It sounds like you want to use one of the water heater objects. Check out the I/O Reference for the WaterHeater:Mixed object (there is also a stratified water heater object). It looks like the WaterUse:Storage object is meant to model storage for collection of rainwater, graywater, etc. The water heater object should be placed on the supply side of the plant system for your domestic hot water system, then use WaterUse:Equipment and WaterUse:Connections to model the DHW demand components.
Regards,Aaron Boranian
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