A super insulated tank, with a heat exchanger built into it. Hot water from a boiler is pumped through the exchanger, which in turn heats the domestic hot water. This creates an abundance of hot water quickly, at an economical price.
An indirect water heater circulates water through a heat exchanger in the boiler. This heated water then flows to an insulated storage tank. Because the boiler does not need to operate frequently, this system is more efficient than the tankless coil. In fact, when an indirect water heater is used with a highly efficient boiler, the combination may provide one of the least expensive methods of water heating.
a heat exchanger and uses heat from an external source (i