The inclusion of solar water heating in a heating system need not be complicated. In fact, the solar system can co-exist happily as an addition to a conventional heating system without interfering with it at all.

The way to keep it simple is to arrange the plumbing so that the solar energy is used to feed the conventional system with pre-heated water. The conventional system is under thermostatic control, so only fires up when the solar has not raised temperatures sufficiently.

There are three arrangements that can achieve this trick:

The simplest to understand is a two cylinder arrangement, where the cold feed to the conventional hot water cylinder is first taken through a cylinder that is heated by the solar panel. On good solar days, the water coming into the second cylinder will already be hot enough, and the thermostat will not call for heat - the boiler will not fire. On less good solar days, the water coming into the second cylinder will not be warm enough to use, and the thermostat will call for the boiler to heat the second cylinder.

The most common, and most cost-effective is to use a cylinder with two heat exchange coils. The coils are arranged one above the other, with the solar coil at the bottom.

Because the coils heat by convection, the boiler can only heat the top part of the cylinder, where a zone of hot water will float on top of the cooler water at the bottom, a phenomenon called stratification. This means that irrespective of the boiler controls, the solar system always has a volume that it can heat.

On good solar energy days, the solar panel will heat the whole tank up to a useable temperature, and the cylinder thermostat will not call for heat from the boiler. On less good solar days, the cylinder thermostat will call for heat and the boiler will top up solar energy.

The boiler controls can be completely independent of the solar system.

See the next section for a more detailed explanation of the twin coil cylinder.

A combi boiler heats water "on demand". A solar heated cylinder can be installed upstream of the combi boiler so that the boiler receives warm water rather than cold as an input, and has to use less fuel to heat up the water to the required temperature. A tempering valve is normally required to ensure that the temperature of the in-feed water to the boiler is not too hot, as many boilers cannot regulate their flame sufficiently to accept water above a certain temperature without over-heating.

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