Indirect steam uses both a storage tank and a shell-and-tube heat exchanger to heat water. Steam supplied from a boiler flows through a heat exchanger’s tubes, and the water is heated as steam passes through the shell on the outside of the tubes. The water is then circulated through the storage tank and held at the desired hot water supply temperature.
A variation of this system is a semi-instantaneous design: it delivers hot water by channeling the incoming cold water directly over the tubes of the heat exchanger in a controlled manner to maximize the heat transfer rate. With a semi-instant design, a large storage tank is typically not needed. However, this system requires a larger steam supply rate than its standard shell-and-tube counterpart.