11/2" Vented Loop - It is worth fitting vented loops to prevent water siphoning back into the toilet through the inlet/outlet pipes. The valve system incorporated in the top of the loop allows air to enter the pipe when not in use, yet seals as water or waste passes through. Comes complete with fixing lugs on the loop for attaching to a bulkhead. Available in 3/4 and 1 1/2" ."