Plumbing for a Kitchen Island Sink
An Island sink is indispensable when remodeling your kitchen. It gives two people a place to work, without bumping into each other. It also gives the kids a sink to wash up, or rinse something off, while you are preparing food at another sink.
One of the big issues when adding an island sink, is how to vent the waste line. If you have a wall behind the cabinet, the plumbing is no problem, but in an island, most of the time the cabinet is free standing with an applied finished back. Which is the case here. Other issues can be the joist placement in the floor. This cabinet sat over an electrical chase in the adjoining joist space, so the plumbing had to come up right in the middle of the cabinet. (Less than Ideal!)
The waste and vent line were stubbed up through the toe kick and floor of the cabinet just far enough to accommodate 45 degree fittings that offset the lines over to the edge of the cabinet. The vent loop - [second picture] - should be as tall possible, beside the sink and up to the counter top. The vent loops and goes back down under the floor. The vent line has a T fitting installed that ties into a waste line to drain any condensation or water that gets trapped in the vent line; from the T fitting the vent line runs to the nearest wall and up from there into the attic and through the roof.
Always install a T fitting after the vent line goes back into the floor, other wise the vent will slowly fill with water and become ineffective over time. There is nothing worse than a slow draining, gurgling, burping sink that vents through the sink drain.
There are after-market mechanical vents available, and these are code approved for mobile homes and repair work, but for this application in a remodel, they really should not be considered. A mechanical vent works with a very weak spring, which holds a rubber diaphragm closed. When draining water runs down from the sink, it creates a suction that pulls on the diaphragm and depresses the spring. These vents work great as long as they are not blocked or obstructed, but spiders build webs. Condensation is corrosive and over time causes mineral buildup, and people throw all kinds of stuff under a sink, so a vent is important and worth doing right.
A little trick to remember when assembling ABS fittings and pipe; make your cuts with a chop saw - wherever they will be visible and wipe off the excess glue with a rag, while assembling the loop. It will be much more attractive and uniform when completed.
Posted by The Cranky Codger