Q: I saw a "rubber diaphram" device marketed as a cap for vent pipes that terminated in the attic. Its advantage was that it did not pierce the roof (highly desirable for expensive roofs like slate and tile), yet it would work to equalize pressures in the vent system. There was no mention of noxious gases or additional ventilation requirements. Do these devices work, and are they accepted by the most common plumbing codes?

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.

The Studor vent can be installed out of sight, beneath a counter or in a wall or attic. However, most inspectors I know will red-flag an in-the-wall installation. Some will not allow the device at all, so always check with your local inspector before installing one.

I commonly use Studor valves when it is either very expensive or downright impossible to run a daylight vent pipe. I’ll also use them for supplementary air in a kitchen or in an existing house that needs more vent air. Although Studor advertises that the system can completely replace the through-the-roof venting system, I prefer to have at least one large-diameter air vent to the outside to provide a way out for positive pressure that might build up in the lines.

For example, I was recently in a home where every time the clothes washer drained, it would blow the trap water and even the strainer right out of the kitchen sink. The washer was upstream from the sink, and the slug of washer water was shoving air ahead of it because the drain line was too small and didn’t allow for any vent air. The air admittance valve installed in this home in lieu of an open air vent could not prevent this problem, because it only reacts to negative pressure, not positive. I’m sure Studor would comment that such things wouldn’t happen if the drain systems were designed properly to begin with. And they would be right — the long 1 1/2-inch unvented washer drain line was against all codes.

Still, even with properly sized drain lines and air admittance valves, I would always include an outdoor vent pipe somewhere in the system as a backup measure. For more information on Studor vents, see www.studor.com.