110mm AAV with wrongly sloped horizontal pipe or 40mm AAV

[jacoscar]

I am in a bit of a pickle with the AAV. The soil stack in on the left corner behind the bath (see pictures).
Previously there was a section of 40mm pipe on top of it with a small AAV. We never had a problem with it.
I understand 40mm don’t provide sufficient air flow (although we never had a problem) so I was keen to install a 110mm AAV.

There isn’t much space to install it directly over the soil pipe and even if I managed, it would be subject to splashes of water and difficult to access, so I bought a flexible 110mm pipe (https://www.screwfix.com/p/alpha-flexi-pipe-110mm-x-455mm/23342), a 1m section, a couple of bends and managed to get the AAV on the other side of the bath, which has more space for possible maintenance. The only problem is that the pipe is sloping downwards before reaching the AAV and I’m worried condensation could fill it with water with no means to escape, blocking airflow and making it useful.

There are no options to extend the 110 mm pipe up to the roof.

My only options are:

- 40mm vent on top of the soil stack: it should be accessible for maintenance for someone with long arms
- 110mm vent on the right side of the bath: fully accessible, but condensation might occur (?)

 

[Hugh Jaleak]

40mm AAV should be enough for what it needs to do, if you still want to upsize, try a 50mm version. By your own admission you have never had an issue so I rather think you are making unnecessary work for yourself.

 

[jacoscar]

40mm AAV should be enough for what it needs to do, if you still want to upsize, try a 50mm version. By your own admission you have never had an issue so I rather think you are making unnecessary work for yourself.

Is 50mm a standard pipe size? I’ve only ever used 40 and 32mm

 

[Madrab]

If you are to follow

Is 50mm a standard pipe size? I’ve only ever used 40 and 32mm

Yep - Largest domestic waste pipe size you'll get

SF actually do a universal AAV for all 3 normal waste pipe sizes

https://www.screwfix.com/p/floplast-solvent-weld-air-admittance-valve-white-32-40-50mm/42968

 

[jacoscar]

If you are to follow

Yep - Largest domestic waste pipe size you'll get

SF actually do a universal AAV for all 3 normal waste pipe sizes

https://www.screwfix.com/p/floplast-solvent-weld-air-admittance-valve-white-32-40-50mm/42968

I’ve seen that one, but if it works with a 32mm pipe doesn’t it just mean that the extra section of a 40 or 50 mm pipe would go unused? I might as well use the 40mm leftover I have then

 

[steven lee]

Go with the 40mm straight up off the stack. You want the AAV as high as you can get it, ideally above the flood level of the highest fitting, and a vertical run avoids the whole wrong slope worry entirely. The 110mm across the bath will work but any sag in that run will hold condensation and slime up over time. 40mm has vented that stack fine for years so I would not overthink it.

 

[denso13]

I understand 40mm don’t provide sufficient air flow (although we never had a problem)

Then just go with that.

 

[Madrab]

I’ve seen that one, but if it works with a 32mm pipe doesn’t it just mean that the extra section of a 40 or 50 mm pipe would go unused? I might as well use the 40mm leftover I have then

That was just explaining that 50mm was an option and is the largest of waste pipe sizes and that the one from SF comes with a reducer I believe that drops the size to either a 40mm or 32mm, so I would presume the size of the AAV itself would be 50mm.

As other have suggested though if you haven't had issues with the 40mm up till now then no reason it should cause issues in the future, if you continued to use it.

 

[cdbe]

I've got one of these with two x 40mm.