Abstract: A gravity feed, natural circulation boiler for an SAGD process using low quality feedwater for carbonatious material recovery, has a large diameter steam drum with downcomers. A furnace of the boiler has individually replaceable membrane wall modules, each with upper and lower headers and membrane roof, wall and floor parts connected to the drum and defining a fire box having an inlet end and an outlet end. The furnace includes a membrane front wall connected to the drum with a windbox upstream of the front wall. Burners at the inlet end of the firebox heat the firebox and riser pipes are connected between the steam drum and the upper header for supplying steam to the steam drum when the firebox in heated, the downcomer pipes being connected to the lower header for supplying water from the stream drum under gravity feed so that each module defines a single circuit.