Probably the least popular solar hot water installation, thermosiphon systems come in two types – direct and indirect – and are passive. They use no pumps but instead rely on the fact that hot fluids rise to provide circulation.

Although they are very simple to install and maintain, thermosiphon solar thermal installations have certain disadvantages that usually push potential customers to choose an alternative. The biggest of these is that the collector array has to be installed below the tank, which is not only unusual but also inconvenient in most residential layouts.

Direct thermosiphon systems

The simplest version of a thermosiphon solar hot water setup uses a single storage tank attached to a flat-plate or evacuated-tube collector. Pipes connect the two from top to top and bottom to bottom, with the collector installed lower than the storage tank.

The collector gathers the sun's heat and warms the domestic water supply directly. This water then rises, exiting the collector at the top and heading up the pipes to the top of the storage tank. This movement causes the cold water in the bottom of the tank to circulate down into the bottom of the collector array without any need for a pump.

The problem with this setup – apart from the need to have a storage tank installed above the collector – is that the domestic water supply passes through the collector. With anything but the softest water supply, mineral buildup will disrupt and quickly clog the flow, so most homes need to install a water softener before the collector.

Because of the unusual layout and the problem with hard water, most people who look at a direct thermosiphon setup end up choosing another type. If the location allows it, an ICS solar thermal system is usually a better choice.

Indirect thermosiphon systems

The alternative to running softened hard water through the collectors is to use an indirect thermosiphon solar hot water setup, which implements a separate solar loop (containing an antifreeze mix ).

The collector array still needs to be below the storage tank but heats the solar fluid instead of the domestic water supply. The solar fluid rises to the tank, where a heat exchanger (internal or external) passes the energy to the domestic supply. This cools the fluid, which then falls back to the collector, creating flow without the need for a pump.

Indirect thermosiphon systems are not popular because they have the same unusual layout and because the extra protection against freezing provided by the solar loop does not extend to the storage tank's pipes connecting to the domestic supply. That problem can be solved by putting the tank in a heated area of the home, but this creates additional layout oddities that rarely match a homeowner's needs.