I have this water heater AO Smith GT15-540NI, I flushed the tank, when I replaced the filter in the drain plug for some reason I lost the O-Ring and it drips so I shut down the heater until I get a new O-Ring. The manufactor says it is a P15 O-ring but I want to go to the local hardware store and get a replacement does anyone know what size O-ring I should get. thanks in advance