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.