The heating boiler has a water-conducting housing with a combustion chamber forming the first draught. Remote from the burner, draught apertures (5) lead to the second draught (4) in channels (4') encompassing the combustion chamber. To the second draught, the third draught (6) in channels (6') encompassing the combustion chamber extends in parallel. The channels of the second draught are dimensioned to the burner side with reducing cross-section (Q) and between these channels the channels of the third draught to the draught side with reducing cross-section (7). The outflow cross-sections (7) of the second draught correspond to the inflow cross-sections (8) of the third draught.
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