This is a diagram showing the main components of a normal central heating system. At the bottom left is a labelled boiler. From this a red flow pipe is shown running horizontally to a labelled pump. An arrow indicates that the flow of water is from the boiler. The flow pipe continues through a labelled control valve and on to a set of four radiators at the right of the diagram. The flow pipe splits to enter each radiator individually. Just before it enters each radiator it passes through a labelled ‘thermostatic radiator valve or ‘T’’R’’V’’. At the opposite end of each radiator is a blue return pipe. The return pipes from each of the radiators combine and are shown returning to the boiler at the left of the diagram with an arrow showing that the water flow is towards the boiler. In the centre of the diagram a further red flow splits from that to the radiators after the pump. It rises to enter a labelled ‘hot water cylinder’ in the centre of the diagram. The flow pipe is shown running through a coiled heat exchanger inside the cylinder before emerging at the bottom as a blue return pipe. Immediately after it emerges it passes through a control valve, which is shown linked by a dotted line to a labelled ‘cylinder thermostat’ on the side of the hot water cylinder. The blue return pipe then passes from the control valve and joins the blue return pipe from the radiators, returning cold water to the boiler. A further white pipe is shown rising from the blue flow pipe immediately before it enters the boiler to a labelled ‘central heating header tank’ at the top of the diagram. This is fed from the left by a white pipe labelled ‘cold water mains supply’. Another white pipe rises from the red flow pipe immediately after leaving the boiler. This is labelled ‘expansion pipe’. It rises and curls over to feed into the top of the central heating header tank.