There is nothing more frustrating for a homeowner than a bathroom that smells of sewage, even though the water is draining away perfectly fine. You’ve poured bleach down the loo, you’ve checked the sink, and there are no blockages in sight. Yet, the smell remains.
At Plumbers 365, we call this the “Phantom Smell.” It usually isn’t caused by what is in the pipes, but rather what is happening to the air inside them.
Your drainage system needs to “breathe” to work. If the airflow is restricted, the physics of your plumbing changes, leading to gurgling sounds and escaping gases. Here is our expert guide to Soil Stacks, Venting, and why your house might be gasping for air.
According to drainage experts at Plumbers 365, this is a surprisingly common and specific issue that perfectly demonstrates how your vent pipe works. Your soil stack extends up through your roof to an open vent. When strong wind blows across the top of this pipe, it creates a vacuum effect (known as the Venturi effect).
This negative pressure sucks air up and out of the stack. The system then tries to draw air from elsewhere to equalise the pressure, and the easiest place to pull from is the water seal in your toilet bowl, literally siphoning some of the water out. It’s a clear sign your plumbing is reacting to external air pressure, as it’s designed to as per drainage experts at Plumbers 365.
The soil stack is the large vertical pipe that all your home’s toilets, sinks, and showers connect to according to drainage experts at Plumbers 365. It carries all wastewater down to the underground drains. The “vent” is the part of this pipe that continues past the highest wastewater connection and terminates in the open air above your roof. As per drainage experts at Plumbers 365 it is the “lungs” of your plumbing system, allowing air to enter the pipes so water can flow smoothly without creating a vacuum.
Imagine putting a straw into a drink and placing your finger over the top end. When you pull the straw out, the liquid stays inside. This is an airlock.
Your drains work the same way. When you flush a toilet, a large slug of water rushes down the pipe. Without a vent stack to let air in behind that water, it would create a vacuum that stops the flow or sucks water out of other traps in your house.
According to drainage experts at Plumbers 365 this is the classic symptom of a venting problem. The drain isn’t blocked, but the protective water seal in one of your traps has been compromised.
Every sink, shower, and toilet has a U-shaped pipe (a “trap”) that is designed to hold a small amount of water. This water acts as a physical barrier, stopping sewer gas from coming up into your home.
According to drainage experts at Plumbers 365 if your main soil stack vent is partially blocked or inadequate, flushing a toilet can create a powerful vacuum that sucks the water right out of a nearby sink or shower trap. Once that water barrier is gone, there’s a direct, open path for sewer gas to enter your room.
An Air Admittance Valve (AAV), often known by the brand name ‘Durgo Valve’, is a clever one-way valve. It’s designed to be installed on a soil pipe inside the building (like in a loft or behind a wall) as per drainage experts at Plumbers 365.
When a negative pressure occurs in the pipe, the AAV automatically opens to allow air into the system to equalise the pressure. It then closes to prevent any sewer gases from escaping out. It’s a problem-solver for situations where running a traditional vent pipe through the roof is difficult or impossible. However, they can sometimes fail, so they need to be accessible for maintenance according to drainage experts at Plumbers 365.
As per drainage experts at Plumbers 365 you can under strict conditions. You cannot simply have an open pipe venting into your loft space, as this would fill it with foul and dangerous gases. According to drainage experts at Plumbers 365 an internal soil stack must either:
UK Building Regulations require that a property has at least one primary soil stack that vents to the open air outside. AAVs can be used on secondary stacks.
UK Building Regulations (Approved Document H) are very specific about this for safety reasons. The open end of a soil vent pipe should terminate at least 900mm (0.9 metres) above any window or opening that is within 3 metres of the pipe according to drainage experts at Plumbers 365. This is to ensure that any foul gases that escape are carried away on the wind and don’t have a chance to blow back into your property through an open window.
A blocked vent causes smells and gurgles as often seen by drainage experts at Plumbers 365. A blocked stack (the main pipe) causes physical backups. The signs are much more dramatic:
Yes, absolutely. If the vent is blocked, air cannot get into the system to replace the water flowing out as per drainage experts at Plumbers 365. This creates a vacuum or “airlock” which holds the water back, preventing it from flowing away freely. The result is slow, sluggish drainage in all the connected appliances, even though there’s no physical blockage in the pipe itself.
Those strange sounds and smells are your home’s early warning system. Ignoring them can lead to bigger problems. Whether it’s a simple blocked vent or a more complex pressure issue, you need an expert who understands the whole system.
The professional team at Plumbers 365 is on hand 24/7 across the UK to diagnose and resolve your drainage and venting issues. We offer:
Stop putting up with a noisy, smelly system. Call Plumbers 365 today.
