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Vent-Free Drains
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Sucking Air
Vent-free drains offer many advantages
"Why spend $100 on a job that you can do for $30 or $40?"
That's the most common argument for using air-admittance vents, or AAVs, in a plumbing system. And while some plumbers and code officials remain wary of this technology, AAVs can save money and make your job easier. That makes them worth a closer look.
An air-admittance vent is a pressure-activated venting port that consists of a rubber diaphragm in a PVC housing. It serves the same purpose as a standard PVC vent stack: It lets air into the plumbing system to break the vacuum caused by water going down the drain. (To understand why this works, consider that you can pour liquid faster from a can with two holes than with one.) But while a standard vent stack stays open to the air all the time, the diaphragm in an AAV opens only when needed. Flush a toilet or open a drain, and water running down the drain creates a vacuum in the vent pipe. The vacuum opens the diaphragm and sucks air into the waste pipe, helping the water drain properly. When the pressure equalizes, the diaphragm falls back into place, sealing the pipe so that sewer gasses can't escape.
You can put AAVs just about anywhere - under sinks, in closets or in the attic. Because each one eliminates a run of PVC vent pipe, using them in a typical home can slash the amount of PVC pipe needed by as much as 75 percent. This may not translate into a lot of money. One plumber saved about $250 by using them on a small, 1-½-bath home in upstate New York. But cost isn't the only benefit. In a standard plumbing system, each drain has to be within a certain distance of a vertical vent stack, depending on the diameter of the vent pipe that serves the drain. Because you don't have to worry about this with AAVs, you have more choice when placing fixtures in new construction, and it's simpler to vent bath and kitchen additions without disturbing the existing space. With fewer vent stacks sticking up through the roof, the house looks better, and there's less chance of roof leaks.
Any obstacles?
Beware that you may have problems getting your plumber to use these devices, or your plumbing inspector to accept them. Some plumbers and inspectors still worry that an AAV will leak sewer gas into the home. The evidence says otherwise: The devices have been accepted by the International Residential Code, and the American Society of Plumbing Engineers was confident enough in their reliability that it issued a statement in favor of them in 1999. Still, some people remain uneasy.
The National Association of Homebuilders' Research Center hopes to change that and has put its name behind an effort to get AAVs accepted by tradespeople and inspectors. Its staffers have sponsored educational sessions and have installed the devices in several model homes. However, they've hit a few bumps along the way. The Research Center's Bob Fuller worked with Carl Franklin Homes in Dallas to put AAVs in a model home last year, "But after the demo, the code authorities forced us to take them out," he said.
But there has also been progress. In one Research Center-sponsored project, the plumber completed the rough-in for a two-bath home in about seven hours, using AAVs. In his words, "That was the easiest two-bath rough I've ever done."
How an AAV works
Draining water creates a vacuum that opens a flapper to equalize pressure in the drain system. After the water drains, gravity closes the flapper, which keeps sewer gasses from leaking into the house.
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