Why a drain needs air
Water flowing through a drain pipe pushes air ahead of it and pulls air along behind it. Especially in a soil stack, where a toilet flush falls at considerable speed, this creates pressure differences. Behind the slug of water negative pressure builds up, in front of it positive pressure. If these pressure differences are not absorbed, they look for a way out via the weakest point in the system: the water seal in your traps.
Good venting lets air flow in and escape freely. That keeps the pressure in the pipes balanced, the water drains away calmly and the traps stay full. The result is a drain that runs quietly and smoothly, traps that do their job and a home that smells fresh, on every floor.
The trap as a water seal
Under every washbasin, kitchen sink and shower and in every toilet pan there is a trap, also called a siphon or U-bend. The water that stays in the bend forms a seal between your home and the sewer. Sewer gases cannot get through. That only works as long as there is enough water in the trap, and venting plays a leading role in that.
With negative pressure in the pipe the water is sucked out of the trap, which is called siphonage. With positive pressure air is forced out through the water seal, which you hear as a bubbling or gurgling sound. In both cases an open connection to the sewer forms and sewer smells enter the home. A trap that is not used for a long time, for example in a guest bathroom, can also run dry through evaporation.
You recognise a dry trap by a sewer smell that suddenly appears in a room that has not been used for a while. Run the tap for half a minute and flush the toilet once. If the smell disappears, evaporation was the cause. If the smell keeps coming back while the drain is used regularly, it makes sense to look at the venting.
Venting through the roof
The soil stack in a home usually continues to above the roof. This top section is called the vent pipe or roof vent. Through this open connection air can flow freely in and out of the system, under both negative and positive pressure. Sewer gases are released above the roof, where they disperse unnoticed. The Dutch standard for internal drainage, NEN 3215, assumes good venting of the system, in practice usually at least one open vent to above the roof.
A roof vent can become blocked over time by leaves, birds' nests or, in winter, ice. During a renovation it is sometimes capped or shortened. If you notice drains starting to gurgle after a roof renovation or loft conversion, it is worth having someone check whether the vent is still clear.
Air admittance valves: a valve that only lets air in
An air admittance valve is a small valve fitted on a drain pipe. With negative pressure the valve opens and lets in air from the room, so the trap is not sucked dry. As soon as the pressure is balanced again, the valve closes and no sewer smell can escape. Air admittance valves are often used on long drain pipes, at a kitchen island or on a soil stack indoors.
An air admittance valve only absorbs negative pressure. It cannot release positive pressure, because the valve does not let air out. That is why an air admittance valve supplements, rather than replaces, the open vent through the roof. Air admittance valves also do not last indefinitely: the membrane can dry out or stick. A valve that stays open lets sewer smells into the room, and one that is stuck no longer protects the traps.
Signs that the venting is not working properly
- Gurgling or bubbling sounds from a drain while water is discharged elsewhere
- The water in the toilet pan drops or moves after another toilet is flushed
- A sewer smell that comes and goes, often around the shower or washbasin
- A drain that empties in fits and starts instead of evenly
- A sewer smell in the meter cupboard or loft, near the soil stack
Gurgling can also be caused by a partial blockage further along the pipe. A narrowing acts like a plug that obstructs the airflow. So with these signs it is sensible to look not only at the venting but also at the flow through the pipe itself. A CCTV drain survey quickly makes that distinction clear.
How to get a drain that runs calmly again
Start with simple checks. Run the tap regularly in drains that are rarely used, so the trap stays full. Check whether the air admittance valves in the house are accessible and whether you smell sewage near them. An air admittance valve that no longer closes properly is usually simple to replace. Preferably have the roof vent checked by someone who can work safely on the roof.
In an apartment building the venting of the soil stack runs via the communal roof. Gurgling or sewer smells for several residents at once then often point to the soil stack or the roof vent, and that usually falls under the owners' association (VvE). In that case report it to the managing agent, so the cause is tackled for all homes in one go.
If the signs keep coming back, Bosmans Rioolservice helps trace the cause of sewer smells and gurgling drains. Our engineers unblock the pipe where needed with spring cable or high pressure equipment and map the soil stack and drains with an inspection camera. That makes it clear whether it is a narrowing, a loose joint or the venting, and you know exactly which step lets your drain breathe again.