COLUMN ENGINEER'S NOTEBOOK FIGURE 5 HVAC HRC COP vs. HHWS reset with fixed CHW setpoint. FIGURE 6 HHW HX% of design DHW load vs. HHWS reset. 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 85 90 95 100 105 110 HHWS Temperature (°F) OAT at 37°F OAT at 65°F FIGURE 7 Baseline DHW design (top) and proposed DHW design (bottom) in plan view. 115 120 100 90 80 70 60 50 40 30 20 10 85 90 95 100 105 110 HHW Supply Temperature (°F) FIGURE 8 Baseline DHW design (top) and proposed DHW design (bottom) in axonometric view. 115 120 heating hot water was simultaneously reduced by a substantial amount (Figures 5 and 6). Given that the building heating and cooling loads are orders of magnitude greater than the domestic hot water load, it is prudent to reset heating hot water and chilled water supply temperatures based on demand to achieve the best overall building energy performance. To accommodate the reduced heat transfer done by the heating hot water heat exchanger within the dual-source domestic hot water system, the author recommends sizing the auxiliary electric resistance heater to handle the full domestic hot water load with silicon-controlled rectifier (SCR) modulating output control to both allow for maximum flexibility in heating hot water supply temperature 62 ASHRAE JOURNAL ashrae.org A U G UST 2021 turndown and provide a margin of redundancy in the event of a service outage from the HVAC heat-recovery chillers. Reduced Mechanical Room Space Requirements Our team found that the proposed dual-source domestic hot water heater design yielded a substantially more compact layout than the baseline design using dedicated air-source heat pumps piped to remote storage tanks. Although this matches our expectations based on the fundamentals of air-source heat pumps requiring Coefficient of Performance (COP) DHW Heating Load (%)http://www.ashrae.org