page_title: Trap (plumbing)
section_title: Venting and auxiliary devices
hierarchical_section_title: Trap (plumbing) / Venting and auxiliary devices
caption_attribution_description: Plumbing trap under 2 hour fire-resistance rated concrete floor slab, showing underside of firestop with rockwool packing.
context_page_description: In plumbing, a trap is a U-shaped portion of pipe designed to trap liquid or gas to prevent unwanted flow; most notably sewer gases from entering buildings while allowing waste materials to pass through. In oil refineries, traps are used to prevent hydrocarbons and other dangerous gases and chemical fumes from escaping through drains. In heating systems, the same feature is used to prevent thermo-siphoning which would allow heat to escape to locations where it is not wanted. Similarly, some pressure gauges are connected to systems using U bends to maintain a local gas while the system uses liquid. For decorative effect, they can be disguised as complete loops of pipe, creating more than one U for added efficacy.
  In domestic applications, traps are typically U, S, Q, or J-shaped pipe located below or within a plumbing fixture. An S-shaped trap is also known as an S-bend. It was invented by Alexander Cumming in 1775 but became known as the U-bend following the introduction of the U-shaped trap by Thomas Crapper in 1880. The U-bend could not jam, so, unlike the S-bend, it did not need an overflow. In the United States, traps are commonly referred to as P-traps.
context_section_description: Maintaining the water seal is critical to trap operation; traps might dry out, and poor venting can suction or blow water out of the traps. This is usually avoided by venting the drain pipes downstream of the trap; by being vented to the atmosphere outside the building, the drain lines never operate at a pressure much higher or lower than atmospheric pressure. In the United States, Plumbing codes usually provide strict limitations on how far a trap may be located from the nearest vent stack.
  When a vent cannot be provided,  an air admittance valve may be used instead. These devices avoid negative pressure in the drain pipe by venting room air into the drain pipe (behind the trap). A "Chicago Loop" is another alternative.
  When a trap is installed on a fixture that is not routinely used—such as a floor drain—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.
caption: Trap with copper drain pipe at underside of firestop packing in 2 hour fire-resistance rated concrete floor slab
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