How a single-stack above-ground drainage system works and why the stack is ventilated.
Most modern above-ground drainage uses a single-stack system, and Level 2 expects you to understand how one vertical stack can take both soil and waste and why it has to be ventilated.
In a single-stack system, the discharge from WCs, the soil, and from basins, baths and sinks, the waste, connects into one vertical discharge stack that carries everything down to the drain. It is simpler and uses less pipework than older two-pipe and separately ventilated arrangements.
The catch is air pressure. When water falls down the stack it drags air with it, creating suction above the flow and a pressure rise below it. Left unmanaged, those pressure swings would siphon or blow out the trap seals on connected appliances.
Ventilation is the answer. The stack is ventilated so air can move in and out to balance those pressures, which keeps the trap seals intact. The branch connections are also designed and positioned to limit how much one appliance disturbs another.
Traditionally the stack continues upward, beyond the highest connection, and terminates in open air above the roof, well clear of any window or opening so smells disperse safely and the open top lets air move in and out.
Where carrying a vent up through the roof is not practical, an air admittance valve can be fitted to let air in when the pressure drops while staying closed otherwise, though it does not relieve positive pressure the way an open vent does. Confirm the arrangement, branch limits and terminal positions against the current drainage guidance.
At the foot of the stack
The bend where the stack turns into the drain should have as large a radius as possible - at least 200 mm at the centreline. In a house of up to three storeys, no branch should join the stack lower than 450 mm above the invert of the tail of that bend. Both rules exist for the same reason: a heavy discharge compresses the air low in the stack, and an easy bend with the lowest branch well clear of it reduces the risk of that pressure disturbing nearby trap seals.
Stub stacks
A stub stack is a short length of discharge stack with no ventilation of its own. It can only be used where it connects into a ventilated stack, or a ventilated drain that is not liable to surcharge, and only within set height limits measured up from the invert of that connection: the floor level of any connected WC no more than 1.3 m, and the centreline of any other branch no more than 2 m. Beyond those limits the stack needs its own ventilation.
Fitting an air admittance valve
Select the valve's classification, airflow capacity and installation height for the system. Some valves require a position above the appliance flood level; BS EN 12380 class A valves may be rated for positions below it. Follow the particular product's instructions and temperature limits. Keep the valve in a ventilated space, accessible and removable for maintenance, and protect it from contamination.
Approved Document H's described arrangement uses internal valves and excludes external or dusty locations. Purpose-designed external products exist, but their use needs the manufacturer's conditions and acceptance for the proposed design; FloPlast advises confirming acceptance with building control before using its external model. An indoor valve is not made suitable for outside simply by adding a cover. The drainage still needs provision for positive pressure and below-ground ventilation, since an AAV admits air but cannot release it.
Ending the vent in open air
Where a stack vents to outside air, the open end must finish at least 900 mm above any opening - a window, for example - that is within 3 m of it, and carry a wire cage or perforated cover that does not restrict airflow. In a one- or two-storey house the dry part of the stack above the highest branch may be a smaller diameter than the stack, but never less than 75 mm.