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

Home Inspector with Structure Tech Home Inspections

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 today the focus is on siphoning. You may have heard that plumbing fixtures will drain faster when they're vented properly, and I know I've said this myself, but that's not the purpose of vents.  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.  To really appreciate the difference, turn up the volume while watching this.

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 are there to help prevent sewer gas from coming in to the home.


Comments (29)

Michael Setunsky
Woodbridge, VA
Your Commercial Real Estate Link to Northern VA

Reuben, learned a lot about plumbing vents today. The videos bring it all together. Thanks.

Jan 29, 2013 09:22 PM
Brian Schulman
Coldwell Banker Residential Brokerage, Lancaster PA - Lancaster, PA
Lancaster County PA RealEstate Expert 717-951-5552

Reuben, thanks for the videos - they really help to make the point!

Jan 29, 2013 09:27 PM
Andrew Mooers | 207.532.6573
Northern Maine Real Estate-Aroostook County Broker

Had one ranch home with a slow moving plumbing and it was butternuts, a vent stack plum full of them from busy squirrels getting ready for winter.

Jan 29, 2013 09:45 PM
Jimmy Faulkner
Florida. Homes Realty & Mortgage - Wantagh, NY
The Best Of St. Augustine

Thank you! You made me aware of the vent situation.  Jim

Jan 29, 2013 10:07 PM
Kathryn Maguire
GreatNorfolkHomes.com (757) 560-0881 - Chesapeake, VA
Serving Chesapeake, Norfolk, VA Beach

Active Rain is such a cool place!  You never know what you are going to learn and this blog was very interesting. Who knew sewer gas leak prevention could be so fascinating?

Jan 29, 2013 10:31 PM
Gary Burleson
Beach Water Realty - www.beachwaterrealty.com - Myrtle Beach, SC
Myrtle Beach Homes, Condos, Foreclosures, Investment Propery

I knew that vents were needed, but never understood the reason. Great post and videos.

Jan 29, 2013 10:32 PM
Anthony Daniels
Coldwell Banker - San Francisco, CA
SF Bay Area REO Specialist

I wonder if that's the sucking sound, that Ross Perot referred to many years ago.

Always good to know about plumbing issues.

Jan 29, 2013 11:29 PM
Brian Rayl
Home Value Leads - Highland Park, TX
Active Agent and Co-founder Of Home Value Leads

I had a couple of buyers purchase a home that had been vacant for 2 years and never properly winterized... When it came time for the inspection, lets just say there was a LOT of sewer gas to be dealt with! (and for the next 3 weeks afterwards!)

Jan 29, 2013 11:34 PM
Marc McMaster
RE/MAX Centre Realty - State College, PA
Putting my clients before myself

Plumbing is something I know very little about so these videos were great.  Thanks for posting them!

Jan 29, 2013 11:42 PM
Richie Alan Naggar
people first...then business Ran Right Realty - Riverside, CA
agent & author

This should be taught in college course or textbooks

Most excellent post and relevant subject matter for the professional and layman alike. Here is something that needed to come out of urban legend and identified clearly as to what it does or does not do. No opinions just facts...thank you

Jan 30, 2013 12:07 AM
Robert L. Brown
www.mrbrownsellsgr.com - Grand Rapids, MI
Grand Rapids Real Estate Bellabay Realty, West Mic

I didn't know that. The things you find out on the way to learning other things.

Jan 30, 2013 12:32 AM
Chip Free
Lake Realty of Charlotte-Lake Norman NC - Huntersville, NC
Chip Free

Hey Reubon,

Great info on vents.  Question:  Since toilets have an s trap built in them, they not require a p-trap below the flange (correct).  This is why you get the sewer gas smell when replacing or installing a toilet because there is no p trap to block gas.  I have come across several toilets that do have p traps below the flange.  Will this make them more likely to clog and should they be removed? 


Another venting question:

I have a friend who bought a sink/facuet from IKEA in Charlotte.  The drain in the sink broke after a few months.  The overflow is intergrated into the sink so they replaced it with a drain without an overflow.  However, now the sink drains very slow.  I guess the old drain had some venting in the design.  Do you have any suggestions on how to fix this issue?  

Jan 30, 2013 01:35 AM
Ronald DiLalla
Century 21 Discovery DRE 01813824 - Anaheim, CA
No. Orange Cty Real Estate

Hi Reuben,,,thanks..great post..I finally understand the purpose now...tks fro sharing with us.

Jan 30, 2013 02:53 AM
Steven Cook
No Longer Processing Mortgages. - Tacoma, WA

Reuben -- when even the people who do this regularly get it mixed up, it is great to see someone in the field who is willing to get to the bottom and illustrate the correct dynamics of the situation. 

Jan 30, 2013 07:25 AM
Scott Nowling
Prudential Starck - Saint Charles, IL
Prudential Starck

Thanks Reuben!  I had one failed attempt at plumbing a bathroom sink.  New drywall in the ceiling below it....I now leave it to the pros!

Jan 30, 2013 09:27 AM
Praful Thakkar
LAER Realty Partners - Burlington, MA
Metro Boston Homes For Sale

Rueben, it's a good idea to learn something new everyday - and frankly, had my buyer asked me this question, I did not have the answer - until now! 

Jan 30, 2013 02:31 PM
Ben Ganje + Partners
Lakes Sotheby's International Realty - Minneapolis, MN
Minneapolis Market Leader

Great comment Reuben and thanks for doing that inspection on this listing I sold.

Jan 30, 2013 09:35 PM
James (Jim) Lawson, DBA
DomainRealty.com LLC - Bonita Springs, FL
Broker Associate, RSPS, BPOR, HI & PE

Nicely done Reuben. For everyone's information, besides standard venting, we also see a lot of Air Admittance Valves (AAVs) down here in South Florida, especially in  newer homes. These are especially useful when there is no nearby vent stack to tie into or you want to avoid penetrating the roof decking by terminating the vent piping in the attic with an AAV. Also, AAVs are often found in remodels. Inquisitive buyers sometimes ask about them and some sellers don't know they have them. Best to have an answer when asked.  

Jan 31, 2013 01:37 AM
Reuben Saltzman
Structure Tech Home Inspections - Minneapolis, MN
Delivering the Unbiased Truth.

Fred, Gita, Ray Michael, & Brian - thanks for reading!

Jay - my five year old son sure didn't think they were annoying.  He made me play the video over and over :)

Andrew - ha!  Stinky nuts.

Anthony - that's gotta be it.

Brian - bad news!



Feb 03, 2013 04:20 AM
Reuben Saltzman
Structure Tech Home Inspections - Minneapolis, MN
Delivering the Unbiased Truth.

Chip - P-traps below toilets?  Yes, I would definitely recommend removing those traps.  As you say, they'll make the toilets much more prone to clogs.

As for your friends with an IKEA sink, my best guess is that the new drain wasn't installed properly.  I've never seen a sink with venting built in to it.

Ben - thanks for calling us!

Jim - I see those AAVs everyone one in a while here in MN as well, but they're not allowed by the MN State Plumbing Code.


Feb 03, 2013 04:30 AM