Plumbing Vents, Why Houses Need Them (forget the soda bottle analogy)

When it comes to first time home buyers, one of the least understood components of a home seems to be plumbing vents.  You know, those pipes sticking up out of the roof that run through the attic and through the rest of the house.  All plumbing fixtures, with the possible exception of floor drains, require a plumbing vent.  Vents are frequently connected together inside the attic, which allows for less penetrations in the roof.

Plumbing vent

Plumbing vents prevent traps from being siphoned.

Let me repeat that – plumbing vents prevent traps from being siphoned.  They also prevent back-pressure on traps, but the big one is siphoning. You may have heard that plumbing fixtures will drain faster when they're vented properly, and I know I've mistakenly said this myself, but it's not true.  The common, improper analogy is to talk about dumping a soda bottle upside down.  You watch the water glug out while air replaces it, and this makes it drain super slow.  Once you put a hole in the other side, the water drains out very quickly.  

This analogy doesn't hold water because the top side of every plumbing fixture is wide open.  The top of a toilet is open.  The top of a sink is open.  The top of a bath tub is open.  If you wanted to re-create the soda bottle analogy, you would need to block off the top of the plumbing fixture and then try to drain the water out.  I can't think of any instance where this could possibly happen.

Every plumbing fixture has a trap, which prevents sewer gas from coming in to the home.  When a lot of water drains through a plumbing fixture, it can be enough water to create a siphon effect, which has the potential to pull water right out of the plumbing trap.  In my blog about S-traps, I included a quick video clip of an unvented drain having water siphoned out of it, leaving the trap with far less water than it should have had.  Here's that same clip again.

 

While writing that post about S-traps, I even set up a home experiment where I was able to get almost all of the water in a trap siphoned out.  This is the same way it works in a house.  When water is siphoned, it typically makes an annoying 'sucking' sound.  To demonstrate this, I cut apart the vent on my own kitchen sink and blocked it off, just to show what a difference a vent will make.

For the record, the water actually drained out of my sink about 8 seconds faster with the vent blocked, because the water was being pulled (or siphoned) through the drain.  When water is siphoned through the drain, the water in the trap gets siphoned.  This can lead to sewer gas coming in to the home.

In short, plumbing vents prevent siphoning, and siphoning can lead to sewer gas in the home.

Reuben Saltzman, Structure Tech Home Inspections

        

 

Written By

Reuben is a second-generation home inspector with a passion for his work. He grew up remodeling homes and learning about carpentry since he was old enough to hold a hammer. Reuben grew up thinking he was going to be a school teacher because he enjoyed teaching others so much. In a sense, that’s a lot of what home inspections are about, so Reuben truly does what he loves. Sharlene has worked with Structure Tech since 2000 and Reuben has been contributing to her blog since 2008.

Related Posts

Plumbing Vents Covered with Frost

Along with all of the cold weather we've been having comes a bunch of non-functional plumbing vents all over Minnesota. The problem is that plumbing vents can get completely blocked...

Why “S” Traps Aren’t Allowed

A common plumbing defect found in many older houses is an "S" trap. An "S" trap is named so because it looks like an "S" on its side - it...

Subscribe to Our Newsletter for Market Updates & Mid-Century Modern Listings

Our weekly HomesMSP Update includes current local market information and a curated list of mid-century modern properties for sale, plus posts from an inspector, a lender, a stager, info about neighborhoods, life in the Twin Cities… even recipes!

Hidden

Blog Categories

Archives

Sharon and John Hensrud

About Us

The HomesMSP Team is committed to meeting you where you are and listening… really listening to understand you so we can use our extensive knowledge of the market and local neighborhoods to give you personalized service.