Solar storage cylinders are used in conjunction with solar collector systems. The heat transfer fluid which is heated by the sun is circulated passed through a heat exchanger coil in an indirect hot water storage cylinder. Solar storage cylinders are fitted with two heat exchangers (coil) circuits allowing the lower coil to be heated by the solar circuit and the upper coil by an secondary heat source (typically a gas, oil or electric system boiler) for used when an additional energy source is required.