Ask the Builder: Don’t underestimate the challenges of plumbing vents

May 27, 2011

I have a remodeling project coming up and I intend to do the plumbing work. I’m good to go on the water lines, but the drain lines and plumbing vents are my weak spot. Is there a fail-safe method of venting so that everything will work fine? —Mandy P., Portland, Maine.

Even apprentice plumbers who are somewhat familiar with the trade often make serious mistakes when it comes to plumbing vent systems.

First, check to see if you’re allowed to install the piping. Some states and towns only allow licensed plumbers to do this work. The reason is public health. If you make mistakes when you plumb, you can get some people seriously ill or even cause death.

Let’s talk about what happens in drain piping when water travels down through the system. In a properly designed plumbing drain and vent system, there is air in the pipes before water is poured down a drain or a toilet is flushed.

As soon as you introduce water — and lots of it quickly — into a drain, the dynamics of the air changes. The water surging into the system displaces the air, often pushing it down the drain in front of the rushing water. This air needs to be replaced so a vacuum doesn’t form in the system.

Vacuums in plumbing drain lines are bad, very bad. You’ve possibly heard a vacuum getting satisfied if you’ve been in a bathroom when a tub or sink drain gurgles when you flush the toilet. At a friend’s house this would happen every time his washing machine would drain. When the washing machine pump came on, his kitchen sink would gurgle and the water in the trap under the sink would be sucked dry. This allowed sewer gas to enter his house — as well as the vermin that crawl around in the sewer lines. Yuck!!

To prevent traps in downstream fixtures from being sucked dry like my friend’s kitchen sink, you install a vent pipe, usually within three feet, close to the fixture trap. This vent pipe rises vertically toward the roof, where it opens to the atmosphere to get the needed replacement air.

Usually a pipe that’s 1 1 / 2 inches in diameter is sufficient to vent any residential fixture. But some plumbing codes have very specific sizing requirements. If you start to collect vent pipes from other fixtures as you head to the roof vent, the pipes will have to get bigger, just as plumbing drain lines and building drain pipes get bigger to accommodate the greater amount of water that enters them.

To be safe, extend a vent pipe from every fixture. Certain fixtures can be wet-vented, which means two fixtures share a common vent. But since I can’t be at your house to mentor you on this complex technique, just install separate vents for each fixture. Be sure that any vent line that has to run horizontal actually has a tilt to it so any condensate water that forms in the pipe drains down to the sewer or septic tank.

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