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The boiler is shown as a rectangular box. It is 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. Its air supply and exhaust flue pass through the brick 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 a blue flame image and labelled ‘gas’. The heated air then travels up the left-hand side of the boiler to a set of red pipes labelled ‘heat exchanger’. From these hot water is shown flowing through a red pipe out to the left to the heating system. The 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 an electric motor shown mounted outside the boiler at the right hand side. The air then descends through a second heat exchanger coloured light blue. The pipework for this is connected at one end to the first heat exchanger. At the other end it is fed by a blue pipe from the heating system and labelled ‘return’. Water droplets labelled ‘condensed water vapour’ are shown falling from this second heat exchanger. They fall into a container and then move down through the bottom of the boiler in a pipe labelled ‘condensate drain’. The air then exits from the boiler through the wall in the inner of the concentric pipes. Its flow is shown with an orange arrow labelled ‘warm exhaust gas’.

 3.1 Gas boilers