Almost all gas boosted solar hot water heaters use a continuous flow gas booster (sometimes called an Instantaneous Gas water heater). These are wall mounted units (although they are often seen mounted on the back of pitching frames where solar hot water systems are placed on flat roofs). The beauty of this method of boosting is that the gas switches off as soon as the solar pre-heated water gets to them in the summer and as soon as you turn off the tap in the winter – so you only heat the amount of water that you use and no more.
It is important to order the correct type of gas booster. There are two types of gas available: Natural gas this is town gas that is piped to your property from the street via a gas meter often found at a front corner of the house. LPG (Liquid Petroleum Gas) is bottled gas which needs to be used if mains gas supply is not available in your area. The two different gasses require different gas jets and the units are not interchangeable.
These are high megajoule (Mj/h)units (20 litre per minute [lpm] gas burners require 160 Mj/j 26 lpm units require 200 Mj/h and 32 lpm units require 250 mj/h) that use a large amount of gas for a short period as they heat cold water as it passes through a heat exchanger. This means that if you are replacing an existing gas storage heater you will almost certainly need to up-grade the size of your gas supply pipe and probably back to the gas meter (or bottle in the case of LPG). The cost of this upgrade will depend on the size of pipe required which in turn is dependent on the distance between the gas booster and the gas meter, but for an easy calculating you can allow $50 per meter for the copper and the labour to lay it.
Continuous flow gas water heaters require the tap / shower to be fully turned on as the gas will switch off (for safety reasons) if there is a flow of less than 6 litres per minute. In the spring and autumn, when the water temperature in the solar storage tank is at about the temperature that the gas booster is set to boost to (70 degrees) then the gas burner sometimes switches on and off causing a temperature fluctuation.
To avoid these two issues and to avoid an expensive gas pipe up-grade, some people use their existing gas storage heaters as the solar booster. This saves the cost of a new continuous flow gas booster, the gas pipe up-grade and the additional cost of plumbing a new booster. However, this means that they will not qualify for a rebate as there are only a few solar hot water systems that have registered pre-heaters for rebate.