European Solar Thermal Industry Federation

Solar Domestic Hot Water preparation (SDHW)


Solar Domestic Hot Water systems (SDHW) are dominating the markets in warmer climates. Around the Mediterranean, as well as in China, SDHW are already installed in vast quantities. Small systems are available for individual dwellings and larger (collective) systems provide hot water for multi-family houses, hotels, office buildings etc. This page introduces the most common SDHW types.


Hot water preparation is still by far the major solar thermal application in Europe. Solar Domestic Hot Water systems (SDHW) are specifically designed to deliver 100% of the hot water requirements in summer and 40-80% of the total annual hot water demand. They include a supplementary heater (e.g. an integrated electric or gas heater). Or they are operated as pre-heaters.

These systems are particularly popular in regions with no or low space heating demand. In Spain and other countries, solar hot water systems are now mandatory in new buildings

Combi Systems and Combi+ Systems are gaining market share where space heating or space cooling is required in addition to domestic hot water. These systems are larger and save more energy

SDHW for one or two dwellings

Two different design principles: Thermosiphons and Forced-Circulation systems

Thermosiphons and Forced-Circulation are the two main SDHW systems. What differentiates them is how the water is circulated between the collector and the tank.

Thermosiphon (or: natural flow) systems

Thermosiphon systems use gravity to circulate the heat transfer medium (usually water) between collector and tank. The medium is heated in the collector, rises to the top of the tank and cools down, then flows back to the bottom of the collector. Domestic hot water is taken either directly from the tank, or indirectly through a heat exchanger in the tank. The main benefit of a thermosiphon system is that it works without a pump and controller. This makes the systems simple, robust and very cost effective. A well designed thermosiphon system is highly efficient.

However, with this type of system the tank must be located above or beside the collector. In most thermosiphon systems, the tank is fastened to the collector and both are situated on the roof. This solar thermal system is most common in the frost-free climates of Southern Europe. The principle can also be used in colder climates, the tank is then installed indoors (e.g. just under the roof).

A typical DHW thermosiphon system for one dwelling has a 2-5m2 of collector area and a 100-200 litre tank.

Thermosiphon system

Forced circulation systems

These are most common in Central and Northern Europe. The tank can be installed anywhere as the heat transfer fluid is circulated by a pump. Therefore, integration with other heating systems - often installed in the cellar - is easier.  The aesthetic advantage associated with these systems is that the tank does not have to be located on the roof

But higher flexibility comes with higher complexity: A forced circulation system needs sensors, a controller and a pump. A well-designed forced circulation system shows the same high performance and reliability as a thermosiphon system.

A typical DHW forced circulation system for one dwelling has 3-6m2 of collector area and a 150-400 litre tank.

1 Collector - 2 Tank - 3 Heat exchanger - 4 Control unit
5 Expansion Tank - 6 Back-up heater - 7 Consumer

Collective SDHW for larger buildings

Central water (and space) heating is common in larger buildings. Increasingly, collective solar domestic hot water systems are being installed into multi-family houses, hotels, office buildings etc. These collective systems have a collector surface ranging from ten to several hundred square metres. Larger SDHW are designed specifically for lower solar input, i.e. they provide a smaller share of the hot water demand with solar energy. This and the fact that they are often operated at lower temperatures results in a high system performance (thermal energy output per square metre of collector area). Most collective SDHW systems are designed as forced-circulation systems, but multiple thermosiphon systems are also used where appropriate.

Further information about collective solar thermal DHW can be found on the SOLARGE project's website.

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