From the Ground Up: Plumbing
Codes dictate the proximity of vents to drains, and the rules should never be ignored. If the vent is too far away, it won't be able to break the vacuum and allow fast drainage. "When you pull the plug on a bathtub," says Richard, "you've got 40 gallons of water that want to scream down that pipe. If there's a vacuum, it can pull the water out of all the traps in the house."