Abstract: A flapper is provided in the inlet and outlet lines to a domestic hot water heater or the like and a permanent magnet is affixed to the bottom portion of each of the flappers. Immediately adjacent each flapper and outside of the pipe, a magnetically operated switch is placed. The magnetically operated switches are connected in circuit with a relay which in turn controls a solenoid valve for cutting off or permitting the entry of water from the source into the inlet pipe. Under normal operating conditions when there is a flow of water in the inlet pipe and the outlet pipe the connection of the magnetically operated switches is such that the relay will not operate and the solenoid actuated valve will be open permitting the entry of water from the source. When a leak occurs, the ensuing movement of the flappers caused by the change in liquid flow will operate their associated magnetic switches, causing the relay to be actuated and closing the solenoid actuated valve, thus cutting off the supply of water.