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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Here’s links to a collection of our other blog posts about PLUMBING:

• How can I protect my pipes to keep them from bursting during a hard winter freeze in North Florida?

• Why is there sand in the bottom of my toilet tank?  

• What causes a gurgling sound when a bathtub or sink drains? 

• Are drop-in toilet bowl cleaner tablets safe? 

• What can I add to my septic tank to help it work better?

• What are the code requirements for layout of drain piping under sinks? 

• Is a hot water faucet handle required to be on the left? 

• Can you live in a house while the plumbing is being replaced? 

• Why is the European-style bottle trap not approved by the plumbing codes in the U.S.? 

• Why can't PVC be used for water pipe inside a house? 

• What are the common problems to look for when the plumbing has been replaced in a house? 

• What's that powdery crust on the pipe connections at the water heater? 

• How can I tell what type of plumbing pipe I have? 

  • What causes low water pressure in a house?

• Should I call a plumber or septic tank contractor when my septic tank backs up into the house? 

• How do I get rid of the sewer gas smell in my house?  

• What are the pipes on my roof? 

• Should I wrap the water heater with an insulation blanket? 

• My water bill went way up last month. How do I look for a leak?

• Why does the water have a rotten-egg smell in some empty houses? 

• What is an "S-Trap" under my sink? Why is it a problem? 

• Where is the septic tank? Are you going to inspect it? 

• What does polybutylene pipe look like? Why is it a problem?

• Which water pipes are an insurance problem and possibly uninsurable? 

• Does a home inspector check the plumbing under the floor slab?

• Is it alright to disconnect the washing machine drain from the septic tank and divert it to the ground in the yard?

      Visit our PLUMBING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.