Find and check the valve

This is a look-and-sniff job. Wear gloves and have a flashlight.

  1. Empty the cabinet

    Clear out everything under the bar or basement sink. Look for a short plastic cylinder with a cap, usually gray or white, rising from the drain line above the trap.

  2. Check its height

    The valve is meant to sit above the trap arm. If it was installed below the trap or lying sideways, it may not seal correctly.

  3. Run water and sniff

    Run the faucet for a minute, then hold your nose close to the valve cap. A whiff of sewer gas right at the valve while water drains, or shortly after, suggests it is not closing.

  4. Check for staining

    Look for crust, dampness or a dark ring on the cabinet floor below the valve or trap. A slow drip can soak particleboard, which then smells on its own.

  5. Look elsewhere too

    Some basement bathrooms, laundry boxes and island sinks also use these valves. Check any fixture added during a remodel.

Air admittance valves in brief

Job
Lets air into the drain line as water flows, so traps don't siphon dry, and closes to keep sewer gas out
Where found
Under sinks in islands, wet bars and remodeled basements where a vent pipe was hard to route
How they fail
The internal seal sticks, warps, gets dirty, or dries out, leaving the valve partly open
Local acceptance
Building codes limit where these may be used; ask the local building department or a licensed plumber
Replacement
Usually a straightforward part swap when installed correctly, but sizing and placement matter

Why these turn up in Hilltop-area lower levels

Basement wet bars are a feature in many of the larger homes built in Hilltop, Crestmoor and around Cherry Creek over the past few decades. Remodels of older brick homes also add bars and kitchenettes to basements that were once laundry and storage only. In both cases, running a new vent pipe up through finished floors to the roof is expensive, so a mechanical vent valve is a common shortcut where allowed.

A bar sink that's used for a party twice a year is a double risk. Its trap can evaporate in dry indoor air, and its vent valve can stick in whatever position it last settled. The smell often shows up as a faint sewer note in a media room or game area that is otherwise spotless.

Another wrinkle is the ice maker or beverage fridge drain. Some are plumbed into the same line, and a dirty or poorly trapped connection can produce a sour, musty smell that is easy to confuse with sewer gas.

Valve problem or trap problem?

More likely the valve

  • Smell is strongest inside the cabinet at the top of the drain assembly
  • Odor appears while water is draining
  • Running water doesn't stop the smell
  • The valve is old, dirty or mounted low

More likely the trap

  • Smell is at the drain opening in the sink
  • Running water makes it fade within a day
  • The sink goes weeks without use
  • No valve is present at all

Common questions

Can I replace the valve myself?

Many homeowners can swap a threaded valve, but ask a plumber if the valve is glued in, mounted in the wrong place, or if you're unsure the fixture is correctly vented at all. A permit may be required for some plumbing changes.

General information for Hilltop, not a quote or professional advice. In an emergency, call 911.