Waterless Trap Valve Self Closing 40mm Female BSP Inlet X 40mm Pvc Compression Outlet can be fitted in a horizontal or vertical position. Membrane opens under water pressure to allow discharge to flow freely and then closes to form a seal to prevent foul odour or sewer gas entering the building. Can be used to take the discharge from a pressure relief valve on an unvented domestic hot water storage system, having no water seal, can never dry out through evaporation or lack of use of an appliance as the membrane remains sealed. Not recommended for installation where there is no access for maintenance.