How to Vent an Island Sink
An island sink, which is set up in a cabinet in the middle of your kitchen, is both a very practical and highly original design solution for your kitchen. However, island sinks have one big problem: outfitting them with efficient venting pipes is a no east feat. In plumbing, venting pipes are a set of air ducts which have some very important functions. Attached to the water flow pipes, they ensure that the right pressure level is maintained throughout the system, that water circulates at an appropriate speed, preventing the accumulation of debris in the pipes, and that pernicious sewage gasses do not enter the kitchen but exit safely through a pipe connected to the roof of your house.
With normal kitchen sinks, like ones that stand next to a wall, proper venting is ensured by mounting a venting pipe inside the wall at a height at least ten inches larger than the overflow level of the sink. This, however, cannot be done with an island sink since it is not adjacent to any wall. Instead, you can proceed as professional plumbers do:
Step 1 - Use Island Vent
Use an island vent, which is the most popular method to vent an island sink. With this method, the vent pipes enter the island cabinet from below the floor, rise as much as possible inside the cabinet, and then join the water drain pipe. This technique allows the water to drain off at a regular speed and provides fresh air to the P-trap, which is situated right below the sink.
Step 2 - Take Care of Cleanout Tees
Be sure to mount cleanout tees in case the vent piping gets plugged. If you want to install a garbage disposer to your water drain system, use flat cleanout tees, which would facilitate the cleaning of the upper vent pipes.
Step 3 - Mount Pipes
Mount horizontal pipes with a downward grade of a quarter of an inch per foot of pipe length. Lead the vent pipes to the nearest wall and have them leave through the stack.
Venting with Air Admittance Valve
Using an air admittance valve is another way to vent your water drain system. These valves open to release fresh air whenever the pressure in the system becomes negative. Otherwise, they remain closed, preventing sewage gas from seeping inside the room. An air admittance valve would greatly simplify the drain system installation, making redundant the mounting of vent pipes under the floor. On the other hand, this valve does not cope very well with high volumes of water, so it should not be used in multi-story buildings occupied by many families.
Tips on Installing an Air Admittance Valve
Make sure you mount as high as possible inside the island cabinet to ensure its optimal operation. Before installation, seal the threads of the valve with Teflon tape to secure the fixture and prevent leakages.
Check the local plumbing regulations to see if the use of the valve is admitted in your area (some US states and European countries have ruled out the installation of air admittance valves into household drainage systems).
Consult a professional plumber for additional tips and advice before you initiate any major installations, regardless of the venting method you have chosen.