The sink 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 is like a vacuum when it is full so it will generally drain slower. Also, it is common to see bubbles floating up from the sink drain as it draws air in when a sink overflow is not there.
If a sink does not have an overflow there is nothing that can be done to improve draining. However, when a sink overflow is a part of the sink it is a good idea to make sure it is working properly by checking it from time to time.
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 inside the overflow opening. This build up can eventually restrict the flow of both air and water. Since access to the sink overflow is restricted it can be difficult to keep it clean. There are, however, a couple of tricks that can help you clean the overflow and keep the sink draining properly.
These are some options for cleaning the overflow.
- A long zip tie - plastic zip ties work very well because they bend around the curve of the sink but they are rigid enough to dislodge any build up. Insert the pointy end of the zip tie into the overflow opening and slowly work it down as far as possible. Move the zip tie up and down and from side to side to knock the build up out of the way. Pull the zip tie back out and run the sink to see if it is draining better.
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.