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.
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 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.
Reuben Saltzman, Structure Tech Home Inspections