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                                                                                                                                                                 Updated 26 January, 2014 > site map > air admittance valves                                                    

Air Admittance Valves (AAV) for Drainage Systems

The air pressure within a drainage system can fluctuate and an air admittance valve is designed to allow air into and out of the system when negative pressure occurs, approved Document H of The Building Regulations 2000 (2002 edition) requires that air admittance valves comply with BS EN 12380:2002

air admittance valve at draindomain.comWhen a drainage system is partially blocked or there is a dipped section of pipe work air becomes trapped between the incoming water and the water standing within the pipe work and it will often be forced up through a water trap on a shower, sink, bath or toilet.

The spring loaded AAV opens to allow air out of the system when required but remains closed and air tight for the remainder of the time in order to prevent internal venting, they can also be used in en suite bathrooms and additional washrooms when it is not practical to either connect into the existing soil vent pipe or to install a new one.

External Air Admittance Valves

air flow contraol in drainage systemsUp until recently these plumbing vent valves were for internal use only as frost and atmospheric debris can affect them however there are manufacturers who state the air admittance valves are tested from -20ºC to + 60ºC so they are therefore suitable for external use

They are also known to occasionally fail in loft and roof spaces due to a build up of dust or insulation fibres within the valve itself so some occasional maintenance may be required, they can be easily dismantled and cleaned before being replaced.but given the price of them it would be prudent to replace an ageing valve with a more modern unit.

Positioning an Internal Air Admittance Valve

We have always been advised to install an internal air admittance valve above the highest flood level in a bathroom, en-suit etc, so if you have a toilet, bath and hand basin in the one room the valve should be higher than the maximum water level achievable within the hand basin. The reason for this is that if the system blocks and the valve is lower than the hand basin you could potentially be losing water out of the boxed in or hidden valve without knowing it.

However some manufacturers have now developed valves that will remain water-tight in this scenario so you can technically put the valve where you want as long as you have the correct product, but unless you have a specific reason not too it would always be good practice to have the valve as high as possible.

Valves are available in 32mm, 40mm, 50mm, 82mm and 100mm in both solvent weld and push fit pipe work, they must always be installed in vertical pipe work.

Air Admittance Valves On Traps

anti-vac bottle trap draindomain.comThere are numerous anti-vac bottle traps available that have a built in AAV and i would always recommend that if your having a full refit of a bathroom or cloakroom that they be used, the additional cost of these valves in nominal and the labour involved in the installation is the same as a traditional trap.

An AAV fitted in the loft can often be sited just out of reach so by fitting one of these traps you can over come a scenario where the existing valve can not be replaced or serviced, if your existing valve is boxed and tiled in or you have an external AAV that fails a couple of times a year due to frost fitting one of these could be a cheaper and less disruptive option compared to dismantling the bathroom.

Basic Investigation Techniques for Faulty Air Admittance Valve

  • If you are experiencing internal venting problems you can do a couple of things to find the cause, first of all try to establish if you have a traditional Soil Vent Pipe or if you have an AAV fitted.
  • Flush a toilet in the problem bathroom and watch the water in the bath/sink or shower trap, if there is plenty of movement or the water level drops this would indicate an air flow issue.
  • When the venting occurs run the water in the bath/sink/shower to top the level in the trap, or put the plug in and cover with an inch of water, if the venting stops this would again suggest an air flow issue.
related pages - internal drain venting problems
  soil vent pipes
  find a drainage contractor


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