A thermosiphon solar water heater moves water between a collector and a storage tank with no pump, because hot water is lighter than cold and rises, and it works only if the bottom of the tank is above the top of the collector, by at least 30 cm. It gives a separate, larger tank than a batch heater and can be built from salvaged parts, but the loop must stay vented, the tank must be insulated, and the delivered water must be tempered.
Part of Solar Thermal Collectors
A thermosiphon system is a flat plate collector joined to a raised storage tank in a loop in which the heat itself drives the circulation.
One Loop, Two Pipes, No Moving Parts
The collector sits low and the tank sits above it. A pipe from the top of the collector enters the upper part of the tank, and a pipe from the bottom of the tank returns to the bottom of the collector. Sun heats the water in the collector; it becomes lighter and rises through the top pipe into the tank; heavier cold water from the bottom of the tank sinks down the return pipe to take its place. The loop turns itself as long as the sun heats the collector (How Thermosiphon Works derives the force).
The tank-above-collector rule also stops the loop running backwards at night. When the collector cools, the water in it becomes the heaviest in the loop and stays below the tank, so heat does not drain from the tank to the collector.
Three Design Choices
- Direct or indirect. In a direct system the water you will use circulates through the collector. In an indirect one an antifreeze loop passes through a heat exchanger coil in the tank (Freeze Protection).
- Tank height and size. At least 30 cm above the collector top; a household tank holds 100-200 L (parent page) (Tank Placement).
- Pipe bore and length. Short, wide, continuously rising pipes (Hot Water Systems works a pipe-friction balance for a fire-heated version of the same loop).
What You Need
- A flat plate collector (Flat Plate Collectors)
- A tank of 100-200 L, insulated, mounted high
- Two pipes of short, wide bore, rising continuously
- A vent at the highest point and a drain at the lowest
- A tempering mixer at the outlet
Method: Assemble The Loop
Step 1: Set the collector at its tilt, and place the tank so its bottom is at least 30 cm above the collector’s top edge.
Step 2: Connect the collector top to the tank upper side, and the tank bottom to the collector bottom.
Step 3: Fill from the bottom; vent trapped air at every high point.
Step 4: On the first sunny day, feel the top pipe: it should be hot while the return is cooler. If neither warms, there is an air lock or a sag.
Step 5: Insulate the tank and pipes.
Common Mistakes
- Tank at or below the collector. The loop does not start, or the tank drains at night.
- Sags and crests in the pipes. Air collects and blocks circulation.
- A valve in the loop. If it closes, the collector becomes a closed boiler.
Safety
A sun-fed loop can stagnate above boiling, and the tank water scalds
If water in the tank is not used, the collector keeps heating and can pass boiling: the loop needs an open vent or relief path to a safe discharge, never a closed valve (Plumbing Connections). Tank water at 60°C or more scalds in seconds; temper it at the tap, because children and the elderly are injured at lower temperatures and shorter exposures than adults (Severity Assessment).
What’s Next
- Learn next: How Thermosiphon Works — the physics and the numbers
- Combine with: Heat Storage Systems — keeping the stored heat