Abstract: This fluid mixing valve is particularly adapted to mix steam and water to operate a liquid nozzle gun as for instance in the food processing industry to wash utensils and equipment. This valve is adapted to be connected upstream of the gun in particular to prevent burns by the steam, to minimize the waste of steam, and to minimize the occurrence and effects of water hammers resulting from mixing steam with water. This valve comprises a body defining a steam inlet, a water inlet, a heated water outlet, a heat exchanger connecting both inlets to the outlet, a piston chamber, and a venturi nozzle merging the chambers of the heat exchanger for flow of water under suction by the steam flow. A valve closes the steam inlet under the action of a piston responsive to the pressure in the liquid inlet and a bleed valve and passage are arranged to bleed unintentional flow of water when the mixing valve is closed to prevent unintentional opening of the valve.