Abstract: An industrial gas operated liquid heater is provided comprising a tubular hearth or furnace a front end face of which is provided with a gas burner and a heat exchanger in which the liquid to be heated and the combustion fumes flow concurrently, the exchanger forming with said tubular hearth a unitary monobloc assembly so designed that the liquid to be heated forms, a first liquid sheet enclosing substantially the whole of the tubular hearth, except for said front end face receiving the gas burner and, in the exchanger, a plurality of second liquid sheets extending substantially parallel to the longitudinal direction of the tubular hearth and adjacent, in said exchanger, to chambers through which flow the combustion fumes issuing from the tubular hearth.