Plumbing vents in houses explained

These vents through the roof are designed to prevent the S-trap under your sinks from being siphoned dry.

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...

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...

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.

Text and image from my favorite home-repair/DIY blog in the StarTribune, which also has two videos illustrating the physics of plumbing vents.  Only homeowners will find this of interest.