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

How would you install the drain pipes on my island if you were my plumber? I’m also interested in other tips you can give to keep my new kitchen installations trouble-free for years to come. – Martha W., Rapid City, SD

A: I’ve been a master plumber since I was 29 and I have to smile at how perfectly Martha summed up the topic! Some new building products are just wonderful; many others are not that great. Even so, I’m a huge fan of not having any moving parts in plumbing systems, so in any of my jobs you will never see an air intake valve, also known as an AAV, nor would I ever use one in my own home.

An AAV is designed so that fresh air enters a plumbing system and no sewage gas enters your home. But sometimes they just don’t work properly and sewage gas can leak out. Over the years, countless homeowners have emailed me complaining about it and asking how to get rid of the AAVs.

In situations like Martha’s kitchen island where you just can’t install a traditional vent pipe hidden behind the faucets, the best solution is a traditional loop vent. These hidden ventilation pipes connect to the drainage pipes and eventually connect higher up your house, often leaving the house through one or more small pipes that you see poking through your roof.

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

When you run water into a sink or flush a toilet, you add a volume of water to the system, and in the case of a heavy addition like a toilet flush, the air in the pipes is forced through the system, much like the way a bullet forces air out of a gun barrel when you pull the trigger. This air must be replaced immediately through the roof vents, otherwise the system will search for air and suck it in through a sink or tub drain near the flush toilet.

You may have heard that sucking or slurping sound from a tub or sink. This is an indication of a problem in your ventilation system and sewage gas can enter your home via the now no water trap.

This is how kitchen loop ventilation works. I would love to meet the late plumber who thought this through because it’s such a simple and elegant solution to a problem. It’s important to know that a kitchen island sink shouldn’t have an ugly vent pipe going through the countertop and up to the kitchen ceiling. Nobody would think that was acceptable, for Christ’s sake.

A loop vent draws the air it needs from the actual drain pipe, just 4 or 5 feet from your island basin. It’s brilliant. Here’s how it works. Martha’s plumber needs to install a 2-inch drain pipe to the kitchen island. In the sink or behind it in an empty room, the plumber has to create a loop. This loop is also done with a 2 inch pipe. You can think of this loop by thinking of a capital letter P lying on its side, with the curved section of the P facing towards the sky and the long leg of the letter representing the drain pipe under the sink.

The actual drain pipe that goes into the sink cabinet cannot be larger than a 1.5-inch pipe. This smaller pipe limits the amount of water that can enter the circuit ventilation system. The water from the drainpipe enters the vertical pipe of the loop, represented on the page by the top of the letter P. The other part of the curved part of the letter P represents the actual vent part of the system.

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

This is what happens when you fill the sink to the brim and pull out the sink plug. The water immediately pours out of the sink, completely filling the 1.5-inch pipe in your closet. But immediately it falls into the 2-inch vertical pipe, which, due to its larger diameter, cannot completely fill it. As the water tumbles to the bottom of the horizontal pipe and begins its way to the sewer, the water begins to smooth out and only partially fill the 2-inch horizontal pipe. The much-needed air is on top of the water.

As the running water passes through the other part of the circuit that leads into the closet, the air needed to satisfy the vacuum created by the draining rinse water flows upstream over the water flowing in the opposite direction to the sewer. The air enters the circuit and quickly drains the water out of the sink. There are no moving parts and you will never have duct gas if the circuit vent is properly installed. It is magic!

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