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What is a "combination waste and vent" in a plumbing system?
Thursday, June 21, 2018
Plumbing drain traps are required to be vented, so that air can be introduced into the drain piping to allow free flow of the liquid. But there is one exception, a combination waste and vent, which is essentially an oversize drain pipe that has enough interior cross-section to allow the drainage of any liquid with room left over for air to flow over it to reach the trap.
A “CW&V” is easily recognizable by the oversize vertical drain pipe, usually 3-inches or more, as in the photo above. Both the Residential Edition of the Florida Building Code (FBC) and the international Residential Code (IRC) have limitations on when a combination waste/vent can be used:
The “food waste grinder” in the code citation is commonly called a kitchen garbage disposal, and the disposal at the left side of the photo above is an example of a common code violation we see.
A defect we see less often is an S-trap connected to a combo waste and vent, like in the photo below. “That configuration can siphon the trap when draining a sink full of water,” according to James Freeman, of JW Freeman Plumbing. “It needs the 3-inch pipe brought up to higher than the trap arm or to have an auto vent installed.”
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