2.1 Drainage Ventilation Systems, Relief Vents & Air Admittance Valves (AAVs)

Drainage ventilation systems

Ventilation protects water seals and gives pressure changes in the foul-water system a controlled path to atmosphere. It is not an optional accessory added after pipe sizing. The certifying drainlayer should identify the compliance path first, then apply that path’s sizing, location, and termination rules consistently.

G13/AS2 drainage ventilation

Under G13/AS2, every main drain must be ventilated, and a branch drain longer than 10 m must also be ventilated. The main vent has a minimum internal diameter of 80 mm. It is positioned so the length of drain upstream of the vent connection is less than 10 m. These are drainage rules and should not be replaced by a DN50 value taken from a different venting table or installation context.

The vent connection and route should rise so condensate can return to the drainage system without forming a water pocket. Protect the pipe against damage and preserve access to components that require maintenance. At design review, check the developed length, offsets, diameter reductions, and whether another compliance path imposes additional sizing rules.

Terminal location

G13/AS1 gives useful terminal clearances for open vents. A terminal is at least 3 m above ground. It is at least 150 mm above the roof surface through which it passes. Around an opening into a building, the terminal must be outside the exclusion zone: not less than 600 mm above the opening and 3 m horizontally or below it. A pedestrian deck has a 3 m exclusion distance in any direction. A mechanical air intake requires 5 m separation.

Apply the actual geometry, not a memorised slogan. For example, “3 m from a window” is incomplete because a terminal sufficiently above an opening is treated differently by the diagram. Confirm which Acceptable Solution governs the system and whether the building design or consent calls for larger separation.

Pressure behaviour

A rapid discharge can entrain air and create negative pressure behind the flow. A downstream restriction or surcharge can create positive pressure. Either can disturb a trap seal if the system lacks effective ventilation. The water seal is the barrier against foul air; therefore, the design checks pipe size, branch length, trap arrangement, and ventilation together.

An air-admittance valve opens under negative pressure and closes when pressure equalises. It cannot release positive pressure and is not an open terminal to atmosphere. Use an AAV only where the selected compliance path, product certification, manufacturer instructions, accessibility, temperature range, and maintenance conditions permit it. Do not use one to erase the G13/AS2 requirement for a main drain vent.

Site inspection

Before enclosure, confirm the vent is connected to the intended foul system, not stormwater; its diameter and route match the plan; it rises continuously where required; the terminal meets the relevant exclusion zones; and no later roof, window, intake, or deck change has made the location non-compliant. Verify penetrations are flashed under the roofing system requirements.

A smoke test may help locate an odour path, but test smoke must be suitable for the task and the process controlled so it does not expose occupants. AAV diagnosis includes checking orientation, accessibility, contamination, and manufacturer limits. Do not assume an odour proves an undersized vent: a dry trap, failed seal, cross-connection, open access cap, or damaged pipe may be the cause.

Worked review

A main drain runs 16 m upstream from the proposed vent connection. Under G13/AS2, that position fails the rule that the upstream length be less than 10 m. Move the vent connection or revise the drainage layout. If the vent passes through a roof 2 m horizontally from an opening at the same level, it is inside the opening exclusion zone; relocate it or extend it so the diagram’s vertical clearance is met. Record the final location on the as-built plan.

Independent source-check exercise

For 2.1 Drainage Ventilation Systems, Relief Vents & Air Admittance Valves (AAVs), practise answering from evidence rather than recall. Write the controlling source, its edition or effective date, its scope, and the exact paragraph, table, figure, or manufacturer instruction that supports the decision. Then state the site inputs that can change the result. In this section those inputs include Under G13/AS2, every main drain and every branch drain longer than 10 m is ventilated.; The main drain vent has a minimum internal diameter of 80 mm under G13/AS2.; Vent-terminal exclusion zones protect openings, pedestrian areas, and air intakes; dimensions must be read from the applicable solution..

Build a four-column check: proposed condition, source requirement, evidence observed, and action. Test the answer with one changed assumption. A different pipe size, discharge-unit load, ground condition, licence status, authority, compliance path, or consent condition may produce a different result. If it does, explain why; if it does not, show which requirement remains controlling.

Finish with a field-verification plan. Name what must be inspected before concealment, what instrument or record demonstrates it, who has authority to accept a change, and what appears on the as-built or completion file. This exercise turns an open-book fact into certifying-level judgement. It also exposes accidental mixing of standards: if the source for a dimension differs from the source for the test or approval, either return to one complete compliance path or document the proposal as an Alternative Solution. Do not promote an example value into a national rule.