Physics of Buildings

Trap seal calculator

Seal depth, drain pressure, a dry spell and wind in; holds or breaks, the margin and the seal left out.

Drain pressure is negative for suction, positive for back-pressure. Defaults: a full 2-inch trap under 1.5 inches of suction, outside the ±1 inch design band but short of breaking. Change any box; the working is shown below the results.

How it is calculated

A trap is a U-tube: the difference between its two water levels equals the pressure difference between the drain and the room, in inches of water [1]. With seal depth d (weir to dip) and s inches of water left, the calculator uses three results from the essay on siphonage:

suction to break = min(d, 2s)
back-pressure to break = 2s
seal after a pull a = min(s, d − min(a, d)/2)(1)

Evaporation lowers the seal by the rate times the weeks unused. Wind adds Cp · ½ρv2 at the vent terminal (Bernoulli) [1], with air at 1.204 kg/m³. Pressures convert at 1 in. w.c. = 249.0889 Pa [2] = 25.4 mm w.c. The ±1 in. w.c. band is the design target for vent systems [3][4].

Worked example

A full 2-inch trap, 1.5 inches of suction. The outlet side cannot rise past the weir, so water spills and the inlet side falls 1.5 inches: 0.5 inches short of the dip, so the seal holds with a 0.5-inch margin. When the suction goes, the water levels out at 2 − 1.5/2 = 1.25 inches. That trap now breaks at min(2, 2.5) = 2 inches of suction again, but a second pull of 1.5 inches does no further damage while a pull of 2 inches gulps air. Now leave it unused for 3 weeks at 0.25 inches a week: 0.5 inches left, and it breaks at only 1 inch, the edge of the design band.

Assumptions and valid range

  • Both sides of the trap have the same bore; the water is in hydrostatic balance (slow pressure changes, no sloshing or momentum); only water that spills over the weir is lost; a broken seal keeps at most half its depth. Real traps lose more, because water keeps moving and bubbles drag it out [5].
  • Pressures are gauge pressures in the drain right at the trap, relative to the room. Negative is suction.
  • The evaporation rate is your input. It varies a great deal with heat, dryness and air movement; experiments in the 1880s found from about one inch a week to more than two in vented traps [6]. The default, 0.25 inches a week, is illustrative.
  • The wind pressure coefficient depends on the vent terminal and the roof around it; −0.5 is illustrative.
  • Accepted inputs: seal 0.5–6 in, water left 0 to the seal depth, pressure ±50 in. w.c., evaporation 0–5 in/week, 0–520 weeks, wind 0–150 mph, Cp −2 to 1.
  • This explains the physics of a trap seal. It is not a vent-sizing method or a code determination: trap and vent requirements differ between the IPC, the UPC and local amendments [7][8], and a licensed plumber decides what is installed.

Sewer gas. Hydrogen sulfide in sewer gas deadens the sense of smell at high levels [9], and methane is explosive between 5 and 15 percent in air [10]. If a smell is strong or comes with headache or dizziness, ventilate and leave. Vent, stack and roof work is for a licensed plumber.

Sources

  1. Samuel J. Ling, Jeff Sanny and William Moebs (2016). University Physics Volume 1, §14.2 Measuring Pressure and §14.6 Bernoulli’s Equation. OpenStax, Rice University. textbook §14.2: a manometer column of height h balances a pressure difference hρg; gauge vs absolute pressure. §14.6 (https://openstax.org/books/university-physics-volume-1/pages/14-6-bernoullis-equation): p + ½ρv² + ρgh = constant along a streamline; pressure drops where a fluid moves fast.
  2. Ambler Thompson and Barry N. Taylor (2008). Guide for the Use of the International System of Units (SI), NIST Special Publication 811, Appendix B.8. National Institute of Standards and Technology. standards body inch of water, conventional = 249.0889 Pa; psi = 6894.757 Pa.
  3. Gary Gauthier, Building Safety Journal (2025). Code Corner: 2024 International Plumbing Code Chapter 9: Vents. International Code Council. code body article IPC 901.2: vent piping permits the admission or emission of air so the pressure difference across a trap seal stays within 1 in. of water column; positive pressure pushes a seal back, negative pressure pulls it over the weir; §1002.4 has provisions for traps subject to evaporation.
  4. Robert S. Wyly and Herbert N. Eaton (1961). Capacities of Stacks in Sanitary Drainage Systems for Buildings, NBS Monograph 31. National Bureau of Standards, Washington, DC. government research report §4.5: vents are sized to hold pneumatic pressures within +1 to −1 in. of water column of atmospheric, so pressure effects on trap seals are negligible; vent sizes for a 1 in. drop computed with Darcy–Weisbach plus loss coefficients. §7: vent stacks sized so pressures do not destroy trap seals. Measured air-to-water flow ratios in a stack.
  5. John L. French and Herbert N. Eaton (1951). Self-Siphonage of Fixture Traps, Building Materials and Structures Report BMS 126. National Bureau of Standards, Washington, DC. government research report Foreword: the water seal is normally 2 to 4 inches deep; lost seal lets drainage-system pressure fluctuations bubble sewer air back through the trap. §1 fig. 1: definitions of trap weir, depth of seal, remaining seal, seal loss; siphonage by other fixtures vs self-siphonage. §2: inertia of the water at the end of the discharge, air bubbles dragging water, the inlet leg pulled down to the dip so air passes; refill by trail discharge. §5.2: discharge set by orifice, trap, drain and vent fitting.
  6. J. Pickering Putnam (1911). Plumbing and Household Sanitation (chapter: The Siphonage and Evaporation of Traps). Doubleday, Page & Co.; reproduced at chestofbooks.com. historical text Reports evaporation experiments signed June 1884 (Putnam and Rice): about 1/7 in. a day in traps vented 3 in. past the crown, over 1/3 in. a day with the vent at the crown; fastest in winter and with heated flues, slowest in damp summer weather; a 1¼ in. S-trap seal lost in four to eleven days.
  7. IAPMO (2018). Uniform Plumbing Code (2018), Chapter 10: Traps and Interceptors (Nevada adoption, printed copy). IAPMO; copy hosted by the City of Lynnwood, WA. plumbing code 1002.1 every trap protected against siphonage and back-pressure by a vent; 1002.2 trap arm length limits (table); 1002.4 vent opening not below the trap weir; 1004.1 S traps, bell traps and crown-vented traps prohibited; 1005.1 liquid seal 2 to 4 in.; 1007.1 trap seal primers for infrequently used floor drains. Described, not reproduced.
  8. International Code Council, PMG Membership Council (2023). CodeNotes: Installation of Air Admittance Valves (based on the 2021 IPC and IRC). International Code Council. code body guidance An AAV lets air into the drainage system under negative pressure to prevent trap siphonage; closes and seals at zero or positive pressure; does not relieve positive pressure, so at least one vent must extend outdoors; ASSE 1051 (branch) and 1050 (stack) valves; must be accessible; manufacturer's instructions apply.
  9. Agency for Toxic Substances and Disease Registry (2024). Hydrogen Sulfide ToxFAQs. U.S. Department of Health and Human Services (CDC/ATSDR). public health agency Rotten-egg smell noticeable at about 0.0005 ppm; often called sewer gas; smell lost after 2–15 min near 100 ppm, so odour is not a reliable warning; very high levels (> 500 ppm) can cause sudden unconsciousness or death; flammable.
  10. ILO and WHO (2018). International Chemical Safety Card 0291: Methane. International Labour Organization. international safety card Colourless, odourless gas; explosive limits 5–15 % by volume in air; can cause suffocation by lowering oxygen in confined areas.