A flow sensor having an inlet and an outlet comprising: a plate moving in response to fluid flow therethrough, being displaced further as flow increases; a sensor for determining moving plate's position and creating an electronic signal related thereto; a biasing device for biasing the plate towards the sensor's inlet end. The sensor may include a sealing device for preventing flow therethrough until a specified differential pressure is reached allowing the device to be used as a bypass mechanism. The sealing device may also constitute a check valve device to prevent flow in an undesired direction. The method using the sensor in multipurpose piping systems includes use of the apparatus in a system incorporating at least one fire sprinkler, the piping system also preferably providing at least one other use in the structure. The other use in the structure may be supplying domestic needs, or supplying heating/cooling water for a heating/air conditioning system.