Why Is the Pipe under the Sink Curved?
The U-shaped pipe under a sink is called a water trap. Every sink, toilet, and bathtub in your house has one. When you pull the plug in the sink or bathtub, or flush the toilet, the waste water drains down through the water trap and into sewer pipes that carry the waste water away.
Some of the water remains in the bottom curve of the water trap. The trapped water forms a water seal that keeps the smelly sewer gases from coming up into the house. A ventilator pipe to the roof dispels the sewer gases harmlessly into the outside air.
If a plumbing fixture is not used over a month or so, the water in the trap will evaporate, allowing noxious odors to escape. When a trap is installed on a fixture that is not routinely used, the eventual evaporation of the water in the trap must be considered.
In these cases, a trap primer may be installed; these are devices that automatically recharge traps with water to maintain their water seals.