D-1.8 Sizing Branch and Circuit Vents

Skilled Trades BC

D-1.8 Sizing Branch and Circuit Vents

Although branch and circuit vents are very different types of vents, they share some common sizing characteristics and have, therefore, been grouped together in this chapter.

Branch Vents

A branch vent is sized according to Table 2.5.8.3 (NPC, 2020, B 2-45). When using this table, you need to know the developed length of the branch vent and the hydraulic load (FUs) it serves at any given point.

The length to consider when sizing a branch vent is from the most distant sanitary drainage pipe connection to the point where it ties into a stack vent, vent stack, or vent header or where it continues as a branch vent to outside air (Figure 1).

"Diagram showing the developed length of a branch vent in a plumbing system. The layout includes connections for kitchen sinks (KS), lavatories (LAV), and fixture drains. Labeled components include branch vents, dual and continuous vents, individual and continuous vents, and the most distant soil or waste pipe connection. The system is connected to a vent stack." — **Figure 1** Developed length of a branch vent. (Skilled Trades BC, 2021) Used with permission.

The load to consider when sizing a branch vent is the sum of the FU loads for the fixtures it serves — in other words, the number of fixture units on the sanitary drainage pipes that it provides air for. As the fixtures are added to a branch vent, the load increases (Figure 2).

"Diagram illustrating the hydraulic load on certain branch vent sections. The layout includes kitchen sinks (KS), lavatories (LAV), and fixture drains. Branch vent loads are specified: one branch vent with 7.5 FUs, another with 4.5 FUs, and another with 2 FUs. The system includes dual and continuous vents with lengths and loads marked as not applicable, as well as an individual and continuous vent at the far right of the diagram. The system is connected to a vent stack." — **Figure 2** Hydraulic load on certain branch vent sections. (Skilled Trades BC, 2021) Used with permission.

Branch vents will increase in size as more vents are connected to them (provided Table 2.5.8.3. dictates) but will never be smaller than permitted by Table 2.5.7.1. and any vent connected to it.

The following example shows how the developed length of the branch vents remains the same (from the most distant sanitary drainage pipe connection) but the loads are additive as the other Category 1 vents join the branch vent (Figure 3 and Table 1).

"Diagram illustrating branch vent additive loads serving Category 1 vents. It shows various connections to the vent stack and stack, including lavatories (LAV), drain fixtures (DF), kitchen sinks (KS), mop sinks, and a 1½-inch service sink with a 2-inch trap. The diagram provides lengths for each connection (ranging from 1 to 5 meters) and indicates that all water closets (WCs) are rated at 6 FUs and all lavatories at 1 FU." — **Figure 3** Branch vent additive loads serving Category 1 vents. (Skilled Trades BC, 2021) Used with permission.

Table 1: Figure 3 Components
Label	Name	Length (m)	Load (FU)	Size (in.)	Notes
A	Dual and continuous vent	N/A	N/A	1.5	Length and load N/A. Vent size from Table 2.5.7.1. using largest trap served
B	Continuous vent	12	3.5	1.5	Vent size from Table 2.5.8.3. Length from the connection of KS to the vent stack. Load is total of KS and mop sink
C	Branch vent	16	8	1.5	Vent size from Table 2.5.8.3. Length from the furthest sanitary drainage pipe connection to the vent stack. Load is total of fixtures served at that point
D	Continuous vent	9	14	1.5	Vent size from Table 2.5.8.3. Length from the connection of lavs to the vent stack. Load is total of lavs and WCs
E	Branch vent	16	22	2	Vent size from Table 2.5.8.3. Length from the furthest sanitary drainage pipe connection to the vent stack. Load is total of fixtures served at that point
F	Individual and continuous vent	N/A	N/A	1.25	Length and load N/A. Vent size from Table 2.5.7.1. using trap size served
G	Dual and continuous vent	N/A	N/A	1.25	Length and load N/A. Vent size from Table 2.5.7.1. using largest trap served
H	Branch vent	16	24	2	Vent size from Table 2.5.8.3. Length from the furthest sanitary drainage pipe connection to the vent stack. Load is total of fixtures served at that point

Circuit Vents

Circuit venting is a form of group venting sometimes confused with wet venting. Instead of using the core of air in a vertical pipe, circuit venting uses the top of a horizontal drainage pipe to vent the fixtures connecting to the horizontal branch. The drainage section is not sized as a wet vent but as a branch from Table 2.4.10.6.-B (NPC, 2020, B 2-35).

To size the circuit vent, you must use Table 2.5.8.3. in the NPC 2020 code book.

Note: As with any vent, the circuit vent can never be smaller than permitted by Table 2.5.7.1. For example, if the circuit vent serves a water closet, Table 2.5.8.3. may permit a 1.25 in. vent. Table 2.5.7.1. overrides this value, as a 1.25 in. vent can only serve 1.25 in. and 1.5 in. traps.

Using Table 2.5.8.3. requires the following information:

The hydraulic load served by the circuit vent — The hydraulic load to consider is the total number of circuit-vented fixtures, not including the load of any fixtures that may be connected to the required relief vent. The hydraulic load is most often identical to the branch immediately upstream of the relief vent connection.
The developed length of the circuit vent — The length to consider is from its lower end — where it connects to the trap arm of the furthest upstream circuit-vented fixture — to its upper end — where it joins a vent stack, stack vent, or header (Category 3 or 4 vent) — or measure through to open air.

To fully understand the circuit vent sizing procedures, a plumber must be well-versed in the Plumbing Code regulations for circuit vents.

The code requires that a section of a horizontal branch may be circuit-vented provided that:

A circuit vent is connected to it.
All fixtures served by the circuit vent are located in the same storey.
No stack is connected to it upstream of a circuit-vented fixture (Figure 4).

"Diagram showing an example of a circuit-vented branch. It illustrates the connection of a branch to a stack, with labeled components including a branch vent, relief vent, and circuit vent. The circuit vent connects to the trap arm of the most upstream circuit-vented fixture." — **Figure 4** Example of a circuit-vented branch. (Skilled Trades BC, 2021) Used with permission.

Fixtures with fixture outlet pipes less than 50 mm (2 in.) in size must be separately vented or separately circuit-vented (Figure 6) as per Clause 2.5.3.1.(2).

"Diagram illustrating a separately vented trap arm connected to a circuit-vented branch. It shows a branch with a relief vent and two 1½-inch traps connected to the circuit vent." — **Figure 5** Separately vented trap arm connected to a circuit-vented branch. (Skilled Trades BC, 2021) Used with permission.

"Diagram illustrating 1½-inch traps being separately circuit-vented when connecting to a circuit-vented branch. The diagram shows a branch vent, combined relief vent, circuit vent, and multiple 1½-inch traps connected to the fixture drain (FD). The installation is permitted because the 1½-inch traps are separately circuit-vented." — **Figure 6** The 1.5 in. traps are being separately circuit-vented when connecting to a circuit-vented branch. (Skilled Trades BC, 2021) Used with permission.

A relief vent must be connected to the branch that forms part of a circuit-vented system, downstream of the connection of the most downstream circuit-vented fixture. This relief vent may be a dry vent (Figures 7 and 8).

"Diagram illustrating a dry vent acting as a relief vent for a circuit-vented branch. The layout includes two 2-inch shower drains connected to the circuit vent, with a branch vent and a dry relief vent." — **Figure 7** A dry vent acting as a relief vent for a circuit-vented branch. (Skilled Trades BC, 2021) Used with permission.

"Diagram showing a vertical branch acting as a relief vent for a circuit-vented branch. The layout includes lavatories (LAV) and 2-inch showers connected to the circuit vent, with a branch vent and relief vent labeled. The branch, carrying 2 FUs, acts as the relief vent." — **Figure 8** A vertical branch acting as a relief vent for a circuit-vented branch. (Skilled Trades BC, 2021) Used with permission.

A sanitary drainage pipe cannot have a hydraulic load of more than 6 FUs when acting as a relief vent. If the sanitary drainage pipe is acting as a relief vent, it may be installed either horizontally or vertically (Figures 9 and 10).

"Diagram illustrating a horizontal fixture drain serving as a relief vent for a circuit vented branch, showing five 2" stall urinals and labeled relief and circuit vents." — **Figure 9** A horizontal fixture drain acting as a relief vent for a circuit-vented branch. (Skilled Trades BC, 2021) Used with permission.

A stack may act as a relief vent for a circuit-vented branch. If there are more than 6 FUs on the sanitary drainage pipe (stack), a relief vent must be added.

"Two diagrams showing examples of relief vents for a circuit vented branch, with labeled urinals, stacks, flush tank WC, and flush valve WC, comparing when a stack can and cannot act as a relief vent." — **Figure 10** Two examples of relief vents for a circuit-vented branch. (Skilled Trades BC, 2021) Used with permission.

A symmetrically connected relief vent can serve as a combined relief vent for a maximum of two circuit-vented branches, provided there are not more than eight circuit-vented fixtures connected between the combined relief vent and each circuit vent (Figure 11).

"Diagram showing a symmetrical connection used to join a combined relief vent, featuring labeled branch vents, circuit vents, combined relief vent, and five 2” showers connected by fixture drains (FD)." — **Figure 11** A symmetrical connection used to join a combined relief vent when serving two circuit-vented branches. (Skilled Trades BC, 2021) Used with permission.

Additional circuit vents are required where:

Each cumulative horizontal change in direction of a branch served by a circuit vent exceeds 45° between vent pipe connections.
More than eight circuit-vented fixtures are connected to a branch between vent pipe connections.

Figure 12 shows additional circuit vents serving a circuit-vented branch, thus allowing more than eight fixtures and the cumulative change of direction between vent pipes.

"Diagram of additional circuit vents added to meet Clause 2.5.3.1., with labeled branch vents, additional circuit vents, relief vent, and more than 5 fixtures connected with a 45-degree offset" — **Figure 12** Additional circuit vents added to meet Clause 2.5.3.1., with labeled branch vents, additional circuit vents, relief vent, and more than 5 fixtures connected with a 45-degree offset. (Skilled Trades BC, 2021) Used with permission.

A sanitary drainage pipe can serve as an additional circuit vent provided:

The sanitary drainage pipe is sized as a wet vent and is not less than 50 mm (2 in.) in size (Figure 13).

"Diagram showing a sanitary drainage pipe acting as an additional circuit vent, with labeled branch vents, circuit vent, relief vent, and individual and continuous vent. A lavatory (LAV) is also labeled with a 2-inch minimum pipe." — **Figure 13** Sanitary drainage pipe acting as an additional circuit vent. (Skilled Trades BC, 2021) Used with permission.

The maximum number of fixtures does not exceed two.
The fixture(s) connected to the vent has a maximum hydraulic load of 1.5 FUs each.
When two fixtures connect to the vent pipe, the connection is made using a double sanitary tee fitting.

Figure 14 shows a sanitary drainage pipe acting as an additional circuit vent. The branch is sized as a wet vent with a minimum diameter of 2 in.

"Diagram showing two maximum FU fixtures connected to an additional circuit vent using a double sanitary tee fitting, with labeled branch vents, circuit vent, relief vent, and wall washout urinals with a 2-inch minimum pipe." — **Figure 14** Two maximum 1.5 FUs fixtures connected to the additional circuit vent by the use of a double sanitary tee fitting. (Skilled Trades BC, 2021) Used with permission.

A trap arm(s) may be connected to a circuit vent (Figure 15), provided that:

The branch or fixture drain is sized as a wet vent and is not less than 50 mm (2 in.) in size.
The maximum number of fixtures does not exceed two.
The fixture(s) connected to the circuit vent has a maximum hydraulic load of 1.5 FUs each.
When two fixtures are connected to the vent pipe, the connection is made using a double sanitary tee fitting, as stated in Clause 2.5.4.5.(1).

"Diagram showing two maximum 1½ FU fixtures connected to the circuit vent by a double sanitary tee fitting, with labeled branch vent, circuit vent, relief vent, branch, and wall washout urinals with a minimum 2-inch wet vent." — **Figure 15** Two maximum 1.5 FUs fixtures connected to the circuit vent by a double sanitary tee fitting. (Skilled Trades BC, 2021) Used with permission.

Circuit-vented branches, including the fixture drain downstream of the circuit vent connection, shall be sized as a branch from Table 2.4.10.6.-B (NPC, 2020, B 2-35; refer to Figures 16–18 below), except that it shall be not less than:

50 mm (2 in.), where traps less than 50 mm in size are circuit-vented.
75 mm (3 in.), where traps 50 mm in size or larger are circuit-vented.

"Diagram showing a circuit vented branch serving water closets, sized as a branch per Table 2.4.10.6.B, with labeled branch vent, circuit vent, relief vent, and flush tank WCs. Various FU values and pipe sizes are indicated for each branch." — **Figure 16** Circuit-vented branch serving water closets sized as a branch from Table 2.4.10.6.-B. (Skilled Trades BC, 2021) Used with permission.

"Diagram showing a circuit vented branch serving 1¼" lavatories, increased to a 2" fixture drain downstream of the circuit vent connection. Labels include trap arm fixture drain, fixture drain (1 FU), branch (3 and 4 FUs), and four lavatories." — **Figure 17** Circuit-vented branch serving 1.25 in. lavatories increased to 2 in. fixture drain downstream of the circuit vent connection. (Skilled Trades BC, 2021) Used with permission.

"Circuit vented branch serving 2” urinals increased to 3” fixture drain downstream of the circuit vent connection. Diagram shows six pedestal urinals, branch vent, relief vent, and trap arm fixture drain." — **Figure 18** Circuit-vented branch serving 2 in. urinals increased to 3 in. fixture drain downstream of the circuit vent connection. (Skilled Trades BC, 2021) Used with permission.

The minimum size of an additional circuit vent or relief vent serving a circuit-vented branch need only be one size smaller than the size of the circuit vent, provided it conforms to the minimum vent requirements set out in Table 2.5.7.1., based on the size of the circuit-vented traps (Figures 19–21). If these requirements are met, the vents need not be larger than 50 mm (2 in.).

"Diagram showing a relief vent serving a circuit vented branch with a 24 FU circuit vent, 6 FU water closets, 2” showers, and labeled branch and relief vents, with specified vent sizes and lengths." — **Figure 19** Relief vent serving a circuit-vented branch one size smaller than circuit vent and in compliance with Table 2.5.7.1. (Skilled Trades BC, 2021) Used with permission.

Figure 20 shows a relief vent and additional circuit vents serving a circuit-vented branch. The vents are sized one size smaller than the circuit vent and are in compliance with Table 2.5.7.1 (NPC, 2020, B 2-43).

"Diagram showing a relief vent and additional circuit vents serving a circuit vented branch with 102 FUs, flush valve WCs, and labeled branch vent, relief vent, and additional circuit vents, with specified sizes and lengths." — **Figure 20** Relief vent and additional circuit vents serving a circuit-vented branch. (Skilled Trades BC, 2021) Used with permission.

Figure 21 shows a relief vent serving a circuit-vented branch. The relief vent is sized equal to the circuit vent in order to comply with Table 2.5.7.1.

"Diagram showing a relief vent serving a circuit vented branch with 18 FUs, 6 FU water closets, 2” showers, and labeled branch vent and relief vent. The diagram references Table 2.5.7.1 for a required relief vent size of 1½ inches." — **Figure 21** Relief vent serving a circuit-vented branch. (Skilled Trades BC, 2021) Used with permission.

The hydraulic load on a circuit vent must include the hydraulic load from fixtures connected to the branch served by the circuit vent but must not include the hydraulic load connected to the sanitary drainage pipe acting as the relief vent (Figures 22–25).

"Diagram showing the hydraulic load on a circuit vented branch with 24 FUs, 6 FU water closets, 3 FU showers, lavatories (1 FU), and labeled relief vent, branch vent, and stack (18 FUs). The diagram also shows additional branches with varying FUs." — **Figure 22** The hydraulic load on a circuit vent is the connected load to the circuit-vented branch, not including the fixtures connected to the relief vent. (Skilled Trades BC, 2021) Used with permission.

"Diagram showing the hydraulic load on a circuit vent with 27 FUs, featuring 1½ FU urinals, 4 FU water closets, and labeled branch vent, relief vent, and additional branches with varying FU loads." — **Figure 23** The hydraulic load on a circuit vent is the connected load to the circuit-vented branch, including fixtures connected to the circuit vent. The load does not include the fixtures connected to the relief vent. (Skilled Trades BC, 2021) Used with permission.

"Diagram showing the hydraulic load on a circuit vent with 48 FUs, featuring lavatories (1 FU), 4 FU water closets, and labeled branch vent, relief vent, additional circuit vent, and branches acting as additional circuit vents with a minimum 2-inch size." — **Figure 24** The hydraulic load on a circuit vent is the connected load to the circuit-vented branch, including fixtures connected to the circuit vent and additional circuit vents. The load does not include the fixtures connected to the relief vent. (Skilled Trades BC, 2021) Used with permission.

"Diagram showing the hydraulic load on a circuit vent with 24 FUs, featuring lavatories (1 FU), 4 FU water closets, and labeled branch vent, individual and continuous vent acting as a relief vent, and branches with varying FU loads." — **Figure 25** The hydraulic load on a circuit vent is the connected load to the circuit-vented branch, including individually vented fixtures connected to the circuit vent. The load does not include the fixtures connected to the relief vent. (Skilled Trades BC, 2021) Used with permission.

Self-Test D-1.8: Sizing Branch and Circuit Vents

Complete Self-Test D-1.8 and check your answers.

If you are using a printed copy, please find Self-Test D-1.8 and Answer Key at the end of this section. If you prefer, you can scan the QR code with your digital device to go directly to the interactive Self-Test.

D-1.8 Self Test QR Code

References

National Research Council of Canada. (2020). National plumbing code of Canada 2020. Canadian Commission on Building and Fire Codes. https://nrc-publications.canada.ca/eng/view/ft/?id=6e7cabf5-d83e-4efd-9a1c-6515fc7cdc71

Skilled Trades BC. (2021). Book 2: Install fixtures and appliances, install sanitary and storm drainage systems. Plumber apprenticeship program level 2 book 2 (Harmonized). Crown Publications: King’s Printer for British Columbia.

Trades Training BC. (2021). D-1: Install sanitary drain, water and vent systems. In: Plumber Apprenticeship Program: Level 2. Industry Training Authority, BC.

Media Attributions

All figures are used with permission from Skilled Trades BC (2021) unless otherwise noted.

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