Solar thermal installations use liquid-to-liquid heat exchangers exclusively: on one side, there's the solar fluid that travels from the collector; on the other there's the water being heated.

Heat exchanger walls

A "wall" is the membrane or surface that separates the two liquids passing through the heat exchanger. Most modern models have a single wall because it's more efficient – the heat has less material to pass through – and which is usually made of copper or stainless steel. Some very particular circumstances allow the use of Pex, though it's rare.

Some areas' rules for solar installations require the use of double-walled heat exchangers. These rules are generally a throwback to the time when solar thermal was still in its infancy and toxic liquids were used as solar fluids. Double-walled exchangers are still available, though they're unnecessary in any system where the solar fluid is non-toxic (which covers virtually every modern system).

Double-walled heat exchangers are less efficient than single-walled models because there's more material between the two fluids. They do have an advantage in that there is usually a gap between the two layers, for "positive leak detection": if either side corrodes or is damaged, the leaking fluid will be quickly visible and the exchanger can be replaced or repaired.

Pipes, plates and plumbing

Exchangers use either pipes or plates to transfer heat. Whichever you choose, the exchanger should always be plumbed so that the two liquids travel in opposite directions. This counter-flow arrangement ensures that as much heat as possible is transferred because the temperatures of the two liquids are kept at a maximum all the way through the exchanger.

The more efficient of the two types of exchanger uses plates. The two liquids pass through alternating rectangular plates stacked in a block, with the liquids always heading in opposite directions.

Given that the amount of heat transferred in the exchanger depends upon the surface area of the walls, plate-style models have an obvious advantage: there's an enormous amount of surface area in a relatively small space. On the down side, the waterways in such compact devices are necessarily small and thus affected by mineral buildup a lot quicker than pipe models. Using a plate heat exchanger in a hard water area will demand regular maintenance to flush out any buildup.

Plate heat exchangers always need two pumps – one on each set of waterways – as they will not convect. It's often best to place isolation valves and boiler drains on the domestic water side of the plumbing, to make for easier maintenance. Flushing the exchanger should always be done with a non-toxic solution that is, preferably, enzyme-based, non-corrosive and safe for septic systems.

A pipe heat exchanger uses a "tube in shell" setup: one or more thin pipes sit inside a bigger tube and the two liquids pass through the inner pipe(s) and outer tube. The walls of the inner pipe(s) act as the heat transfer medium.

Pipe exchangers are less efficient than plate exchangers but they only need one pump. They come in single- and double-walled versions,