These facilitate solar heat exchange between the transfer fluid to the home water supply. Solar heat exchangers are often made from copper, a good thermal conductor and less likely to corrode, but can also be made from other metals like steel. The main type of heat exchanger is liquid-to-liquid, which uses transfer fluid, with one or two barriers between the transfer fluid and the water supply. These are described below:
- single-wall: fluid in the pipe and outside of the pipe are different.
- double-wall: used as a safety measure when the transfer fluid is toxic. These are less efficient because the heat must go through two surfaces instead of one.
The most common designs of heat exchangers are:
- coil-in-tank where the exchanger is a tube in the storage tank
- shell-and-tube, where two tubes with water and heat-transfer fluid circulating in opposite directions are encased in a metal shell outside the storage tank.
- tube-in-tube, with the two tubes of water and heat-transfer fluid in direct thermal contact with each other, is the most efficient type.