This is a typical building recirculation system with pump and instantaneous water heater system. The expansion tank is typically installed on the cold water inlet but can be installed on the hot water supply after the water heater. The use of strainer is highly recommended. The purge and pressure/temperature relief valve are typically used when the building outlets are safety fixtures. The recirculation pump can be controlled by temperature (aquastat), timer, or occupancy sensor to maintain loop at usage temperature.