Traps & Trap Seal Protection
Trap purpose, seal depth, trap siphonage, trap arm lengths, trap primers, and prohibited traps.
- Explain the purpose of a plumbing trap and the required seal depth
- Identify the causes of trap siphonage and methods to prevent it
- State maximum trap arm lengths by pipe size
- Describe trap primers, building traps, and fresh air inlets
Lección 1
Trap Purpose, Types & Seal Depth Requirements
Why Every Fixture Needs a Trap
A plumbing trap provides a water seal that prevents sewer gases from entering the building. Every plumbing fixture that connects to the DWV system must have a trap. The small amount of water that remains in the curved section of the trap creates a barrier between the living space and the sewer system.
Without traps, methane, hydrogen sulfide, and other toxic and flammable gases from the sewer would freely enter buildings through every drain opening.
Exam Definition
The purpose of a plumbing trap is to provide a water seal to prevent sewer gases from entering the building. This is the single most tested concept about traps.
Trap Seal Depth
The trap seal is the vertical distance between the crown weir and the dip of the trap. The IPC requires a trap seal depth of 2 to 4 inches for standard fixture traps.
- Minimum seal depth: 2 inches
- Maximum seal depth: 4 inches (deeper seals impede flow and can self-siphon)
Types of Traps
The most common trap is the P-trap, which is required for virtually all fixture installations. The P-trap consists of a J-bend and a trap arm, creating the water seal in the curved section.
Other approved traps include:
- P-trap - Standard for sinks, lavatories, tubs, showers
- Integral trap - Built into the fixture (water closets, some urinals)
- Drum trap - Limited to specific applications where access is provided
The minimum size trap for a floor drain is 2 inches.
A trap provides a water seal of 2 to 4 inches to prevent sewer gases from entering the building. The minimum floor drain trap size is 2 inches.