The influence of return loop flow rate on stratification in a vertical hot water storage tank connected to a heat pump water heater

A temperature-controlled hot water heat pump was simulated using heating in a vertical, domestic hot water storage tank. The influence of the return loop flow rate on stratification was investigated experimentally. The return loop is the water line that supplies a long line of consumers with hot water and returns colder water to the middle of the hot water storage tank. The return temperature is a function of the length of the loop, insulation, and ambient conditions. Temperatures were measured as a function of time at different vertical locations on the centerline of the storage tank. The temperature distributions in the tank were compared for different return flow rates. A return flow rate of three tank volumes per day was identified as preferable, although good results were also obtained for less than three tank volumes per day.