What is a Trap Seal?

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  • Written By: M. McGee
  • Edited By: Lauren Fritsky
  • Last Modified Date: 14 January 2020
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A trap seal is a pluming term where water is used to both seal a pipe and aid in the movement of water. Traps are found in nearly every building with internal plumbing. The most common style of trap in a house is the U-trap found under almost every sink, while the second most common is the P-trap. These traps hold water inside a bent pipe, creating a seal between the fixture’s drain and the house’s pipe system. This pipe seal keeps gasses from moving into the house from the internal pipes and creates a siphon effect when water flows down the drain.

A trap consists of a handful of areas. The inlet and outlet are the openings where water flows into and out of the system. The cup is the portion where the sitting water creates a trap seal. The crown sits at the junction of the outlet and cup; this is the area where water is almost flowing from the trap, but it is prevented by gravity.

The construction of a trap is what creates the trap seal. In a common U-trap, the drainage pipe from the fixture goes straight down into the U-shaped trap. The outlet connects to the other side of the U, allowing water to flow out of the fixture. When water isn’t flowing through the U-shaped cup, water is trapped there by gravity.


This held water creates the actual trap seal. The water is a physical barrier between the house and the pipes. This barrier is made of water so it doesn’t prevent the usage of the pipe. In addition, the water is refreshed every time water flows through the trap, so it doesn’t smell stagnant.

The trap seal has two basic purposes. Its main use is preventing bad smells from moving out of the pipes and into the house. Most household pipes have a very strong and unpleasant odor. The physical barrier created by water present in the trap prevents the inner air from moving into the house.

The trap’s second function is as a water-siphoning system. When water enters the trap from the inlet, it pushes water upwards in the crown. This is enough to make the water start to flow, which pulls liquid through the trap. This serves two purposes: it makes the water drain faster and smoother than a simple straight pipe and moves material trapped in the water along at a faster pace, which makes it less likely to get stuck in the pipes.


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