- This topic has 1 reply, 1 voice, and was last updated 2 days, 1 hour ago by
.
-
Posts
-
Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.
On a residential drainage design I need to size the vent pipes and decide whether air admittance valves (AAVs) are acceptable instead of extending every vent through the roof.
- How are the stack and vent sizes chosen under EN 12056-2, and how does that compare with the IPC approach?
- Where are AAVs allowed, and where are they not?
- What are the usual rules for the vent terminal on the roof?
A short worked example for a typical apartment stack would help.
Size the stack for its waste water flow so it stays within the capacity of a ventilated stack of that diameter, and size vents from the connected load and length. AAVs can replace many branch and stack vents, but they only admit air: the system still needs at least one open vent to atmosphere and a way to relieve positive pressure.
EN 12056-2 approach
- Add the discharge units (DU) of the fixtures on the stack, using the values for your system type (System I is common in continental Europe and Turkey, System III in the UK).
- Waste water flow: Qww = K·√ΣDU, with K = 0.5 for dwellings.
- Choose a stack whose permitted flow for the chosen ventilation arrangement (primary or secondary ventilated, with or without swept entries) is at least Qww, and never smaller than the largest connected drain (DN100 where a WC is connected).
Worked example
A stack serves 10 flats, each with a 6 l WC (2.0 DU), basin (0.5), shower (0.6), kitchen sink (0.8) and washing machine (0.8), System I values.
- Per flat: 2.0 + 0.5 + 0.6 + 0.8 + 0.8 = 4.7 DU; 10 flats: ΣDU = 47.
- Qww = 0.5 × √47 = 0.5 × 6.86 ≈ 3.4 l/s.
- EN 12056-2 gives a primary ventilated DN100 stack with ordinary branch entries about 4.0 l/s, so DN100 with the stack vent continued to the roof at full size is adequate.
On the drawings, label each branch that serves two or more fixtures with its total DU and DN so the checker can follow the sizing.
IPC approach
- Vents are sized from the drainage fixture units (DFU) served and the developed length of the vent (IPC Chapter 9).
- A vent is not smaller than half the diameter of the drain it serves and not smaller than 1¼ in (32 mm).
- Terminals extend at least 6 in (150 mm) above the roof, more where the roof is occupied or snow is deep, and are not within 10 ft (3 m) horizontally of an openable window or air intake unless at least 2 ft (0.6 m) above it. Local amendments often increase these.
Air admittance valves
- Product standards: EN 12380 in Europe; ASSE 1050 (stack type) and ASSE 1051 (fixture and branch type) under the IPC.
- Install them accessible, in a ventilated space, above the drain they protect (IPC: branch type at least 4 in / 100 mm above the horizontal branch or fixture drain; stack type at least 6 in / 150 mm above the flood-level rim of the highest fixture).
- Keep at least one open vent to atmosphere on each building drain, normally the stack vent of the largest or furthest stack.
- AAVs do not relieve positive pressure. In tall stacks, where surges are expected, add relief venting or a positive pressure attenuator.
- Do not use them on sumps, ejector pits or grease interceptor vents unless the product and the local code explicitly allow it.
Common mistakes
- Reducing the stack vent diameter above the highest branch.
- Replacing every roof vent with AAVs, leaving no open vent.
- Placing AAVs in sealed boxes without access or ventilation.
- Terminating vents next to openable windows or fresh air intakes.
Related: Plumbing Vent Pipes: Why, Where and How Big, Drainage Pipe Sizing: EN 12056 DU vs IPC DFU.
Check the edition of the code that applies to your project.
- You must be logged in to reply to this topic.