The boiler has a burner (160) supplied with an air-gas mixture by a mixing chamber (100) provided with an air supply line (120) and a gas line (260) from a gas block (200). The block is connected with a gas supply line (210) over a gas valve (220). A modulation flap (240) is controlled by a membrane subjected to gas pressure on one side and atmospheric pressure on the other side. A transmission link (270) of a pressure wave (delta P) connects a line of the mixture behind a gas nozzle structure (110) and a neck (111) with an air connection (253) i.e. pipe, of the flap with a free opening (254).
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