It works by taking advantage of the fact that hotter water floats on cooler water. The cylinder has two heat exchanger coils arranged one above the other. The boiler (or other auxiliary heater) is connected to the upper coil, with the solar circuit connected to the lower one. When the boiler is providing heat to the cylinder, a heating fluid is pumped from the boiler to the cylinder, where it flows inside the coil. The coil has thin metal walls which conduct the heat into the surrounding water. The heated water near the coil expands and becomes less dense than the surrounding cooler water, and so rises. Cooler water at the top of the cylinder falls to replace the rising hot water. This so-called "convection current" means that the boiler heats the top part of the cylinder above the boiler coil.