A. Master plumber Rex Cauldwell responds: The device you refer to is called an air admittance valve, and was invented by Sture Ericson in Sweden more than 25 years ago. Since then, millions have been sold worldwide by the manufacturer, Studor. According to Studor, the air admittance valve can effectively replace a through-the-roof vent pipe.

Here’s how it works. The valve keeps the system closed until it senses negative pressure within the drain system, such as behind a pipe filled with running water. The pressure pulls the rubber diaphragm down as long as the system needs air, then a spring reseats the diaphragm.

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