air admittance valve not happy!

[barryplumb]

Went to customer this week who has had a downstairs cloakroom built about three years ago as a new build extension with a toilet and basin.
The toilet is connected to a sub stack with an aav fitted, then waste runs underground the full length of the house to the main waste .
Thay also have an upstairs bathroom with waste outlets going to a full height stack with vent at roof height.
The cloakroom toilet has started to back up when flushed and drains away slowly but if the aav is removed it flushes perfectly! the moving part of the aav is being pulled down by suction rather than lifting to let air in! there is no blockage between the pan and soil pipe.
It has been working ok since installed for three years but only started this recently.

Any ideas!

Thanks in advance for any replies

 

[Resolute]

Nackered AAV, replace it.

 

[Guest123]

Replace it with a Floplast A rated AAV. These can be fitted anywhere, even below the flood level of the highest appliance and are brilliant. Available from Screwfix.

If the AAV is boxed in, it will need some form of air flow for it to work. Fit a grille to allow air to circulate to it.

 

[barryplumb]

Thanks for the replies, I did fit a new aav but it made no difference. Anyway I went back today and its working fine, maybe it was ablockage further down the soil pipe that has cleared itself.