A thermosiphon solar water heater circulates on density difference alone: fluid heated in the collector becomes lighter and rises into a tank mounted above it, while cooler fluid sinks back down to be reheated. The tank base must sit at least 30 cm above the top of the absorber. Thermosiphon systems dominate solar water heating across the Mediterranean, Asia outside China, Latin America and Sub-Saharan Africa.
How does thermosiphon circulation work without a pump?
Heat makes water lighter, and that is the whole mechanism. Water at 80 °C is roughly 3% less dense than water at 20 °C, and in a closed vertical loop that difference is enough to move fluid: sun-warmed liquid in the absorber rises into the store above it, and cooler liquid from the bottom of the store sinks to take its place.
The driving pressure is measured in millibar, not bar, so a thermosiphon design has to remove every obstacle a pumped loop can ignore. That means generous pipe bores (22–28 mm rather than 15 mm), pipe runs kept short, a continuous upward slope from collector to tank with no dips that can trap air, and as few bends, valves and fittings as possible. The flow then regulates itself: bright sun produces a bigger temperature difference, faster circulation and more heat delivered; weak sun lets the loop idle. A forced-circulation system needs a solar controller and a pump to imitate exactly this behaviour.
Where does the tank have to sit?
Above the collector, always. The rule of thumb is that the base of the store sits at least 30 cm higher than the top of the absorber; below that, circulation stalls and the system needs a pump. That is why thermosiphon units are recognisable at a glance — a horizontal cylinder bolted to a frame directly above the panel.
The consequence is structural, not hydraulic. A filled 200 L thermosiphon unit weighs 250–300 kg once tank, frame, water and collector are counted, concentrated on a small roof area. On a pitched tile roof the frame must land on rafters, not battens, and in high-wind or seismic regions the mounting frame is the part that needs engineering attention. Solimpeks TSM thermosiphon systems are built in 120–350 L sizes for exactly this reason: the store is matched to household demand rather than to the largest collector that will fit.
What stops the tank cooling down again at night?
A check valve or a heat-trap loop in the pipework. Once the sun sets, the temperature gradient reverses: the collector becomes the coldest part of the circuit, and unless something blocks it, the loop runs backwards and dumps the day's heat to the night sky. Well-built thermosiphon systems use a one-way valve, an inverted heat-trap loop at the tank connections, or both.
Night losses are also a tank problem. A thermosiphon store sits outdoors in the wind, so its standing loss matters more than for an indoor cylinder; 50 mm of polyurethane and a UV-stable outer casing are the minimum worth buying.
Thermosiphon or pumped — which suits your site?
| Criterion | Thermosiphon | Pumped (forced circulation) |
|---|---|---|
| Electricity needed | None | 20–50 W pump, 6–10 h/day |
| Tank position | Above the collector, outdoors | Anywhere, usually indoors |
| Frost tolerance | Direct types: frost-free climates only; indirect types with glycol tolerate mild frost | Full protection with propylene glycol or drain-back |
| Roof load | 250–300 kg concentrated | 20–40 kg per collector |
| Installed cost | Lowest — a factory kit | Higher: pump station, controller, indoor cylinder |
| Best fit | Mediterranean, Middle East, Africa, Turkey | Central and northern Europe, architecturally sensitive roofs |
Thermosiphon dominates in sunny markets because it fits the climates where solar water heating is most economic. Where winters are hard, the same fluid that circulates by gravity also freezes by gravity, and the pumped indirect layout wins.
What certification should a thermosiphon system carry?
Factory-made solar water heaters — the complete kit of collector, tank, frame and fittings — are tested and certified as a system to EN 12976, not just as a collector. The Solar Keymark can be granted on that basis, with the collector itself tested to ISO 9806. Custom-built pumped systems fall under the separate EN 12977 series.
Ask for the system certificate, not only the collector certificate. A Solar Keymark number covering the whole factory-made system is what proves the tank insulation, the frost behaviour and the annual yield of the assembled product have actually been measured — and it is what grant schemes across Europe ask to see.
Frequently asked questions
Does a thermosiphon solar water heater work in winter?
Yes, in mild winters. Output falls with irradiation, so a Mediterranean system that covers 90% of demand in July may cover 30–40% in January, with the electric backup element making up the rest. In climates with hard frost, only an indirect thermosiphon system with a glycol loop and a jacketed store is safe, and pumped systems are usually the better answer.
Can the tank be installed inside the loft instead of on the roof?
Only if the tank base still sits above the top of the collector and the pipe run rises continuously. In practice a loft installation means the collectors have to be low on the roof, which rarely works. Once the tank has to go lower, switch to a pumped system.
How long does a thermosiphon system take to heat up?
Circulation starts within minutes of the collector warming above tank temperature, but a full 200 L store typically needs 4–6 hours of good sun to reach 55–65 °C from cold. Because there is no controller to switch off, the store keeps charging all day and reaches its highest temperature in the late afternoon.
Do thermosiphon systems need maintenance?
Less than pumped systems, because there is no pump, controller or expansion vessel to fail. Direct systems in hard-water areas need descaling of the absorber circuit; indirect systems need the glycol tested every two years. The tank's anode and the frame fixings should be inspected on the same visit.