Trap (plumbing)

A trap in plumbing is a U-shaped or S-shaped pipe fitting designed to prevent sewer gases from entering a building. It does this by holding a quantity of water within the bend. This water acts as a seal, blocking the passage of gases while still allowing wastewater to flow through. The effectiveness of a trap depends on maintaining a sufficient water seal; evaporation or siphoning can render it ineffective, allowing sewer gases to escape.

Different types of traps exist, each suited to specific plumbing fixtures:

• P-trap: The most common type, resembling the letter "P". It's typically used under sinks and lavatories.
• S-trap: Similar to a P-trap, but with a more pronounced curve. While less efficient than P-traps at preventing siphoning, they're sometimes used due to space constraints.
• Bottle trap: A more complex trap with a vertical pipe connected to a horizontal section. Often used for bathtubs and showers.
• Drum trap: A larger, more robust trap typically used for floor drains and larger fixtures.

Proper ventilation of the drainage system is crucial to the effectiveness of traps. This ventilation prevents negative pressure from siphoning the water out of the trap. A vent pipe allows air to enter the drainage system, preventing a vacuum from forming and drawing the water seal from the trap.

Insufficient venting or improperly installed traps can lead to sewer gases entering the building, posing a health hazard. Regular maintenance, such as checking for and replenishing the water seal if necessary, is important to ensure the trap's continued effectiveness. Clogged traps can also lead to backups and plumbing issues.