The water heater has an upright water tank (1), an exhaust flue (3) extending through the water tank, a combustion chamber (2) arranged under the water tank, an atmospheric gas burner (5) in the combustion chamber and a complete pre-mixing gas/air supply system with outside gas and air intakes. The gas burner has a burner mat (7) with a pressure drop less than 5 to up to 45 Pa. The combustion chamber (2) is provided with an opening (2b), which is not sealed, but enclosed in a flame-blocking manner. The gas burner (5) is attached to a wall section (2a), which is mounted releasably on the: combustion chamber wall (W) over opening (2b) so that a gap of predetermined width is formed between the wall section and wall (W), which prevents backfire of flames into the surroundings but admits secondary air.