Gas boiler, coloured X-ray. This type of boiler is typically found in a kitchen, where it heats water used in a household central heating system. The electronic control system (bottom centre) is used to set the temperature and may also have a timer, so that the system can be set to start and stop heating the house at specific times. The heat exchanger (centre, purple) transfers the heat from the gas flame to the water. A flue (chimney, upper right) is also seen. If this becomes blocked and the boiler is working inefficiently, carbon monoxide gas can be vented into the household. Should this gas build up it can cause asphyxiation.