What type of heater uses the main furnace or boiler to heat a fluid that's circulated through a heat exchanger in the storage tank?

An indirect water heater utilizes the main furnace or boiler to heat a fluid, typically water or a heat transfer fluid, which is then circulated through a heat exchanger located within the storage tank. This process allows the storage tank to gain heat from the fluid without being directly connected to the heating source. This design enables the indirect water heater to efficiently use existing heating systems, such as a furnace or boiler, to supply hot water for domestic use.

In contrast, a direct water heater heats water directly using an electric or gas element within the tank, so it doesn't rely on a separate furnace or boiler. A solar water heater harnesses solar energy to heat water, while an instantaneous water heater heats water on demand as it flows through the unit. These different types of water heating systems highlight the unique design and function of indirect water heaters in household heating solutions.