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Running Plumbing Vent Lines

Master plumber Rex Cauldwell presents a number of solutions to common problems experienced when running vent lines.

By Rex Cauldwell
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The purpose of a vent line is to prevent the water in the drain traps from being pulled out by a vacuum caused when rapidly running waste water flows down a pipe.

Vent line theory and technology have evolved substantially over the years. Early system design called for one vent for every fixture. Then we found out we could have a common vent system for several fixtures and vent them with a single pipe. Another innovation is the air admittance valve (AAV), which is a one-way diaphragm valve that gets installed just above a drain line and can replace individual vent lines running through the roof.

AAVs are accepted by all the accredited agencies but are still prohibited in some localities. If codes prohibit their use in your area, you’ll have to run an individual vent from a fixture or install drainpipe from the fixtures to a line sized to prohibit the formation of a vacuum. You can also create a very expensive loop of pipe as some do for an island drain in a kitchen. The illustrations on these pages demonstrate a number of solutions to common problems experienced when running vent lines.

Typical Old-Style Venting

Typical-Old-Style-Venting
Typical old-style venting—circa 1940 and earlier—employed the installation of fixture vent pipes in the interior wall cavity.

 

Working Around Obstructions

oversized-drainpipe
If you cannot run an individual vent because of a window, use an oversized drainpipe. Because the pipe can’t fill with water, it won’t siphon the trap.

admittance valve (AAV)
Or install an AAV in the fixture drain line before the line enters the wall. Such an installation avoids the necessity of running a vertical vent line.

 

Installing a Fixture Vent

Installing a Fixture Vent
Before running a vent line horizontally, the vertical vent section must travel at least 6 in. above the flood rim of the fixture it is venting. This keeps sludge from settling in the vent line in case of a backup.

 

Venting with an Inverted T-Fitting

Venting with an Inverted T-Fitting
Install a sanitary T-fitting upside down to allow the vent gas to flow upward and out the vertical vent more easily.

 

Venting in Joist Spans

Venting in Joist Spans

 

Avoiding Vent Clogs

Avoiding Vent Clogs
Never run a take off for vent air beyond 45° from vertical. Below 45° from vertical, a vent line can plug up with water and debris.

 

install an AAV in the drain line
Instead of running expensive re-vent loops, install an AAV in the drain line at the fixture.

 

Large AAVs can vent an entire branch
Large AAVs can vent an entire branch (a pipe off the main line going to several fixtures) at one time.

Taunton's Plumbing Complete bookExcerpted from Taunton’s Plumbing Complete by Rex Cauldwell.

Available at Amazon.com.

Also available for download.

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