April 16, 2026
From the 2024 UPC Illustrated Training Manual, Chapter 9, VENTS
901.3 Trap Seal Protection. The vent system shall be designed to prevent a trap seal from being exposed to a pressure differential that exceeds 1 inch water column (0.24 kPa) on the outlet side of the trap.
The commentary in Section 901.2 explained that positive and negative air pressures within the drainage system may affect fixture trap seals. At what point will the pressures produced be detrimental to the trap seal? This was the question raised by the subcommittee on plumbing under the direction of the National Bureau of Standards in the 1920s [see Recommended Minimum Requirements for Plumbing (BH13) p. 198]. The subcommittee agreed that any trap having a full seal depth of two inches should not be reduced to less than one inch. This is equivalent to a variation of plus or minus 2-inches of water column (0.07 psi). A factor of safety was introduced into the testing criterion whereby the pressure variations within the branch lines were not to exceed plus or minus 1-inch of water column (.04 psi), which in a trap with a two-inch seal would leave a 1 1/2 inch seal under suction and approximately a full seal under back pressure. This factor of safety has determined vent sizing in all plumbing codes.
From the 2024 UMC Illustrated Training Manual, Chapter 13 – FUEL GAS PIPING
1310.9.1 Location of Drips. All drips shall be installed only in such locations that they are readily accessible to permit cleaning or emptying. A drip shall not be located where the condensate is likely to freeze. [NFPA 54:7.6.2]
Drips are usually located near the outlet of the meter or the service entrance and at other locations where condensate could collect. The drip is installed at the bottom of a downward-flowing line by placing a tee at the bottom of the line, installing a nipple and cap in the run of the tee and continuing the pipe run out the side of the tee. Drips must be located so that they can be emptied to prevent liquid from causing an obstruction to the flow of gas. Drip locations must also be protected from freezing.
(This is not to be considered the official position of IAPMO, nor is it an official interpretation of the Codes.)
IAPMO
IAPMO develops and publishes the Uniform Plumbing Code®,the most widely recognized code of practice used by the plumbing industry worldwide; Uniform Mechanical Code®; Uniform Swimming Pool, Spa and Hot Tub Code®; and Uniform Solar Energy, Hydronics and Geothermal Code™ — the only plumbing, mechanical, solar energy and swimming pool codes designated by ANSI as American National Standards — and the Water Efficiency Standard (WE-Stand)™. IAPMO works with government, contractors, labor force, and manufacturers to produce product standards, technical manuals, personnel certification/educational programs and additional resources in order to meet the ever-evolving demands of the industry in protecting public health and safety.
Last modified: April 15, 2026