Abstract: A pressure relief cap for a water supply comprises an outer portion defining a front face for generally covering an outlet of the supply. The outer portion includes a seal for being urged against and sealingly engaging a periphery of the outlet. An inner portion includes a connector for coupling and decoupling to an outlet. The inner portion is rotatably coupled to the outer portion such that upon at least partial rotation of the outer portion in a first direction relative to the inner portion the seal is moved axially inwardly relative to the inner portion from a disengaged position to an engaged position to sealingly engage a periphery of an outlet, and upon at least partial rotation of the outer portion in an opposite direction the seal is moved axially outwardly from the engaged position to the disengaged position to thereby permit back pressure within an outlet to vent.