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Ninety percent of the time, the heat pumps ran for less than twenty-four hours (whether there were three or four operable units). In the most extreme cases the HPWHs ran for several days consecutively. This was observed in late May 2019 as a leak was detected at the site and again in February 2020, when two HPWHs were inoperable, and the backup system not responding. At that time, the operable equipment ran for five days. Figure 13. HPWH Run Times One consideration in future designs would be to include an additional heat pump beyond what is strictly needed to serve the design load. The costs of additional equipment should be balanced against the contribution of the added heat pump: reduced run times when operating normally, further reducing the contribution of a backup system, and resiliency during periods when one or more heat pumps may be inoperable. Temperature Maintenance and Swing Tank Operation The “swing tank” at the Site is “minimum- sized” in that a larger storage tank could not fit through the mechanical room door. However, due to the low recirculation loop losses at this Site, the tank sizing is almost perfect. Intended to minimize the use of backup electric water heaters to maintain the recirculation loop delivery temperature, the “swing tank” does this most effectively when all HPWHs are operable (and the design capacity of 150°F water is provided), the occupant draw pattern is approximately 20 gallons per apartment per day, and occupant usage keeps the “swing tank” primed and above the 125°F delivery temperature. When the environment, or draw pattern, differ from the design standards, the electric resistance water heaters initiate to keep the recirculation temperatures within setpoint parameters. Additionally, if the HPWHs are not operating as intended or draw patterns exceed the design load, the ERWH boilers can serve the building’s hot water needs. Although the majority of the HPWHs operated flawlessly during the monitoring period, occasionally, one or more HPWHs have been inoperable. The red line in Figure 14 demarcates the ERWH usage during a period with three operable HPWHs (before April 11, 2019) and with four operable HPWHs (until April 22). With the HPWH equipment operating as designed (to the right of the red line) only minimal operation of the backup boiler equipment was observed. Additionally, the “swing tank” is kept at a more stable temperature when all HPWHs are available. Periods when only three HPWHs were operable show lower temperatures and a greater variability in average daily temperatures of water leaving the “swing tank”. This is as expected as it would take longer for three HPWHs to recharge the BONNEVILLE POWER ADMINISTRATION 17PDF Image | CO2 Heat Pump Water Heater Multifamily Retrofit
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