Reducing soil stack vent, fitting AAV, potential future loft conversion problem

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Hi all,

Long time lurker but not hugely active on here. Hoping for some advice on the below.

I'm currently redoing the first floor bathroom. The current soil vent stack (110mm) is boxed in the corner and vents through roof. The boxing in is 250mm x 300mm and I'd like to minimise this if possible. I was always proposing to reduce soil vent to just above sink height and fit an AAV, boxing this in with a removable cill for access with ventilation coming from roof space below.

However, there is potential for a loft conversion in the future and I don't want to lose my only means of disposing of waste from the roof space, so I was wondering if it's possible to run a 40mm waste from the existing stack, below A1 rated AAV (suitable for installation below flood level) and then if a loft conversion does go ahead, using a macerator toilet for toilet waste. With another AAV installed above the last entry point.

I've attached a couple of rather crude drawings (original & proposed) to make things as clear as mud.

Just wondering if there's any reason I cannot do this? Or if there's any alternative ways you can propose?

Thanks for any help!
 

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Macerator toilets are not as reliable as a WC and soil pipe. Why not leave it until the loft plans actually materialise, and change then as necessary, it is a small part of a much larger job.

Blup
 
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I would strongly advise against removing an open vent and replacing with an AAV, the vent is there for a reason, and does an essential job. Using an AAV means you are essentially capping off and sealing the drainage system, which is poor practice, drains need to breathe. Any positive pressure would then be looking for another route out, which is usually through the shallowest trap and then causes odour issues in the bathroom.

Secondly, no-one in their right mind removes a 110mm pipe and replaces with a Macerator. If need be, look at getting the 110mm pipe as tight into the corner as possible and reduce the size of the boxing in.
 
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Thanks for the replies. Lots of food for thought.

The bathroom will be tiled floor to ceiling so I'd like to deal with minimising the boxing in now whilst not creating a huge hassle if we end up staying and converting the loft in the future.

If I reduce the 110mm vent to 82mm and leave open vented at roof level for time being, is this sufficient ventilation as far as any regs go? Or would I still need to install an AAV on the 110mm? I'm aware of the downsides of AAV's and Macerators and how they'd be in line with "bad practice," but regulation wise, is there anything wrong with what I've suggested? Out of curiosity more than anything. The likelihood is I will go the 82mm option and reduce the boxing in as much as possible.
 
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Vent will be fine at 82mm, should also be ok to connect a WC to at a later date if you get a pan with the appropriate sized outlet. Keeping a vent removes the need for an AAV.
 
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Vent will be fine at 82mm, should also be ok to connect a WC to at a later date if you get a pan with the appropriate sized outlet. Keeping a vent removes the need for an AAV.

Thank you sir.
82mm pipe and reducer being delivered Monday.

After demolition of the boxing in I can see now why it's not tight to the corner. Ceiling joist has taken that privilege away. 82mm will sit lovely in the corner and open up the wall above the vanity unit nicely.
 

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