A water treatment system including an automatic shut-off control valve. The system preferably includes inflow and outflow sensors that monitor flow through the system and a pressure sensor that senses back pressure in a downstream plumbing system. If the sensors sense different flows, a leak in the system is assumed and the control valve is closed. In one embodiment, the flowing back pressure is monitored by the pressure sensor to detect leaks in the plumbing system. In another embodiment, the valve is closed after a specific volume of water flow is sensed and the pressure sensor measures static back pressure in the plumbing system to detect a leak. In yet another embodiment, incremental amounts of water are released by the valve and subsequent back pressures are tested to detect a leak. Optionally, the system includes a control to shut-off a water supply when a water treatment device enclosure is accessed.