What is plumbing venting? When was venting first required for plumbing drain systems?
Tuesday, June 4, 2019
Just about every time we call out an unvented “S-trap” in a home inspection, like the one shown above, someone says “Is that some new code thing? It probably wasn’t required when this house was built.” But actually it's been around for quite awhile.
The original 1948 edition of the Uniform Plumbing Code (UPC) required venting for all drains, and all the codes we know of that address plumbing standards require venting. Properly designed venting is necessary because unvented systems can have all of these problems:
- The fixture may drain slowly.
- It may make a gurgling sound while draining because it is trying to gulp air and drain liquid out of the same drain opening.
- An unvented S-trap also sometimes sucks all the water out of the trap as it drains. The trap water seals the drain against sewer gas rising up, so a stinky sewer smell wafts through the house. The methane gas can also be dangerous. No fart jokes here, this is serious.
And, of course, another reason is that the building codes—all of them that include plumbing standards—require venting. The traditional way to vent plumbing fixtures is with pipes through the roof. A home is required to have at least one, and it is often encased in a lead sleeve “roofing boot.” But it is also possible to install an air admittance valve near the plumbing fixture, which opens as necessary to provide air in the pipes for drainage, and then closes afterwards.
To learn more, see our blog posts What is an auto vent, air admittance valve, or check vent? and What is a P-trap? and What is an "S-Trap" under my sink? Why is it a problem? and What is a "combination waste and vent" in a plumbing system?
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