How to install the proper venting in a double-bowl sink in a kitchen island

How would you install the sink drain pipes in my island if you were my plumber? I’m also interested in any other tips you can provide to make my new kitchen plumbing trouble-free for years. — Martha W., Rapid City, S.D.

A: I’ve been a master plumber since age 29, and I have to chuckle at how perfectly Martha has summed up the issue! Some new building products are simply wonderful; many others are not that great. That said, I’m a big fan of not having moving parts in plumbing systems, so you’ll never see an air-admittance valve, a.k.a. AAV, on one of my jobs, nor would I ever use one in my own home.

An AAV is designed to allow fresh air into a plumbing system and not let sewer gas leak into your home. But sometimes they just don’t work right and sewer gas can leak. I’ve had countless homeowners over the years send me emails complaining about this and asking how to eliminate the AAVs.

In situations like Martha’s kitchen island, where you simply can’t install a traditional vent pipe that is hidden behind the plumbing fixtures, the best solution is a traditional loop vent. These hidden vent pipes are connected to the drain pipes and eventually connect together up higher in your home and often exit the house through one or more little pipes you see poking through your roof.

The purpose of the roof vent pipes is to allow air into your plumbing system. When your plumbing drain system is not in use, all the pipes are filled with air except for the water that’s in the traps under sinks, tubs, showers, floor drains and toilets.

When you run water in a sink or flush a toilet, you add a volume of water to the system and in the case of a violent addition like a toilet flushing, the air in the pipes is forced through the system much like a bullet forces air out of a rifle barrel when you pull the trigger. This air must be replaced instantly through the roof vents or the system will go hunting for the air and suck it through a sink or tub drain nearby the flushing toilet.

Perhaps you have heard this sucking or slurping noise from a tub or sink. This is an indication of a problem in your vent system, and sewer gas can enter your home via the trap that now has no water in it.

Here’s how the kitchen loop vent works. I’d love to meet the long-dead plumber that thought this through because it’s such a simple and elegant solution to a problem. It’s important to realize that with a kitchen island sink, you don’t want some ugly vent pipe to extend through the countertop and run up to your kitchen ceiling. No one would find that acceptable, for goodness sake.

A loop vent gets the needed air from the actual drain pipe just 4 or 5 feet away from your island sink. It’s ingenious. Here’s how it works. Martha’s plumber needs to install a 2-inch drain pipe to the kitchen island. Inside the kitchen sink cabinet or behind it in a void space, the plumber needs to create a loop. This loop is also done with a 2-inch pipe. You can envision this loop by thinking of a capital letter P that’s laying on its side with the curved section of the P aiming to the sky and the long leg of the letter representing the drain pipe under the sink.

The actual drain pipe that enters the kitchen sink cabinet must be no larger than a 1.5-inch pipe. This smaller pipe restricts the amount of water that enters the loop-vent system. The drain pipe water enters the vertical pipe of the loop that is represented by the top of the letter P when it’s on its side. The other part of the curved part of the letter P represents the actual vent part of the system.

I’m sure you can envision how the curved part of the letter P attaches to the horizontal drain pipe under the floor. Can you see this in your head? It’s best to have these two vertical pipes that are in the cabinet or behind the cabinet at least 32 inches apart, with 3 or 4 feet being ideal.

Here’s what happens when you fill the sink to the brim and pull out the sink stopper. The water immediately rushes from the sink and completely fills the 1.5-inch pipe that is in your cabinet. But right away it starts to fall into the vertical 2-inch pipe that it can’t fill all the way because of its larger diameter. As the water crashes to the bottom of the horizontal pipe and starts its journey to the sewer, the water starts to level out and only partially fill the 2-inch horizontal pipe. Much needed air is on top of the water.

As the flowing water passes the other part of the loop that is going up into the cabinet, the air that’s needed to satisfy the vacuum created by the draining sink water is flowing upstream across the water that’s going the opposite direction toward the sewer. The air enters the loop and allows the water to drain quickly from the sink. There are no moving parts and you’ll never have sewer gas if the loop vent is installed correctly. It’s magic!

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