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Single Pass: The design is based around a “Single Pass” heat exchange strategy as opposed to the typical “Multi Pass” strategy employed in most hydronic space heating applications. This means that the flow of water through the heat pump is regulated by a control valve or variable speed pump to maintain a target output temperature of 149°F. This results in a variable flow rate and variable temperature rise across the heat pump, as opposed to the typical fixed flow rate and fixed 10-20°F temperature rise on the water. The heat pump can therefore output 149°F water with incoming water temperatures ranging from 40-122°F2. The advantage of the “Single Pass” is that a usable water temperature is always delivered to the top of the storage reservoir. Multiple Storage Tanks: This design used multiple storage tanks plumbed in series. The arrangement enabled a high degree of temperature stratification throughout the system, with the hottest water at the end of the primary storage system (ST-3). It also used of smaller, less expensive tanks that were easier to install and fit through doorways. The three existing tanks were reused for the primary storage tanks. A fourth tank, ST- 4, was added to act as a dedicated temperature maintenance tank (“swing tank”). This fourth tank is in series with the three primary tanks. See Figure 1. Storage Temperature: The water was heated to a relatively high temperature (149°F) to effectively increase the stored 2 Heat pumps can continue to function at incoming water temperatures below 40°F. heating capacity of the plant. This controls possible legionella bacteria, and increases the effectiveness of the “swing tank” (ST-4). To prevent scalding, outgoing water is tempered with recirculation water and/or incoming municipal water down to approximately 120°F before delivery to the apartments. Backup Electric Water Heaters: This design used the three existing instantaneous electric water heaters for backup. Configured in parallel and operating in unison to deliver 135°F water to ST-4. The backup instantaneous electric water heaters operated any time the final storage tank dropped below 120°F either due to inadequate capacity coming from the HPWHs or due to extended periods of time with no hot water draws and continuous cooling from the recirculation system. Temperature Maintenance Swing Tank: This tank (ST-4) was designed to swing in temperature between 120°F and 150°F. During periods with hot water use, over-heated (~149°F) water moves from the primary storage tank to the “swing tank”. These periodic draws keep the “swing tank” primed above 120°F. If the “swing tank” drops below 120°F the backup electric water heaters raised the temperature above 125°F. Serial Primary and Temperature Maintenance Tank Arrangement: The series configuration enables a “swing tank” concept, which is defined as providing over-heated water from the primary tanks to be mixed with cooler BONNEVILLE POWER ADMINISTRATION 6PDF Image | CO2 Heat Pump Water Heater Multifamily Retrofit
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