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Reuben Saltzman

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 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 it’s not necessarily 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 top, the water drains out very quickly. Just like a vent in a gas can.

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 into 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 into the home.

In short, plumbing vents are there to help prevent sewer gas from coming into the home.

Reuben Saltzman, Structure Tech Home Inspections



8 responses to “Plumbing Vents, Why Houses Need Them (forget the soda bottle analogy)”

  1. Adam Tope
    January 29, 2013, 5:15 pm

    I don’t know in your area of the country, but here tons of laundry drains were installed in the basement without a vent at all connected to them from day one. The soda bottle analogy I haven’t heard, but the way I describe to customers is to compare old canned drinks like V8 Juice. If you only put one hole in the can, instead of a vent at the top, it will glug out, much like a sluggish drain in plumbing with no vent. Laundry drains with no vent will talk to you from the floor drain nearby or sometimes even at the stand pipe after the water is stopped pumping out. Water wants to have scouring action as it goes down the pipe, with no vent you cause all sorts of problems in the way the water flows down the drain. I think this is a good analogy to use vs. the soda bottle thing. I would add at the end that obviously vents are there for proper draining of the pipes as well, unless of course you enjoy sluggish drains and cleaning them out all the time :D. Good read, maybe will people will realize some of this trade isn’t all *bleep* runs downhill, payday’s friday 😀

  2. gary havens
    February 1, 2013, 9:57 pm

    I thought that vent pipes were to be only 6″ to 12″ above the roof line. I don’t get why that should be true, but your photo looks like that one is high.

  3. Reuben Saltzman
    February 2, 2013, 7:17 am

    Adam – thanks for reading. I encourage you to do a few tests on the your own to see how much of a difference a vent makes on the amount of time it takes a fixture to drain. I think you might be surprised.

    Gary – the MN State Plumbing Code requires vents to terminate at least 12″ above the roof line – . Higher is acceptable, but this will also increase the potential for frost to close off the end of the vent. I’m planning to write to very short blog post on this topic in the near future.

  4. karey
    February 5, 2013, 8:10 am

    Is it a good idea to put spray foam around pipes that go out the foundation of the house? I have a stone foundation and where pipes leave the house the foundation has big gaps. Not directly around the pipes but in the area

  5. Reuben Saltzman
    February 5, 2013, 2:41 pm

    Karey – if air is leaking around the pipes, yes.

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