There are two basic types of water heater – storage systems and instantaneous (or continuous flow) systems. Each system can use a variety of energy sources to heat water.
Storage water heaters
Water is heated and stored in an insulated tank for use when it is required. These systems can operate on mains pressure or from a gravity feed (constant pressure) tank.
Mains Pressure – Hot water is delivered at a similar pressure and flow rate to cold water so more than one outlet can usually be turned on without greatly affecting the pressure. The storage tank is usually located at ground level inside or outside the house. Mains pressure systems have been the most popular systems in recent decades.
Constant Pressure or gravity feed – Hot water is delivered at lower than mains pressure from a tank located in the roof of the house. Pressure depends on the height difference between the tank and the point of use. Gravity feed systems are most common for older properties and properties not connected to mains water. They are often cheaper to purchase and last longer than mains pressure systems. For either type of system, storage tanks may be made of copper, glass (enamel) lined steel or stainless steel.
Copper and glass-lined tanks typically have a sacrificial anode to reduce tank corrosion, which needs to be replaced every few years. Warranties offered for tanks typically range from five to 10 years.