A device for preventing the back flow of water from a point of use into a water supply line. The device includes a unitary casing having an inlet, a central chamber, an outlet, a removal port and a drain vent opening. The inlet, the central chamber and the outlet are coaxially aligned. A relief valve is located in the central chamber and offset from the coaxial alignment of the inlet, the central chamber and the outlet. There is a cover for the removal port. There is also a pair of check valves. One is mounted between the inlet and the central chamber and the other is mounted between the outlet and the central chamber. The device allows water to pass unobstructed in a straight-through flow path through the device. The back flow of water is prevented.