Although solar is most often used to make hot water for washing, solar thermal technology can also be used to keep the building warm (space heating). The most common way to implement this is to use a heat store.

A heat store differs from a conventional hot water cylinder in that the water in the store is not the water that will come out of the tap. Instead the store is filled with a mixture of water and inhibitors (additives to prevent corrosion) that remain there indefinitely.

Solar energy is added to the heat store via an indirect coil at its base which can heat the whole height of the store by convection (see also previous section). Backup heat for lower solar energy days is provided by a conventional boiler or alternative such as a heat pump. The fluid from the heat store is circulated to the boiler, where it is heated to be returned at a higher temperature. The tapping point where the fluid leaves the heat store is some way up the store, to leave a solar dedicated volume at the bottom that the boiler cannot heat.

Domestic hot water is heated on demand by running the cold incoming water through a coiled pipe inside the heat store. As the the cold water on its way to the hot outlet is warmed up, the volume in the store is cooled down.

To achieve a desired hot water output temperature of 50C for the hot tap, the heat store has to be kept at a higher temperature than a hot water cylinder would be (typically 80C as opposed to 60C). This means that heat stores have to be better insulated if they are not to suffer from higher heat losses, though because they are often larger in size, a reduction in the surface:volume ratio mitigates against this.

Space heating is provided by pumping heat store fluid from a tapping high up the heat store around the heating circuit, and returning to a tapping near the bottom. Low temperature heating systems such as under-floor heating will give the best results, because low return temperatures mean that the solar panel is more often operating at lower temperatures and therefore higher efficiencies (see Section 3.2: The Efficiency of Solar Panels).

Heat stores are also typically larger than a hot water store for a similar size building, the reason being that the heat store is storing energy for both hot water and space heating needs, and it is this, probably more than any other factor, that has limited the uptake of solar water heating in small UK homes.

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