Have you ever wondered why sinks have an overflow opening, since it is obvious that it can't really stop the sink from overflowing? The answer is that this plumbing feature serves a different purpose than its name implies.
The overflow can help divert a small amount of water but its main purpose is to allow air into the drain when the sink is filled with water. The air helps water in the sink to drain faster. A sink without an overflow opening will generally drain slower and may create bubbles as it draws air in through the drain.
To ensure that the overflow can serve its purpose well, it needs to stay clean and clear. The occasional water along with ambient dust can build up restricting the flow of both air and water. These are some options for clearing it out.
- A long zip tie - plastic zip ties work great because they bend around the curve of the sink but they are rigid enough to dislodge any build up. Note: Make sure you don't drop the zip tie into the overflow. A longer zip tie is easier to hold on to.
- A Rubber hose to blow air through - fill the sink up with water slightly higher than the overflow opening. Let some of the water run down the overflow. Put one end of the rubber hose against the overflow opening and blow several puffs of air through the other end. Drain the sink and blow air through the hose again. The air should blow through clearly without resistance. Repeat as needed. Note: I use a 7/8" dishwasher hose but any sturdy hose will work as long as it fits over the overflow opening.
Clearing your sink overflow should help the drainage considerably. Keep this in mind as another step in troubleshooting a slow drain.