How to Fit an Air Admittance Valve — UK Trade Guide
An AAV lets air into your drainage without letting sewer gas out. Fitted at the top of a soil stack or under a hidden basin. Here is the compliant UK method.
Step-by-step method
1. Choose the right AAV size
32/40mm for a single sink/basin. 82/110mm for a full soil stack. McAlpine MRAA and Durgo are the UK go-tos.
2. Position above the highest branch
Must be at least 200mm above the top waste branch entering the stack — otherwise it will get flooded.
3. Ventilated space
Loft installation needs a warmed roof space or the valve seizes in winter. Alternatively fit inside a boxed-in vent within the building envelope.
4. Solvent weld or push-fit
On 110mm soil pipe, use a strap-boss and solvent-weld boss adapter with the AAV on top. On 40mm waste, push-fit is fine.
Frequently asked questions
Are AAVs allowed under UK Building Regs?+
Yes — permitted under BS EN 12380 and Building Regs Part H. Cannot be the ONLY vent on a whole drainage system — main stack must terminate as an open vent OR AAV, but a secondary main vent is often required.
Can I replace an open vent with an AAV?+
On a single-dwelling stack, yes if the drainage system has adequate ventilation. Best to consult Building Control for anything complex.