An apparatus for controlling the temperature of a supply of water by selective injection of steam is disclosed. The apparatus comprises a water supply conduit which includes a water inlet end and a water outlet end. Steam is supplied to the apparatus by a steam supply conduit which supplies steam to the water supply conduit at a predetermined location between the water inlet end and the water outlet end of the water supply conduit. The steam entering the water supply conduit is controlled by a steam supply control valve located on the steam supply conduit. A pressure differential sensing means senses the difference in pressure along the water supply conduit between a first point in said water supply conduit, upstream of said predetermined location and a second point in the water supply conduit downstream of the predetermined location. A means for controlling the control valve means in response to this sensed pressure differential is also required.