Symptoms
- Sink drains slowly despite the overflow hole being open and clear
- Water rises close to the overflow hole before slowly going down
- Overflow hole itself has no visible blockage
- Slow drainage has been present for a while
Likely causes
- CommonClog in the main drain unrelated to the overflowThe overflow is a safety vent, not a secondary drain path, so it doesn't help clear the primary clog.
- PossibleOverflow channel itself clogged with grime, contributing to smell but not to drainage speedCan add odor even though it doesn't affect drain speed directly.
- CommonBelieving the overflow should assist drainage (misunderstanding, not a malfunction)It's a normal design; a clear overflow doesn't mean the drain itself is clear.
What to do
- Understand that the overflow hole exists to prevent the sink from flooding over the counter if the drain is blocked while the faucet runs, not to speed up normal drainage.
- Focus troubleshooting on the main drain and stopper rather than the overflow hole itself.
- Remove the stopper and clean out hair and soap scum from the main drain opening.
- Use a plastic zip-style drain tool or plunge the sink as you would for any slow bathroom sink clog.
- Separately, clean the overflow channel with a bottle brush and a baking soda and vinegar flush if it smells musty, even though this won't speed up drainage.
- If the main drain is cleared and flow is restored, no further action on the overflow is typically needed beyond occasional cleaning for odor.
Tools and parts
- Plastic zip-style drain tool
- Plunger
- Bottle brush
- Baking soda
- Vinegar
When to call a pro
Call a plumber if the main drain remains slow after standard cleaning, since the overflow hole itself is not the cause of slow drainage.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.