Everything You Need to Know About Plumbing a Kitchen Island Sink

An island sink presents a unique plumbing problem: Like all drains, the sink drain has to be vented to allow air into the pipes to equalize pressure, but how do you connect it to the building’s vent network if it’s sitting in the middle of the floor? Vent pipes must always rise from the drain, and you can’t go horizontally from the sink into a wall without creating an obstruction in the kitchen.

Plumbers have two solutions. One allows them to run a vent pipe under the floor by first creating a high loop inside the cabinet to ensure the vent can never be flooded. The other uses a mechanical vent known variously as an air admittance valve (AAV), Studor valve or “cheater” valve. So which method is better?

Myles Robinson, a home renovator, prefers the AAV because “it’s so efficient and easy to install in tight spaces. Plumber Kelly Russum has a different take: “The high loop is the preferable venting method I always recommend for island sinks if you have all the necessary conditions to make it work well.” Russum reasons that AAVs are mechanical devices that can fail and are less reliable in the long run. Some (not all) state and local plumbing authorities agree with Russum and allow AAVs only in very specific circumstances, if at all.

If you live in a community that allows AAVs, both methods are options, and Robinson is right: an AAV is easier to install than a high loop. Here’s what you need to know about plumbing an island sink using one or the other of these methods.

Also, find out why sinks stop working.

The Importance of Venting

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, allowing harmful sewer gas to enter your home.

The High Loop Method for Venting an Island Sink

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A regular kitchen sink adjacent to a wall has a vent pipe that connects to the drain hidden in the wall. To prevent it from plugging up, the pipe must rise six inches vertically above the sink’s overflow level before it goes horizontal. Obviously, a vent pipe in a kitchen island can’t do that.

One solution to this quandary is a special type of vent called a high loop. It connects to the waste pipe after the P-trap like a regular vent does, and then rises inside the cabinet to form a loop just under the countertop. From there, it drops under the floor and connects to a horizontal pipe that runs to the main vent network behind the wall.

For this to work, you need enough space under the cabinet to raise the bottom of the loop above the sink drain. As Russum explains, “If you have at least 32 inches of vertical space under the sink to create the loop. Also, keep in mind that using the high loop method is more practical for one-floor homes or first-floor installations where there’s easy access to the vent stack without needing to go through multiple floors.”

Always get approval from your local plumbing inspector before installing this type of vent. A second clean-out may be required in the vertical wall vent pipe.

Using an Air Admittance Valve

An AAV is a plumbing fitting that you can glue onto the drain pipe. It typically goes on the horizontal waste arm extending from the P-trap to the drain, which means you can install it right inside the cabinet.

“It works by allowing air into the drainage system when it needs it, but sealing shut to prevent any foul smells from escaping,” says Robinson. “This makes it particularly useful in situations where traditional venting might be difficult—like in a kitchen island.”

The type of AAV you need isn’t the spring-loaded device you sometimes find in mobile homes and trailers. The AAV that’s most likely to be approved for an island sink is gravity-operated and opens when water flow creates negative pressure, allowing air to enter to equalize pressure. With no metal parts to rust or corrode, it’s less likely to fail than a common “cheater” valve. For it to work properly, airflow can’t be restricted by anything inside the cabinet, and plumbing codes require it to be accessible at all times in case of failure.

AAVs do have their drawbacks. They sometimes can’t keep up with the venting needs of high-volume (18 to 22 gallons per minute) discharge washers and dishwashers, and they have a tougher time relieving pressure in the drainage and vent system of buildings five stories or taller.

“As a plumber, I always start by checking whether the high loop method is possible, and only then do I consider the installation of an AAV,” says Russum. “High loop and AAV are not two options to choose from — you do the high loop if it’s possible and AAV only if the high loop method is not possible in your situation.”

About the Experts

• Myles Robinson has over 15 years experience in home improvement and is the CEO of UK Composite Doors.
• Kelly Russum founded KC’s 23 ½ Hour Plumbing and Air Conditioning way back in 1978. He services Palm Springs, the Coachella Valley and other California communities.