Two ways to vent an island sink
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Go with the standard island vent method first.
An island vent rises as high as possible
under an island before running to a regular
vertical vent. It keeps air in the drain system
and prevents siphoning of the P-trap.
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Use an AAV vent as an alternative.
An air admittance valve, approved for use in certain
states, provides air for drainage but remains
shut to sewer gases. And it eliminates the need to
run a vent pipe under the floor.
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A typical Air Admittance Valve (AAV)
Plumbing an island sink is challenging. It can't be
vented the same way as a regular kitchen sink. Here's how it
Plumbing vents (a network of pipes that carry air and gas outdoors
through a pipe exiting your roof) are essential to supply enough air to
keep equal pressure in the plumbing system. They keep water and waste
moving through the pipes at the right speed. A flow that's too slow leaves
behind debris that clogs pipes. If it's too fast, suction siphons water from
the P-trap (see photo), allowing harmful sewer gas to enter your home.
A regular kitchen sink, adjacent to a wall, has a vent hidden in the wall
that connects to a vent. So it won't plug up, the vent must rise 6 in. above
the overflow level of the sink before the pipe goes horizontal. Obviously, a
vent pipe in a kitchen island can't do that.
As an alternative, most plumbers recommend a special type of vent
(photo) that loops as high as it can go inside the cabinet before
heading under the floor and over to the main vent in the wall. Always get
approval from your local plumbing inspector before installing this type
of vent. A second cleanout may be required in the vertical wall vent pipe.
A second option—one that requires no outside venting, makes installation
much easier, and is either loved or hated by plumbers—is called an
air admittance valve (AAV).These are not the spring-operated
cheater vents used in trailers; they're gravity-operated valves that open
when water flow creates negative pressure, allowing air to enter to equalize
pressure. But before you opt for this choice, you need to understand
the pros and cons, as well as contact your local plumbing inspector to
determine if an AAV is allowed (many local plumbing codes in the
United States currently accept AAVs).
If installed correctly, they can work in most single-family homes, as
long as there's one primary main vent that penetrates the roof to outside
air. Drawbacks? Sometimes AAVs can't keep up with
the venting needs of high-volume (18 to 22 gpm)
discharge washers and dishwashers. And AAVs have a tougher time relieving
pressure in the drainage and vent system of five-story buildings and higher.
Tip: If you plan to attach a
garbage disposer to an island sink,
replace the sanitary tee and cleanout
adapter with a flat-patterned
cleanout tee. That makes it easier to
snake the top of the loop if it gets