Improper venting for plumbing in an island sink drain can cause a myriad of unpleasant problems: strange drain noises and gurgling; sewer gases entering the premises; and waste water backups. Vent stacks in a plumbing system eliminate these problems, since they introduce air and equalise pressure in the pipes. However, an island sink drain poses a dilemma, since the plumbing does not allow for a vertical vent stack pipe directly up and through the island.
The Uniform Plumbing Code is a regulatory code adopted by most municipalities. According to the code, drain traps must sit above the finished floor; the system must have a trap cleanout; and the vent may be either a loop vent or have an air-admittance valve situated six inches above the horizontal drain pipe.
The loop vent system directs waste water out of one pipe and air in through another pipe; both these pipes are connected by a loop at the top of both pipes. The air-admittance valve, or AAV as it is commonly called, is a small mechanical device connected with a tee fitting to the sink's drain pipe.
The loop vent system requires adequate space under the sink to allow for the two columns of pipes. The AAV system requires frequent maintenance, as the mechanical device wears out over time.
According to the Uniform Plumber Code, the island sink drain pipes must be three inches in diameter, consistent from the beginning to end of the run. The drain line must be upstream of the return vent line. The drain and vent may serve no other fixtures.