A bypass valve includes a body with first and second chambers connected by a bridge passage, an untreated water inlet and an untreated water outlet both open into the first chamber, and a treated water inlet and a treated water outlet both open into the second chamber. A first valve element is rotatably received in the first chamber alternately to connect the inlet to the untreated water outlet or the bridge passage. A second valve element is rotatably received in the second chamber alternately to connect the outlet to the treated water outlet or the bridge passage. A manually operable mechanism rotates the first valve element in the first chamber and the second valve element in the second chamber. A flow sensor is received within either the first or second valve element to measure the amount of water flowing through the bypass valve.