Believe it or not, one of the most important parts of your drainage system is up on your roof. Those things that look a bit like anorexic chimneys are vents — the V in your drain-waste-vent (DWV) system.
Vents help protect you from gases in the sewer system. To begin with, they allow sewage gases to leave the house far from sensitive noses. But a vent alone wouldn’t stop smells from sneaking through the drains, and that’s what drain traps are for. Every drain has a U-shaped pipe beneath it. This trap — which can also catch lost wedding rings — stays filled with water at all times, blocking the gases. And this is where the vents perform their second duty.
As water travels down your drainpipes it creates a bit of a vacuum, or low pressure area behind it. Without vents, this vacuum would suck a lot of the water out of the trap, and you’d smell the results.
To relieve this pressure, a vent pipe extends vertically from the drainpipe within a few feet of each trap. The vents connect to one of the roof stacks and allow air in to equalize the pressure.
So now you know why they’re important, and why if you’re installing a plumbing fixture away from existing vents, you need to put in a new one. Easier said than done, since you likely need to break into an existing wall and ceiling to do it. There’s hope. A one-way air admittance valve lets you put a short vertical pipe off the drain to do the job of a vent. The valve opens when water is draining and stays closed otherwise.
Position the vent higher than the top of the sink to prevent the chance of water backing up through the vent.