A vent pipe is a critical part of a soil and vent system in plumbing. It plays a key role in regulating the pressure within the drainage system, helping to maintain a smooth flow of waste water and preventing unpleasant smells from entering the home.
What Does the Vent Pipe Do?
The primary purpose of a vent pipe is to regulate pressure in the drainage system. Here’s how it works:
Positive Pressure: When water is flushed down the soil pipe, a surge of positive pressure can form in front of the water flow. In extreme cases, this pressure can cause wastewater to flow back into fixtures, like sinks or toilets, creating a health hazard. While this is typically a concern in high-rise buildings, it’s avoided by using long-radius bends at the foot of the soil pipe to maintain smooth flow.
Negative Pressure: More commonly, negative pressure occurs when the water flow creates a siphoning effect, pulling the water seal out of a trap (such as in a sink or toilet). This breaks the seal, allowing unpleasant drain smells to enter the home. The vent pipe prevents this by letting air into the system, balancing the pressure and preventing siphoning.
Does the Vent Pipe Need to Finish Above the Eaves?
Yes, vent pipes generally need to terminate above the eaves of a building, as per building regulations. If the vent pipe is located within 3 meters of an open window, it must terminate at least 900mm above the eaves. This ensures that no foul smells are drawn into the home through the window.
However, if you want to avoid the vent pipe extending high above the eaves, an External Air Admittance Valve (AAV) can be installed. The AAV allows air to enter the pipe and balance pressure but prevents foul gases from escaping, essentially acting as a one-way valve.
Is the Vent Pipe Different from a Normal Soil Pipe?
The vent pipe is essentially part of the soil system, but it is specifically the section above the highest wet point (the highest point where water can flow). The pipe itself is identical to the rest of the soil pipe in terms of material and design. It’s typically made from UPVC, which is durable and resistant to corrosion.
Colours of Soil and Vent Pipes
Soil and vent pipes come in a range of colours to suit different aesthetic and functional needs:
The black and grey versions are popular for external installations, while white is a common choice for interiors.
What Happens If the Vent Pipe Becomes Blocked?
If a vent pipe becomes blocked, the system cannot regulate air pressure properly, which can lead to siphonage. This siphoning effect causes trap seals to fail, allowing drain odours to enter the home, particularly in bathrooms and kitchens. Additionally, you might notice air bubbles appearing in the WC pan when the cistern is flushed.
Most blockages in vent pipes are caused by nesting birds. To prevent this, you can install a Vent Terminal on top of the pipe. This is a simple, inexpensive cage-style fitting that prevents birds from entering and blocking the pipe.
Key Points to Remember
Vent pipes help regulate air pressure in the drainage system, preventing issues like siphonage and unpleasant smells.
Regulations require vent pipes to be above the eaves if located near windows.
An Air Admittance Valve can replace the need for a high-level vent pipe in some installations.
Vent pipes are the same as regular soil pipes, made from high-quality UPVC.
They come in a variety of colours, including black, white, grey, and anthracite grey.
A blocked vent pipe can cause air pressure issues in the drainage system, often due to nesting birds, which can be prevented by installing a Vent Terminal.
Understanding how vent pipes work and when they are necessary can help ensure your drainage system functions correctly, preventing potential issues like odours and backflow.
