The vent systems of a sanitary drainage systems is one of the most least understood component in sanitary drainage system design. Lack of understanding on the principle on these 2 kinds of vents leads to over design which is costly or worse under design which results to sewer odor coming out of the system.
In this article, we will explore the function of a circuit vent and relief vent as well as know the difference between the two. We will also try to learn the proper installation in accordance with the following codes and standards used as reference to this article:
Revised National Plumbing Code of the Philippines (RNPCP)
International Plumbing Code (IPC)
BS EN 12056 Standard for Sanitary Drainage inside Buildings.
What is a Circuit Vent?
According with the Revised National Plumbing Code of the Philippines (RNPCP), a circuit vent is defined as:
"A Loop or Circuit Vent is a vertical vent connection on a horizontal soil or waste pipe branch at a point downtream of the last fixture connection and turning horizontal line above the highest overflow level of the highest fixture connected thereat; the terminus connected to the stack vent in case of loop venting or the vent stack stack nearby in case of circuit venting."
On the other hand, the International Plumbing Code define a circuit vent as:
"Circuit Vent. A vent that connects to a horizontal drainage branch and vents two traps to a maximum of eight traps or trapped fixtures connected into a battery."
By these definitions, we learned that a circuit vent is a vent connected to a horizontal soil or waste branch at a point downstream of the last fixture connection and is connected to a vent stack. However, it does not provide the Master Plumbers the reasons and function of a circuit vent and why is it installed vertically.
Basically, a circuit vent is a method of venting which uses the upper portion of a horizontal soil or waste branch as a vent. In general, horizontal waste and soil pipes are assumed and sized half way full only which means that the liquid level is only up to half of the pipe and the other upper half is air. This is important in order to prevent self siphonage to occur as the liquid travels through the pipeline. This means that we could utilize the upper portion of the pipe as a vent for multiple or a battery of fixtures.
Using a circuit vent therefore reduces the amount of pipe material used for venting since we could just provide 1 vent for up to 8 trapped fixtures. However, before we implement this method of venting, it is important to know the code requirements of its installation to avoid failure of the vent system or wrong circuit vent installation.
The following illustration shows an example if the same sanitary drainage does not utilize circuit venting. In this image, more vent pipes and fittings are used since all fixtures without an individual vent shall be provided with a vent connection to the branch vent.
Wet Vent and Dry Vent defined.
A wet vent is a vent pipe which also serves as a waste drain of no more than one fixture which is usually waste coming from a lavatory. Unlike wet vent, a dry vent does not carry any wastewater and dedicated only for the passage of air.
In IPC, in a matter of its installation, a dry vent connection to the branch drain shall be 45 degrees or more from the horizontal plane. A dry vent is best installed using a tee reducer installed vertically, although, a sanitary wye may also be used provided the 45 degree minimum angle is met. Figure 4 shows the different ways to install a dry vent.
Requirements of a Circuit Vent Installation
As mentioned earlier, a circuit vent uses the top half of the branch drain as a vent for a battery of fixtures. This condition necessitate that the upper half vent portion of the branch drain needs to be protected against blockage due to water turbulence and waste flows from connected fixture drains. Therefore, the following code requirements are set by IPC to address that air blockage probability to any circuit-vented branch drain:
All drain connections in a Circuit-vented branch shall connect to the drain horizontally. Figure 5 explains this requirements. This is required because if the drain connection will be located at the top portion, then any waste flow will considerably block the upper vent portion of the branch drain.
Figure 5 - Shows a common mistake in connecting a floor drain to a circuit-vented branch drain pipe. Vertical and Horizontal offsets shall not be allowed within a circuit vented branch. This provision is directed at eliminating the creation of turbulence as the waste is subjected to horizontal turns as well as drops in elevation. Figure 6 illustrates this conditions taking place.
Figure 6 - Shows an example of a vertical offset and horizontal offset. Circuit Vent shall be installed on or before the upstream fixture. The main purpose of a circuit vent is to vent all fixture within the the circuit-vented branch. If for example the circuit vent is installed downstream of two or more fixtures, a siphonage will occur at the trap of the last fixture when the second to the last fixture drains significant amount of waste.
What is a Relief Vent?
A relief vent is a vent, by definition in accordance with NPCP is:
"RELIEF VENT - A vertical vent line, the primary function of which is to provide additional circulation of air between drainage and vent systems or act as an auxiliary vent on a specially designed system such as a "yoke vent" connection between the soil and vent stacks.
IPC defines relief vent as the following:
"RELIEF VENT. A vent whose primary function is to provide circulation of air between drainage and vent systems."
Using these definitions alone will not help RMPs decide when it is appropriate to use or provide a relief vent. To simplify, the function of a relief vent is to relieve any excess pressure at certain areas in the sanitary drainage system. The objective is to have a balanced atmospheric pressure within and at any part of the drainage system to protect the trap seals which prevent unwanted sewer odor from coming out of the system.
There are different areas in a sanitary drainage system where relief vent is required where positive atmospheric pressure is expected to develop.
The following are code requirements on relief vents:
Relief Vent at the base of the stack.
The purpose of this relief vent is to counter hydraulic jump. A hydraulic jump is a sudden jump of atmospheric pressure at the base of the stack as waste is draining from very high points of the stack.
Figure 7 - Shows an example of relief vent connection at the base of a stack (2 Stacks System) Relief Vent at Stack Offsets.
Whenever there is a stack offset, a relief vent shall be provided when the offset exceeds 6 meters in lenght. A relief vent shall be provided 10 pipe diameters from the stack connection to the horizontal drain.
Figure 8 - Shows the relief vent connections at a Stack Offset. Relief Vent along the stack at every 10 branch intervals.
The function of a relief vent at these locations of the stack is to maintain a continuous free opening of air within the soil stack. As more and more waste connections are connected to the stack, the combined flow of these waste creates a plug which would create a positive pressure and imbalance of atmospheric pressure within the stack.
In its installation, IPC states that its connection to the soil stack shall be below the drain connection of the floor served, and its connection to the vent stack shall extend 900mm above the floor level. The illustration in Figure 8 shows the correct installation of a pressure relief vent.
Figure 9. Shows correct installation of a relief vent along branch intervals. 
Figure 10. Based on the IPC code requirement, the relief vent installed this way is unacceptable. Relief Vent at the Branch Drain connecting to a stack.
For branches connecting to the stack more than 5 branch intervals, a relief vent between the last downstream fixture and the stack shall be provided. Its function is to relieve the positive pressure caused by the combined waste flow from the upper stack connections as discussed earlier. As the combined flow creates a plug of wastewater, the positive pressure created will try to escape and will pass through the connected horizontal drains. The function of this relief vent is to serve as a diverter of positive pressure so that it would not affect the trap seals upstream of that branch drain.
This relief vent can be a wet vent. By our definition of a wet vent above, a pressure relief vent can therefore accept wastes, however it should be noted that the allowed waste flow shall not exceed 4 drainage fixture units (dfu).
The Figure 11 below shows a relief vent which accepts wastes coming from a lavatory.
Figure 11 - Shows a lavatory drain serves as a relief vent for the circuit vented branch with more than 4 water closets.
By reading this article, I hope you now have completely understood the intricacies of these 2 kinds of vents commonly used in sanitary drainage systems. If you have questions or comments about this article, feel free to comment in the comment section below.
Author:
RMP James Ray L. Magdadaro
Registered Master Plumber | Plumbing Consultant
PRC Reg. No. 6491 (RMP)
Managing Partner - MEP Global Co.
Principal Architect - JLM Architects
Contact: jlm@jlmarchstudio.com
Thank You Archt