Drainage Air Admittance Valve

Air admittance valves in 40/50 mm for waste and 110 mm for soil. The valve lets air into the pipe to break the vacuum that would otherwise siphon a trap dry, without needing a vent pipe up through the roof. Check before ordering: It admits air but does not let foul air out, so it cannot replace the main soil vent. It must also stay accessible — a valve buried in a sealed void cannot be replaced.