Most builders and plumbers are familiar with two ways to vent kitchen-island sinks: the time-tested island fixture vent, and the newer air-admittance valve, or AAV. Neither method is exactly foolproof-they're allowed in specific circumstances, and their acceptance and design are not universal across municipalities, states, or even code-writing entities. There is an arguably better method of venting kitchen-island fixtures, though-we'll get to that in a minute. As sophisticated as modern houses are, a simple cup of water is essentially the only thing keeping sewer gases from backing up into our homes. This cup of water sits in what is commonly called a P-trap. Contrary to some assumptions, its name has nothing to do with the kind of liquid in it, but rather the shape of the plumbing, which is similar to a letter P that's fallen on its face. Every time a fixture is used, the water in the trap is cleared and replaced. If the drain isn't adequately vented, the rush of water from a draining sink or bathtub can suck the last bit of water out of the trap. Venting eliminates this siphoning action, ensuring that the last cup or so of water that enters the trap gets stuck there until the next time the fixture is used. This is tested science. You'd think everything in the code would be determined this way, but that's not always true.
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