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The boiler is shown as a white box mounted on a cavity brick wall which is shown on the right-hand side of the picture. The boiler assembly is fixed to the left side, on the inside of the house. At the left there are pipe connections to the heating system. The air supply and exhaust flue pass through the wall to the outside. This takes the form of two concentric pipes. Fresh air is shown being drawn into the boiler through the outer concentric pipe. It then travels down to the bottom of the boiler and upwards on the left-hand side. Near the bottom of the left side of the boiler is a burner shown with blue flames and labelled ‘gas’. The heated air then travels up the left-hand side of the boiler to a set of finned pipes labelled ‘heat exchanger’. From these hot water is shown flowing through a red pipe labelled ‘water flow’ out to the left to the heating system. Another blue pipe labelled ‘water return’ feeds cold water from the left back to the heat exchanger. The combustion air continues rising to the top of the inside of the boiler and across to the right-hand side. It is pumped by a fan inside the boiler housing driven by a labelled electric motor shown mounted in a grey area inside the centre of the boiler. The air then exits from the boiler to the right through the inner concentric pipe where it is labelled ‘hot exhaust gas’.

 3.1 Gas boilers