Abstract: A heater having a combustion chamber in which a burner and a heat exchanger for fluid to be heated are arranged. The burner is oblong and includes a mixing chamber with a venturishaped cross-section. A converging inlet part of the mixing chamber is connected to a feeding device for supplying air and fuel to the burner. The air and fuel are each supplied through a restriction plate of the feeding device. The restriction plate has two rows of calibrated openings so that a row of incoming air jets and a row of incoming fuel jets are formed. The row of air jets is directed against a deflecting surface incorporated in the upper wall of the inlet part for producing a whirling effect for obtaining an appropriate mixing of air and fuel. A diverging outlet part of the mixing chamber is closed by a burner plate with evenly divided ports.