (CO2) are used for DHW generation in international markets, and these products should be brought to the U.S. market. Why: Benefits of this Typology The transition from fuel-fired DHW plants to heat pumps can be a low-cost way (relative to space heating) to electrify building systems. The tie-in to the existing DHW water distribution system can be reused from the existing system, so very little distribution work needs to be done specifically for this retrofit. Central fuel-fired DHW plants are typically located somewhere in the building with access to outdoor air for ventilation and exhaust. A central AWHP plant also needs ventilation and exhaust. The ventilation required for fuel-fired equipment intake and exhaust may be reused (with some modification based on AWHP airflow requirements). Because a heat pump water heater operates year-round and heats water by cooling air, the plant can also serve as a kind of air chiller. With careful planning and design, the DHW plant can serve to provide cooling to a space in the building that needs it, such as a building lobby or community room. These integrations of systems can double the electrical efficiency of the new heat pump plant when cooling is needed. Underground garages, basement spaces containing condensers from other systems, or other spaces with heat gains and no conditioning needs can be middle ground options. These spaces are warm enough for the heat pump to operate at high efficiency, but the space cooling does not offset a mechanical cooling load. With easy access to equipment, operational adjustments and maintenance is less challenging for central HPWH plants than for HPWHs in each apartment. The only tenant-dependent component of the DHW energy use is consumption, which can be lowered with low-flow water fixtures and education about water conservation. Why Not: Plausibility Concerns AWHPs need enough airflow to operate properly. Indoor locations for a central plant may be challenging for adequate airflow ducting, since the heating plant exhaust may still be in use or can’t be reused as a pathway for AWHP airflow. Many older buildings have DHW infrastructure in the basement, which may have limited opportunities for adequate airflow without significant ducting. Putting the AWHP at ground level for shorter piping connections to the basement could be difficult if there is insufficient outdoor space. One option is to put the AWHP on the roof, but this can result in long water piping and expensive electrical service to the roof if existing service is not adequate. System design is important to enable the plant to operate as efficiently as expected. Inlet water temperature to the heat pumps needs to be appropriate for the product. Adequate thermal storage is needed with stratification to control water temperature requirements. Outdoor water piping to and from the AWHP needs to be adequately protected from freezing in the winter. This can be accomplished with insulation, heat trace wiring, and drain-back systems in the event of power failure. Nevertheless, outdoor water piping should be minimized as much as possible. Steven Winter Associates,Inc. 39PDF Image | HEAT PUMP RETROFIT STRATEGIES FOR MULTIFAMILY BUILDINGS
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