Air Admittance Valves (AAV)

When an upstairs toilet is flushed a plug of water can be formed as it drops down the soil vent pipe, this plug of water can momentarily leave a vacuum behind it as it drops unless there is a sufficient air flow through the system. This vacuum will empty internal sink, bath and shower traps leaving a direct vent into the property.

Likewise if a system is partially blocked air becomes trapped between the incoming water and the water standing within the pipe work, this foul air is then displaced and normally it would vent through the soil vent pipe and up into the atmosphere.

As most properties have external soil vent pipes which vent above the eaves this is not an issue however internal SVP`s often finish beneath the roof line and an air admittance valve is fitted. The spring loaded valve opens to allow air into or 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 SVP or to install a new one.

Air Admittance Valves For Internal Use Only

These valves are for internal use only unless the manufacturer states otherwise as they fail when subject to a heavy frost of freezing weather, 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 but they can be easily dismantled and cleaned before being replaced.

Siting An internal AAV

When installing an internal air admittance valve it should be sited above the highest flood level, or in other words if you have a toilet, bath and hand basin the valve should be higher than the maximum water level achievable within the hand basin.

Air Admittance Valves On Traps

There are 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.

Basic Investigation Technique For A 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.