I have an AAV connected to my stack in the loft and it has 'ventapipe 50' markings on the top. I noticed some water on the top of the pipe where the seal is on the pipe and wondered how it was getting there. I have then looked up into the AAV and there is quite a bit of condensation (well i think its condensation?!) on the inside of the valve. I have some pictures below showing this - I don't know if this is normal on them or not?? The nearest branch connection to this is approximately 2.5m below the AAV in the bathroom below..