COLUMN ENGINEER'S NOTEBOOK FIGURE 1 Baseline Design Approach. Medical office building with dedicated heat pump water heaters. FM T dP BTU T HRC HRC HRC HRC CH CH CH FM T dP dP BTU T VFD VFD BTU T HHWS VFD VFD DHW HPWH HPWH HPWH HPWH CW HWR Storage Tanks CW HWR E E HHWS dP VFD VFD CHWR HRC HRC HRC HRC HHWR CH CH CH T FM BTU T CHWS FIGURE 2 Proposed Design Approach. Medical office building dhw production integrated with HRCs. FM T VFD VFD CHWS CHWR HHWR DHW Dual Source Storage Tanks supplemental air-source chiller (CH) modules to accommodate peak cooling demands. Each heat-recovery chiller and cooling-only chiller module is furnished with a set of integral automated isolation valves that are controlled by a factory controller to modulate output and prevent bypass of chilled and heating hot water through inactive modules under normal operation. Four identically sized air-source heat pump domestic water heaters (HPWH) with remote storage tanks are provided to meet the demand for domestic hot water. The owner's RFP encouraged design-build teams to consider and propose alternative design solutions based on merit to the project and owner. To that end, our team considered options that had the potential to achieve as many of the following attributes as possible: * Lower fi rst cost; * Lower maintenance; * Better or similar energy-effi ciency performance; and * Reduced mechanical room space requirements. The integrated domestic water heater design approach shown in Figure 2 provided a compelling option to simultaneously achieve as many of the aspirational goals listed above as possible. The proposed approach consists of two identically sized dual-source tank-type water heaters that use the HVAC heating hot water utility to preheat the domestic cold water (CW) feed from ~60°F (~16°C) to ~110°F (~43°C) with auxiliary electric resistance element heaters as the second heat source to lift the water to its storage temperature of 140°F (60°C). The heat-recovery chillers were initially sized to provide an aggregate heating capacity of 2,495 MBtu/h (731 kW) based on a selection of four equally sized " 60 ton " modules to meet a design heating load of 2,106 MBtu/h (617 kW) during a design heating condition, providing a peak heating hot water supply temperature of 115°F (46.1°C). This yields 389 MBtu/h (114 kW) of " spare " capacity that enables consideration of the proposed approach of integrating the domestic hot water production. Additionally, because the chiller modules have hardware similar to that of the heat-recovery chiller modules, it is possible to consider conversion of cooling-only chiller modules to heat-recovery modules to provide additional heating capacity, if needed, without adding extra compressors or heat exchangers to the proposed system. A U G U S T 2 0 2 1 ashrae.org ASHRAE JOURNAL 59http://www.ashrae.org