How to Drain and Vent a Bathroom Sink | Ask This Old House

23July 2018

Ask This Old House plumbing and heating expert Richard Trethewey helps add ventilation to a bathroom sink that is unvented.
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Time: 2 hours

Cost: $100

Skill Level: Moderate

1. An unvented bathroom sink may have a full S-trap, which may siphon and expose the home to sewer gas.
2. Before doing any plumbing work, turn off the water at the main water supply or at local service valves.
3. Remove the old drain piping.
4. If replacing the faucet, remove that as well.
5. Use plumber’s putty to make a watertight seal on the faucet base plate.
6. Install the faucet per the manufacturer’s instructions.
7. Add flexible supply tubing between the faucet and the shut off valves.
8. Dryfit the PVC drain piping. A P-trap should be used. If venting through the roof is not an option, an air emittance valve can be added at the highest possible place in the cabinet below the sink.
9. Use plumber’s putty on the drain connections to the sink.
10. Prime and glue all PVC pieces for the remainder of the drain.

Richard installed the Vega Single Control Centerset Bathroom Faucet in Brushed Nickel, which is manufactured by Pfister Faucets (

All other parts for this project, including PVC drain piping and an air admittance valve can be purchased from a home center or plumbing supply store.

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Homeowners have a virtual truckload of questions for us on smaller projects, and we're ready to answer. Ask This Old House solves the steady stream of home improvement problems faced by our viewers—and we make house calls! Ask This Old House features some familiar faces from This Old House, including Kevin O'Connor, general contractor Tom Silva, plumbing and heating expert Richard Trethewey, and landscape contractor Roger Cook.

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How to Drain and Vent a Bathroom Sink | Ask This Old House

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