Every fixture drains through a trap, which is the bend holding a small volume of water. That water is the seal preventing gas from the drainage system entering the room, which is why a fixture that has not been used for a long time can smell: the trap has evaporated dry.
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For a burst pipe or a serious leak, turn the water off at the mains tap before you call. If water is anywhere near power points, the switchboard or light fittings, stay clear and treat it as an electrical hazard. Do not enter standing water in a room with live power.
