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This is shown as a rectangular white box. In the centre is a labelled ‘finned heat exchanger’. A blue pipe labelled ‘water return from heating system’ is shown feeding cold water to the top of the heat exchanger. A red pipe labelled ‘water flow to heating system’ is shown carrying hot water away from the heat exchanger. Inside the boiler a gas burner with four blue flames is shown heating the heat exchanger. At the right is a smaller flame labelled ‘pilot light’. Beneath the burner an upward pointing vertical blue arrow is labelled ‘combustion air from house’. At the top of the boiler a vertical flue is shown with an upward pointing vertical arrow labelled ‘hot exhaust gas’.

 3.1 Gas boilers