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equipment can be successfully adapted to serve multifamily applications, and that the “swing tank” concept was an efficient way to treat the building hot water circulation loop heat losses. Although this demonstration was a 60-unit building, this same approach could be extended to serve larger buildings. Annual measurement and verification at Site provided the following insights: Based on annual monitoring, the average DHW system coefficient of performance was 3.3. Annual operation showed that the HPWHs used an average of 68kWh/day. The DHW System COP is defined as the heat energy required to heat the incoming water to delivery temperature plus the heat energy required to make-up the losses in the recirculation loop divided by all of the electrical energy needed to power the heat pumps and back-up electric boilers. The average water usage at this site is 20 GPD/apt, or 20 GPD/person. This is a little higher than usages measured in previous multifamily studies from larger market-rate buildings which reported 13-19 GPD per person (Heller 2015). Additionally, this site has less dramatic peak demand periods and almost no difference between weekend and weekday usage. Increased usage and shifted daily patterns maybe because the Site is a senior facility versus workforce demographic at the other sites. The M&V metering equipment was installed primarily to evaluate equipment performance. However, the M&V served as a diagnostic tool for identifying and solving operational problems. Without the M&V equipment, it would not have been possible to detect and address system issues swiftly. Future central heat pump water heating system designs should incorporate some means for automatic remote alarms to be sent to building owners and maintenance personnel to maintain operations and avoid inefficient back up system operations. The “swing tank” concept was a successful method to isolate the warm recirculation water from the HPWHs. The dedicated temperature maintenance tank was designed and sized to be a “swing tank”, utilizing over- heated water in the primary storage tanks to mix with the cooler water returning from the recirculation loop. The periodic draw of the over-heated water into the temperature maintenance “swing tank” kept the tank primed above the temperature at which the backup electrical boiler system will engage. This design minimized the electric resistance reheat of the hot water circulation system, and was a successful strategy at this site, due to the building’s low recirculation losses. The existing electric system was preserved during the HPWH retrofit to provide support for the “swing tank” operation, and backup for water heating in the event of equipment outages. Over the course of monitoring at this site, the electric boilers were needed for both these tasks; however, the boilers only contributed approximately three percent of the total heat to the water. At future installations, the need for full backup could be avoided by installing additional, redundant stages of heat pumps and a larger “swing tank”. Median run times with four operable heat pumps were 15 hours per day or a 63% duty cycle. This includes summer and winter periods representing the warmest and coldest ambient conditions. Fewer operable units resulted in longer median duty cycles (83% or BONNEVILLE POWER ADMINISTRATION 22PDF Image | CO2 Heat Pump Water Heater Multifamily Retrofit
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