How a solar water heater works
Collector, storage tank, pump and controller. What each part does, the difference between a pressurised and a drain-back system, and what to look out for.
A solar water heater heats your tap water with the sun. A collector on the roof captures heat, a pump carries it to a storage tank, and a controller decides when that is worth doing. When the water isn't hot enough, your boiler or an electric element tops it up.
The parts
The collector sits on the roof, usually facing south. A fluid runs through it that, in the sun, gets tens of degrees warmer than the air.
The storage tank stands indoors, often in the attic or a utility room. At the bottom is a heat exchanger: a coil the warm fluid from the collector runs through. Your tap water never touches it. At the top, the water is heated further when needed.
The pump moves the fluid between collector and tank. In principle it only runs when there is heat to gain.
The controller measures two temperatures: the collector's and the bottom of the tank. When the collector is a few degrees warmer, it switches the pump on. When the difference gets small, it stops it again. Its display shows those temperatures, and whether the pump is running.
Two kinds of system
A pressurised system is closed and filled with a mix of water and glycol, which doesn't freeze. You recognise it by a pressure gauge and an expansion vessel, a round tank that absorbs changes in pressure. It is the standard in Germany and common in the Netherlands too.
A drain-back system is filled with water. When the pump stops, the water runs back from the collector into a tank indoors, so it can't freeze or boil. In the Netherlands you mostly find it with solar water heaters for a single household. Hearing the water flow back when the pump stops is normal.
Which type you have is in the manual or on the tank's rating plate. If you don't know, ask your installer: it decides what is normal, such as the pressure.
What a solar water heater yields
In summer, a solar water heater can make almost all the hot water a household needs. In winter it makes much less, and the boiler does most of the heating. Over a whole year, a solar water heater in good order often covers about half of a family's hot water. How much exactly depends on the orientation, the number of people and how much hot water you use.
How to tell it is working
A solar water heater stays quiet when something is wrong: you still have hot water, because the boiler takes over. A broken pump or sensor often only shows on the energy bill. On a sunny day, the controller shows whether the collector gets warmer than the tank and whether the pump runs then. Check it now and then, or have it measured.