Reducing stack size & venting with AAV.

[Kasheen]

Hello all,

Quick Version:
A ~110mm cast iron stack appears from my basement, can I reduce it to 40mm pvc and put a 40mm AAV on top of it if all appliances connected only need 40mm?

Full Version:
I have a bathroom which is being turned into a laundry. Currently the WC in the bathroom is serviced by a cast iron stack (around 110mm but not quite) which comes up from the basement below.

Currently the cast stack bends off through the bathroom and goes out of the exterior wall to vent. What's odd is that it just appears out of the exterior wall, it doesn't turn and go up and is below opening windows. Perhaps the rules for this sort of thing were different when this house was built (1900s).

The stack has a crack in it where it appears from basement floor, so I'm going to join in some PVC. For space saving in the laundry room instead of bringing the stack up to the room in full size I'd like to only bring up 40mm for a washing machine. I also want to eliminate this odd vent pipe coming out the side of the house and use an AAV instead.

If I convert the stack down to 40mm and bring it up into the laundry in this size, can I cap it off with a 40mm AAV, or do I need to vent the stack suitably for the widest part (or 75mm minimum if I'm reading regs correctly)? The stack would service a single appliance in 40mm only.

I've drawn a picture to help explain, thanks for any insights.

 

[muggles]

Unless you have a second stack vent somewhere serving your property, you should retain the existing one as it will be ventilating your entire waste system - everything is connected together underground. 75mm is indeed the minimum size, and it needs to be an open end which lets air both out and in, not an AAV which only lets air in.

Be aware that alterations to a soil stack are notifiable to Building Control

 

[Kasheen]

There are other stacks on the house which are open ended to allow for air release, does this specific stack still require to be open ended, my understanding is the house just needs at least one?

If this one needs to be open ended as well, then I'm a bit lost on why larger sized AAVs exist, but perhaps that's a wider topic than I realise .

Assuming it's ok to use the AAV since there are other open ended stacks on the building, and I need to keep a diameter of at least 75mm, does anyone know if it's allowable to have the 40mm waste branch extend above the vent of the stack in a bid to have only 40mm come up into the laundry room?

Like this:

I'd intend to use A1 rated AAVs since the 75mm one on the large part of the stack would be below flood level.

Failing this and assuming the AAV is usable in my scenario I'll look into just bringing the 75mm section of the stack up through the floor and putting a 40mm boss on it.

Doing a bit of reading I think the regs on this might be different for Scotland, I'm going to go read the following:
http://www.gov.scot/resource/buildingstandards/2016Domestic/chunks/ch04s08.html
http://www.amac.md/Biblioteca/data/17/10/27.pdf

In the mean time if anyone else has any further comments, I really appreciate them. Thank you for your advice so far @muggles.

 

[Kasheen]

Reading the Scottish building standards further it seems those are what should be used rather than part H of building regulations.

I think I've figured out the following:

I'm hoping to attempt something that will look like the middle of this diagram with the AAV at the top of the stack and that seems to be allowable.

There doesn't seem to be a minimum stack size (unless I've missed something important elsewhere), only that a stack must be as wide as the widest connected branch. The vent on the stack must be the size of the stack diameter, or reduced to 80mm (75?) only. Using this I believe that since I'm intending to limit the stack diameter so close to where it appears from the basement floor it should be allowable to vent in 40mm if the stack and all branches to it are also 40mm...

But there's a snag which is that the airflow rate of the AAV on a stack must be 8 x the l/s of the fixtures/appliances connected. I've calculated Qtot x 8 to be around 4l/s and 40mm AAVs only do around 3/ls but a 50mm AAV could do the required flow rate.

No where in the building standards did I see anything that would allow (or disallow) my other diagram where a 40mm branch comes up higher than the main stack's vent, but given that all the diagrams showed the vent at the highest point I think my options are now: reduce stack to 50mm and come up through the floor with AAV on top OR reduce stack to 75mm and come up through the floor with AAV on top.

Can anyone give me a sanity check here, am I about to hang myself?

 

[denso13]

Can anyone give me a sanity check here, am I about to hang myself?

Probably just overthinking it. Do what you planned in your first post, I can't see there being an issue.

 

[John D v2.0]

You could probably get away with connecting directly to the drain from the ground floor, especially if you use 50mm pipe after the washing machine. Technically 40mm pipe isn't allowed for a vertical stack. Then you could do away with the aav.