Vent Cowls & Air Admittance Valves
Every discharge stack must be ventilated so that water seals are not siphoned or blown. It is terminated either to open air with a vent cowl, or inside the building with an air admittance valve where a roof termination is not practical.
Also searched as: vent cowl, vent terminal, balloon grating, AAV, air admittance valve, durgo valve, weathering slate
Vent cowl and weathering slate
The vent cowl caps the stack above roof level and keeps birds and debris out while leaving the full bore open to atmosphere. It is paired with a weathering slate or collar that seals the pipe where it passes through the roof covering. The terminal must be positioned clear of any opening window or air intake by the distance the authority requires.
Air admittance valves
An AAV lets air into the stack when negative pressure develops, but seals against foul air escaping. It is the answer for a stack that cannot reasonably be taken through the roof — a refurbished apartment, an island kitchen run, a stack terminating in a ceiling void. Note that an AAV admits air only; every drainage system still needs at least one open vent to atmosphere to relieve positive pressure.
Active drainage
On tall stacks with heavy simultaneous discharge, positive transient pressures are as damaging to seals as negative ones. Discuss stack design with our technical team before relying on AAVs alone in a high-rise.