Abstract: A direct contact water heating system and process are disclosed which provide for the direct contact of hot flue gases with water-to-be-heated without the hot flue gases being washed or quenched prior to their contact with the water in such a manner that maximum thermodynamic efficiency is achieved while delivering water heated to the equilibrium temperature corresponding to the vessel pressure and free of dissolved gases, and requiring, under most conditions, no make-up water. A variation will produce water at a higher temperature. The system discharges scrubbed environmentally-acceptable flue gas to the atmosphere with essentially zero chemical, particulate or thermal pollution.