32 then through a space having a restricted cross sectional area around flue tube 25 and into the space having a large cross sectional area of the vent portion 31 the products of combustion enter flue tube 25 through rectangular flue opening 43 communicating with combustion chamber vent portion 31 and are exhausted through flue tube 25 the flow of the flame produced by burner 39 through the obstructed combustion chamber c causes turbulent combustion this turbulent combustion is believed to reduce production of oxides of nitrogen this turbulence also causes the gases of combustion to repeatedly contact the walls of chamber c which results in good heat transfer to the water while at the same time helping to limit the maximum temperature of the gases as the products of combustion go up flue tube 25 heat is extracted from them by the relatively cool surfaces of the flue tube wall and baffle 51 and transferred to the water in tank a water vapor contained in the products of combustion condenses on these surfaces transferring the latent heat of vaporization to them and thus to the water in tank a the condensed water flows down flue tube 25 into receptable 46 the water cools the lower portion 24 of flue tube 25 and is evaporated by the heat of combustion it is believed that the lower portion 24 of flue tube 25 projecting into the combustion chamber c reduces formation of oxides of nitrogen through a number of mechanisms.