Traps & Venting Systems
Trap types, seal depths, venting methods, vent sizing, and preventing siphonage in plumbing systems.
- Identify trap types and explain why each is used or prohibited
- State the minimum and maximum trap seal depths required by code
- Compare stack, wet, relief, and loop venting methods
- Explain siphonage causes and how trap arm length limits prevent it
Lesson 1
Trap Types, Seal Depth & Prohibited Traps
The Purpose of Traps
Every plumbing fixture connected to the DWV system must have a trap that maintains a water seal between the fixture and the drainage system. This water seal prevents sewer gases from entering the building. Without traps, toxic and flammable gases like methane and hydrogen sulfide would flow freely into occupied spaces.
Approved Trap Types
The NPC approves specific trap types for use:
- P-trap - the most common trap, used on lavatories, sinks, and showers. The water seal sits in the curved section
- Integral trap - built into the fixture itself, such as a water closet or bidet
- Running trap - an inline trap on a building drain, sometimes used before the building sewer
- Drum trap - allowed only for specific applications like bathtubs in some jurisdictions
Approved Traps
P-trap - standard for most fixtures
Integral trap - water closets, bidets
Running trap - specific code applications
Prohibited Traps
S-trap - self-siphoning design
Bell trap - difficult to clean
Crown-vented trap - promotes siphonage
Mechanical trap - moving parts fail
Why S-Traps Are Prohibited
An S-trap is prohibited because its design creates a siphon condition during fixture discharge. As water flows down the vertical leg, it can pull the entire trap seal out, leaving no barrier against sewer gas. A properly installed P-trap with a horizontal trap arm to a vent prevents this.
The minimum trap seal depth is 50 mm and the standard maximum is 75 mm. S-traps are prohibited because they self-siphon and lose their seal. Every fixture must have an approved trap - most commonly a P-trap - to prevent sewer gas entry.