Any time you see bubbles coming from a toilet that isn't in use, it means air is trapped in the waste pipe. If the bubbling happens when you use a sink, the sink vent may be blocked or improperly installed. A sluggish toilet is another sign of venting problems.
Plumbing code requires each plumbing fixture to have its own vent, which is an unobstructed air passage that usually ties in with the main vent that exits through the roof. The code requires a minimum diameter for each vent pipe as well as minimum installation distance from the fixture trap. A vent that doesn't fulfill these requirements fails to allow sufficient air movement for drainage, and negative pressure can push or pull air through the P-traps of fixtures near the improperly vented one.
Stopping the Bubbling
When draining a sink affects the toilet, it means their waste pipes probably are connected, and they likely share a common vent. The problem may be as easy to fix as removing debris from the vent opening on the roof, or it may involve re-plumbing the sink vent to bring it up to code. You may be able to install an air admittance valve near the sink P-trap to compensate for inadequate venting, if local plumbing regulations allow it.
- Stockbyte/Stockbyte/Getty Images