A guide to vent cowls

Sewer network planning and choosing the right vent cowl option may not be a topic of conversation that you’d have every day, yet the process’s importance affects everyone.  As vent shaft design, fabrication, installation and maintenance specialists here at SVSR, here is a guide to vent cowls, what vent cowls are and why they are important as choosing the wrong vent cowl could make a sewer network problem worse.

Rusty and broken vent cowl post

The science behind vent cowls

To understand vent cowls and their purpose, you need to understand the science behind sewer network infrastructure and design. One of the most critical elements of any sewer network is its ventilation infrastructure. Not only for the obvious reasons of corrosion and odour control but also for the maintenance of the entire system.

Hydrogen sulphide (H2S) is also known as sewer gas or swamp gas. Sewers naturally ‘breathe’ and gravity sewers drag the air with the water flow that provides some level of circulation. With insufficient ventilation or airflow, anaerobic decomposition occurs, which generates hydrogen sulphide..

Ventilation and wind flow rates in a sewer network are vital, and using quality steel and materials is key.

Stocks of exhaust vent cowls

Educt vs Induct

Educts or exhaust vents allow air flow out of a sewer system, while induct vents are designed for air intake.

Educt sewer vent cowls can be active or passive, depending on the requirement.

Induct sewer vent cowls can be fitted with a wind vane to the rear that ensures the cowl spins to face the direction of the wind, forcing wind into the vent, which reduces septicity, corrosion and concrete cancer. They can also be customised ground-mounted vent cowls.

Vent cowl placed at the backyard

Which vent cowl type is used where?

Generally speaking, active educt cowls often referred to as ‘whirlybirds’ are most useful when installed on the top of a tall vent as they are wind-driven and tend to increase the amount of gas leaving a sewer network. These educt cowls have a service life of 10-15 years, due to the moving parts and corrosion risk.

Passive educt cowls are recommended in most cases. They require lower maintenance as there are no moving parts, therefore they last longer.

With careful consideration, we recommend using induct cowls if a vent is located at the top of a gravity sewer. Gas flows with the water so an induct vent cowl is critical to dilute the gas along the network.

Cowls are also vital to help keep birds from falling into vent lines. In some parts of Australia, cowls are actually called ‘Birdstops’.

mini crane supporting a vent cowl post

A changing sewer network

Duncan Reynolds, our R and D Manager believes that managing a network involves a careful balance of long-term cost with managing residential and stakeholder requirements.

This has to occur within a changing network and with access to the best possible data to inform good decision making.

The expert approach

As vent shaft design, fabrication, installation and maintenance specialists, we believe there is no ‘one size fits all’ solution to vent cowls. A detailed analysis of the data is required to make informed decisions. The age of an area, size of the property, the number of people in a specific area, whether the sewer network is new or old and whether it feeds into an older sewer network are all educated, data-based decisions that need to be made when establishing and maintaining a sewer network.

If you would like expert vent cowl advice for your sewer network, we would love for you to contact us.


Enquiry for: Sewer Vent Design, Fabrication, Installation

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29 Shepherd St, Liverpool
NSW 2170

02 8798 8788