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Key Design Principles Some of the key design principles that emerged from the HPWH retrofit design at the Site include: Storage and HPWHs should be sized per ASHRAE 2015 “Low water usage” methodology. This project demonstrated that domestic hot water for a large building can be supplied by a remarkably small central heat pump plant. Hot water for 60 apartments is supplied by just 5- tons of nominal capacity and functioned for long periods with less than 4-tons of capacity. Industry standard design practices lead to oversized systems. This is evidenced by the 120KW of electric resistance capacity designed for the existing system; over five times the output capacity. Storage and controls must be configured to allow a large volume of cold water to be stored before turning on heat pumps. This allows for a longer cycle length8 without having too-hot water enter the heat pump and potentially decrease efficiency and possible high head pressure. Aquastat location is important. Locate Aquastat far enough away from incoming water to avoid triggering Aquastat every time water is used. Building hot water temperature maintenance recirculation systems represent a significant energy usage. Using electric resistance will significantly decrease efficiency. However, a large volume of warm water from the recirculation loop returning to the primary 8 Minimum run times of one hour are recommended per John Miles, General Manager, Eco Products, Sanden International. storage and HPWHs also results in lower performance. An effective strategy is to separate the temperature maintenance load from the primary load, allowing the primary HPWH system to run at peak efficiency. Typical non-electric tempering valves do not function well with varying inlet water temperatures. Consider use of electronic tempering valves. Heat lost from the circulation loop and distribution piping can account for 25 to 50 percent of the heat needed for the water heating system. Insulate the circulation and distribution piping. Eliminate all areas of thermal bridging. The “swing tank” design philosophy is a proven concept and will minimize the electric resistance temperature maintenance recirculation loop load. Repurposing existing equipment in retrofit projects, (storage tanks, the original system for backup, or recirculation reheat) saves upfront costs. Include robust measurement, verification, monitoring with automatic alerts to building to assist in diagnosing issues and improving future designs. Conclusion and Recommendations This project demonstrated that HPWHs can yield significant energy savings for multifamily buildings in the Pacific Northwest climate. This demonstration also showed that Sanden BONNEVILLE POWER ADMINISTRATION 21PDF Image | CO2 Heat Pump Water Heater Multifamily Retrofit
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