A sink overflow is a hidden channel running along the underside of the sink basin. It's typically unseen from both below and above, but you may have seen it as the two small holes close to the rim of your sink. The overflow is always open, even when the sink is stoppered.
How a Sink Overflow Works
There are two main purposes of a sink overflow.
1. Prevents Water from Overflowing
It's a common misconception that the sink overflow prevents overflowing water in the event that your sink is clogged. The reality is that because the overflow outlet on most sinks is very close to the drain opening in the basin, it's unlikely that a clog can occur between the drain opening and the overflow outlet that won't affect both lines.
A bathroom sink overflow really protects against overflow when the stopper is closed, and the faucet is accidentally left running, not in the event of clogs.
2. Improves Drainage
The sink overflow helps the basin drain faster by allowing air into the drainpipe via the overflow channel. When the basin fills with enough water to cover the drain overflow openings, the drain can't get air through the water, creating a suction effect that slows the flow of water going down the drain. But with the overflow feeding air to the drainpipe, the suction effect is reduced, and the water drains much faster. Some modern designs of sinks that don't have overflow openings will send up bubbles as they drain (and typically drain more slowly).
Got drain problems? Contact EJ Plumbing at (650) 513-8852 today to schedule an appointment!