A pair of water conditioning tanks each having a service port. The service ports are connected to each other and to a service outlet conduit. A pair of semibalanced, direct-acting control valves are provided, each having an inlet port, a tank port connected to one of the conditioning tanks, and a drain port. Each valve has a pressure responsive piston engaging the valve stem and a conduit for supplying line pressure to the piston to assist in operating the valve. Timed motor operators alternately maintain one of the valves in service position and the other in backwash position whereby each tank can be backwashed with conditioned water from the other.