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What Are Plumbing Vents and Why Do You Need Them?

Do you need plumbing vents? Why are they used? Everything you need to know about plumbing vents and more. All you need from Fixed Today Plumbing.

Plumbing vents are also referred to as vent stacks or plumbing air vents. Your vent system does not deal with water even though it is part of a plumbing system. It is a vertical pipe that generally goes through your home’s roof, providing an entry point for air and an exit for odours and gas.

A vent has two primary purposes:

1. It regulates the airflow in your plumbing system, ensuring waste and water flow through your drain pipes that drain out of your house. It prevents a vacuum that causes slow or no drainage.

Each plumbing fixture in your home requires air to move the water through the drainage pipes. Clean drain pipes can only do their job when the vent pipe works.

2. It removes sewer gases, allowing them to escape and providing an entry point for fresh and clean air into the sewer system.

It’s common for such gases to flow from the sewer system into your home. This build-up of gases can include methane, hydrogen sulphide, nitrogen oxides, sulphur dioxide, ammonia, etc. They cause a foul odour, but they can also be dangerous.

A vent allows fresh air and lets smell and sewer gasses out of your plumbing, stopping your home from smelling bad due to everything flowing through your pipes.

Plumbing Vents


Traps are serving every drain in your house.

P-Traps are a modern invention to hold a small amount of water that prevents sewer gases from infiltrating your home. They can also trap debris and prevent clogs. And they can catch your valuables like wedding rings, necklaces or other small prized belongings.

Traps and vents must work together. An adequately vented trap and plumbing system won’t cause air pressures to push and pull air through the trap. A properly vented system will redirect those pressures leaving the water reservoir intact.

Vent Pipe System Specifications

Many plumbing codes will define where a vent should connect to a fixture. Typical examples would be more than 6 inches above flood level for the fixture.

The vent stack should lead from the main bathroom to the outside and extend between 1  and 2 feet above your home.

The diameter of this vent pipe should not be smaller than 2.5 inches if it feeds to at least two drainage fixtures. Generally, in a home for a small family, the pipe needs to be at least 3 – 4 inches to serve the whole house.

The vent stack is the central vent, and other attached vents can be smaller, like a tree trunk and branches coming off it.

For the smaller vents to reach the fixtures they serve, they can be installed horizontally. But many are sloped, which helps airflow work better in the system. Plumbing codes usually define how sloped these must be.

Types of Plumbing Vents

Residential Plumbing Vent Roof Scaled

Here are the different types of vent pipes and where you can find them:

  1. True vent: It’s a vertical pipe attached to your drain line and the most common vent type. Because no water runs through it, it vents from the roof.
  2. Common vent — Used between two fixtures installed on opposite sides of a wall, like back-to-back sinks. They’re linked to the stack with a sanitary cross.
  3. Auxiliary vent: Attached to the drain line or behind the plumbing fixture, it runs up and over to the central vent that goes to the roof.

Special Venting Systems

You may need a particular type of vent for your specific system. Here are a few examples:

  1. Circuit Vents - Places with many fixtures, such as apartments and buildings may need a circuit vent system. They can minimise the number of exit points required for the many vent stacks of regular vent systems. Each horizontal vent in a circuit system can be connected to eight fixtures. You can connect many circuit vents, which works well in large buildings.
  2. Chicago Loop - These are for some fixtures with specific configurations and incompatible with standard vents. For example, some kitchen islands feature a drain line that cannot run horizontally above floor level. Using a Chicago loop can be the answer. A Chicago loop sees a vent pipe connected under the sink to create a circle above the sink’s floor rim, leading right down into the kitchen’s floor. Then, the pipe can extend out horizontally to connect with the main venting pipe.
  3. Studor Valves/Air Admittance - Rather than using a standard vent for your sink drain, you can use a studor valve. Check your local regulations about these types of valves as they cannot be used everywhere. When water drains down the pipe, the valves open, allowing air in and releasing pressure building up in the pipes. The downside of these is that they can get stuck closed and wear out rather quickly.

What Are the Signs Your Plumbing Vent Pipe Is Blocked?

When vents become blocked, this causes negative pressure to build up in the plumbing system, interrupting water flow.

Here are some signs to watch out for:

  • Water is draining very slowly.
  • Potent smells like sewage are coming up through the drain. If this happens, your P-trap has likely emptied due to negative pressure.
  • Gurgling sounds are coming from your drains and pipes, with water coming out of another drain. The sound happens when air gets sucked out of your P-trap.
  • Stagnant water is sitting in your bathtub, sink or other drains.

Need a Plumbing Service?

Unblocking Vents

The way you clear your vents will depend on why it is blocked. The best method for finding out what is blocking it is using a CCTV drains camera. This must be used by a highly-trained technician, such as one of our plumbers at Fixed Today.

We snake this high-tech camera down your drain pipes. It feeds us live images of the drain line to see what’s happening and helps us come up with a solution specific to your problem.

It will also confirm something wrong with your vents or if the issue is related to something else.

We don’t recommend you deal with vent issues yourself. If you suspect that you are having vent problems in your plumbing system, call us ASAP. We’ll be right over to check if your vents are blocked and fix the issue.

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