Heatexchangers are used in closed-loop solar hot water systems. They enablethe transfer of heat from one fluid to another without the two mixing.Internal heat exchangers are inside the tank and not visible. They canbe as simple as a coil of pipe resting in the bottom of the tank, orwrapped around the outside beneath the insulation and cover. As theheated fluid from the solar collector travels through the coil, theheat is passed from the hotter fluid to the cooler potable water.
Anexternal heat exchanger is usually a pipe within a pipe. The solarfluid and potable water flow counter to one another, and heat istransferred within the heat exchanger pipe. Fluid may be moved withpumps, thermosyphoning, or a combination of the two.