Which water heater?

Which solar water heating system you choose depends on several factors, including the availability of gas for boosting, the design of your house, and your available budget.

Solar water heaters come in several different system configurations. The most common is the ‘close coupled’ system, with the panels mounted on the roof and the storage tank mounted horizontally directly above them. These are usually mains pressure systems, though some close-coupled systems are gravity feed systems.

Some systems have the tank within the roof space so that only the collector panels can be seen from outside. These systems are nearly always gravity feed systems.

It is also possible to have the collector panels mounted on the roof, and the tank located at ground level. Water is pumped around the system by a small electric pump. This type of system is always a mains-pressure system, as a gravity feed system needs the tank to be well above tap level in order to work. This ‘split-level’ system has less visual impact, which may be desirable if the water heater needs to be discrete, such as when heritage laws govern home modifications.

Determining the correct size for a solar water heater unit is most important. On average, each person will use around 50 litres of hot water per day. It is advisable to have 1½ days’ reserve capacity. The system should be sized for the dwelling, and not necessarily its occupants—people sell houses and move on, but the hot water system stays. A three bedroom house can comfortably accommodate four people, so should have a 300 litre system, even though it may be occupied by a retired couple. An undersized system will have a disappointing output and require excessive boosting from non-renewable sources.

Mains pressure solar hot water systems have become very popular over the last thirty years and are strongly promoted by the manufacturers. Mains pressure systems allow the use of flick-mixer taps and give a strong flow in the shower. They are suitable for use with water supplied by a pressure pump, a tank high up on a hill (provided the pressure is great enough) and, of course, water supplied from the mains. With gravity feed systems, the pressure at the taps is supplied at a constant rate governed by the height of the header tank. Larger diameter water pipes are often required in the house to allow for the reduced water pressure, though this also depends on the position of the water heater and the height of the roof. The advantages of gravity feed systems include longer tank life due to the greatly reduced pressure and the ability of these systems to be connected directly to the heat exchanger (wet back) of a slow combustion stove, which provides auxiliary heating. Gravity feed systems are also generally less expensive to purchase.