Thermosyphon Solar Hot Water Systems

Whenever you see a solar collector and a cylinder located immediately above it, you are very probably in the presence of a thermosyphon solar system. Thermosyphon systems are simple and reliable and the most common solar water heater worldwide.

How thermosyphon solar systems work

Thermosyphon systems in their typical configuration do not have pumps and other controls. They are based on the natural convection principle: hot water rises and cold water sinks. In the case of thermosyphon systems: as the potable water (or a antifreeze fluid, in colder climates) entered into the collector it warms and rises to the top of the collector and to the tank, where it is stored (that’s the reason why tanks are always above the collector).

Thermosyphon Systems & Collectors

Thermosyphon systems can be equipped with either evacuated tubes collectors or flat-plate collectors, which gives them the possibility of responding satisfactorily to different needs and climates. That’s an advantage.

Thermosyphon Systems & Advantages

Thermosyphon systems are cheap, and since there are no pumps or controls they are reliable systems without technical or maintenance problems.

Relatively to the batch ICS systems, thermosyphon systems allow a much higher flexibility: since the hot water is stored in a well-insulated tank - instead of being kept in the collector itself, like in batch systems – it can be used many hours after, without the problem of nighttime losses.

Thermosyphon Image credit: EERE

Thermosyphon Systems & Limitations

The big disadvantage or limitation of thermosyphon systems is that they depend deeply on air temperatures. To be more precise, they will stop working if temperatures drop below the freezing point and stay there for long. In that case the system needs to be drained or warmed. To overcome this limitation, in cold climates, some manufacturers equip their thermosyphon systems with small circulating pumps - pumps that are turned on in very cold weather conditions to move water and to prevent the one in the collector from freezing. Only in this case, thermosyphon systems are truly an all-climate solution.

Another possible disadvantage of this system – when installed on a roof – is their high weight. A tank full of water weights many gallons, and that may require the reinforcement of roof structures.

Installation

It’s easy to install a thermosyphon system and connect it into existing hot water lines. Some manufacturers offer pre-packaged systems, ready to be connected to those lines. The installation involves basically mounting a cold water supply line, a cold water inlet and a hot water outlet.

Prices

Thermosyphon systems are relatively inexpensive. Recent prices in the most competitive and larger manufacturers are much cheaper than some years ago. In USA you may find a 40 gallons system with an evacuated-tubes collector (standard 20 tubes) for around $1,000-$1,200. A larger thermosyphon system (80 gallons) with the same 20 evacuatedtubes collector may cost you around $1,200-$1,500.