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How to size the sanitary vent piping system?

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16 views asked May 23, 2018 by SUMIT

1 Answer

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More specifically: 

  • How much is the slope percentage in a horizontal vent pipe?
  • How to estimate the load in venting?
  • How to size the vent pipe?


1-  Vent systems are installed to eliminate trap siphonage, reduce back pressure and vacuum surges, promote rapid and silent flow of wastes, and ventilate the sewer.

2-  Sizing the system: The sizing of vent system piping is based on fixture unit values and length of the piping, then refer to pipe sizing table in Uniform Plumbing Code (UPC) or State Plumbing Code book. See below:



3- Vent system types:

  • End Venting: system of floor drains where branch arms do not exceed 15 ft and are sloped at 1%  (1/8 in./foot) to a main drain that is sized two pipe diameters larger.
  • Common Vent:  system may be used for two fixtures set on the same floor level but connecting at different levels in the stack.
  • Stack Venting: A group of fixtures, consisting of one bathroom group and a kitchen sink or combination fixtures, installed without individual fixture vents in a one story building or on the top floor of a building and each fixture drain connects independently to the stack.
  • Wet Venting: A vent that also serves as drain. In this case the drain pipe should be one or two size larger than the actual required size for regular drain to allow the upper part of drain pipe to be used for the venting.
  • Circuit Venting :  A branch vent that serves two or more traps and extends from the downstream side of the highest fixture connection of a horizontal branch to the vent stack.  
  • Loop Venting: A circuit vent that loops back to connect with a stack vent instead of a vent stack.

3-  Vent stacks:

  • Each drainage stack that extends 10 feet or more stories above the building drain or other horizontal drain, shall be served by a parallel vent stack, which shall extend undiminished in size from its upper terminal and connect to the drainage stack at or immediately below the lowest fixture drain. Each such vent stack shall also be connected to the drainage stack at each fifth floor, counting down from the uppermost fixture drain, by means of a yoke vent. Size of the yoke vent shall be not less in diameter than either the drainage or the vent stack whichever is smaller.
  • Vent terminals should not be located within 10 feet and 3 feet (check local codes) above any operable window, door, opening, air intake or vent shaft, nor less than 3 feet (check local code) in every direction from any lot line, valley and street.
  • Where frost or snow closure is likely to occur in locations having minimum design temperature below 0° F, vent terminals shall be a minimum of 2 inches in diameter, but in no event smaller than the required vent pipe.   
answered May 30, 2018 by Devan Talebi

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