The flood preventer using a float, operates like a check valve to seal off water back-up caused by overloaded sewers.Water flows normally through the drain until the sewer begins to back up. Then the Flood-Guard float rises to seal off the drain opening until the water recedes.The valve can be installed by the property owner, or by a licensed plumber. Simply insert it into your 3" drain and tighten the screws on the top to expand the rubber seal and hold the valve in place.