Circuit Vent

Section 1 of the OBC:

Circuit vent means a vent pipe that functions for two or more traps connecting to a horizontal branch where the lowest end of the vent pipe connects to the waste pipe on the downstream side of the highest fixture of the group and the upper end connects to branch vent, vent stack, stack vent or extends to open air independently and "circuit vented" has a corresponding meaning.



A circuit vented horizontal branch connected to a soil stack



A circuit vented horizontal branch connected to a sanitary building drain



Requirements for a Circuit Vent

7.5.2.2.(1):

A horizontal branch off a stack or off a sanitary building drain may be circuit vented where

  1. the circuit vented branch is of a size 3" or larger,
  2. the minimum number of circuit vented fixtures is less than 7 per vent and in any group of 6 or less circuit vented fixtures all but the last of the group are downstream of the point where the circuit vent connects to the branch.




A circuit vent may protect up to sex fixtures installed on a circuit vented horizontal branch. The circuit vent must connect downstream of the last fixture in the circuit vented group. If seven to twelve fixtures are to be circuit vented, two vents are required. If thirteen to eighteen, three vents, and so on.


  1. the circuit vented fixture is sanitary unit, floor drain, hub drain, shower drain or trap standard slop sink,


A sanitary unit is: water closet, urinal, bidet, bedpan washer


  1. all fixtures connected to the circuit vent are on the same floor level,
  2. no soil or waste stack connects to the circuit vented branch:


  1. No circuit vented trap has a horizontal run of waste pipe of more than 1 500 mm:

You should note that the standard vent to trap requirements apply.

  • When installing fixture drains to a circuit vented branch, the minimum distance between a trap and the horizontal branch is 2 pipe diameters, and the maximum distance is 1 500 mm.
  • The maximum fall between a P-trap and the horizontal branch is one pipe diameter, and the minimum fall is 1:50.
  • The maximum change of direction between a fixture trap and the horizontal branch is 135o  for a P-trap fixture and 135o for a siphonic fixture


  1. in a water closet installation, no circuit vented trap has a horizontal run of waste pipe of more than 1 500 mm nor a vertical run of more than 900 mm:




7.5.2.2.(2)

Where a stack or a sanitary building drain has a circuit vented branch connected to it and

  • the stack or sanitary building drain carries more than eight fixture units of drainage upstream of the connection:



  • has connected to a sanitary drainage pipe larger than 2” size:



It is not clear from the Code at what point this sanitary drainage pipe  larger than 2" should connect to the stack. Let's assume it should be located at the same floor level as the circuit vented branch. But on the other hand I'd like to see how they are going to insert 8 fixture units into 2" pipe.


see drainage pipes sizing


  • receives drainage from a higher floor level:




the circuit vented branch shall be relief vented.


7.5.2.2.(3):

A relief vent required by sentence (2) may be a wet vent if it is of 2" size and greater and not more than one fixture having a maximum of 1 1/2 fixture units is drained into it:




7.5.2.2.(4):

Where circuit vented fixture traps are connected to 2 or more horizontal branches that connect to the same horizontal branch, the horizontal branches may have a combined relief vent:

Are you sure you was born to be a plumber? Maybe job of a  Ph.D. suits you better? What a great piece of esoteric wisdom found in a little OBC sentence!




7.4.2.1(e):

fixtures that have a hydraulic load totalling not more than 1 1/2 fixture units may be connected to a vertical section of a of a circuit vent provided

( i )

the fixtures are located in the same storey as the fixtures served by the vent pipes,

( ii )

not more than 2 fixtures are connected to the vent pipe:


( iii )

and where 2 fixtures are connected to the vent pipe, the connection is done with a double fitting in conformance with Table 7.2.4.5.


Circuit Vent Sizing

7.5.6.1.(2):

Every relief and circuit vent shall be at least 2" size.


7.5.7.2.(2):

Where Table 7.4.10.5.B is not applicable to a branch vent, headers or circuit vent, no vent or header shall have branch connections in excess of the number that are permitted by the combinations in Table 7.5.7.2.A and, where one vent pipe protects more than one trap, each trap shall be counted as one vent.


"The enlargement must take place as the two circuit vents join together", - Plumbing Apprenticeship Training, Intermediate Plumbing Book, page 181, year of the issue is unknown.


7.5.7.2.(4)(a):

For the purpose of Table 7.4.10.5.B and Table 7.5.7.2.B, the length of a circuit vent shall be the developed length of vent piping from the horizontal soil or waste pipe to the vent stack, stack vent, header or open air.