Not a particularly green topic, but I'm renovating our upstairs bathroom. It has a vented soil stack that in it's current status has the WC connected, a double sink and a bath. The bath drain run has an additional vertical vent pipe connected which exits the roof.

In the new layout there will be a single sink, a shower, a bath and WC. What I want to know is, do I have to have the additional vertical air vent?

Whether you need additional vents etc depends on the length of the horizontal runs. Details are in Building Regs Part H. They can join the main SVP (i.e. vented soil stack) rather than be led upwards separately if you wish.

The current layout has the vented soil pipe in one corner of the bathroom. Branching off this is another 110mm diameter grey pipe that runs 4m along one wall under the floorboards, it has a stop end. The sink is connected to the main vertical stack, the bath is connected by a 4m long 40mm pipe to the far end of the branching grey 110mm pipe. The current additional vent branches up from the 40mm pipe where it joins the branching 110mm pipe.

My new layout would have the shower furthest from the main stack, where the bath use to be. The bath would be above the branching 110mm pipe and would have a short connection to it.

Thanks for info re air admittance valves, I didn't know such things existed. I always wondered why modern houses never seem to have pipes poking out the roof.

Posted By: mitchinoThanks for info re air admittance valves, I didn't know such things existed. I always wondered why modern houses never seem to have pipes poking out the roof.

That's not necessarily the reason. It is still necessary for some soil pipes to end in vents (to prevent a build up of explosive gases in the sewers - air admittance vales only allow air in, not out) but these days they mostly terminate in roof tile vents, rather than with pipes that project above the roof.

and the vent pipe doesn't even need to go on the house - I have one shared between two houses on a garage and it looks like it's a gutter down pipe - unless you look up (it's a dry stack and therefore only needs to be 75mm)

I was not aware that SVP could be shared amongst houses. I am a single dwelling with septic tank which is vented and have an outside toilet which is open vented. I had quite a lengthy conversation with the BCO about not having one go through the roof of our house or as an outside ugly stack. In order to maintain as much air tightness as possible my plan was not to have holes in walls and roof etc. He eventually agreed I could vent via the Septic tank and outside loo and use the AAV's but he said at my risk. I have not had an issue with the setup
I have recently done a downstairs WC and cloakroom and space in the cabinets would not allow me to fit a 110 0r 80 mm AAV. So I fitted an AA basin trap and a 40 mm AAV on an extension piece behind the cloak cabinets. I was informed it would not work as when the toilet was flushed the AAV had to allow the same volume of air as the water. However when I ran some trials there was no issue. Thinking about it the air flow into the system will move faster than water out of the WC which is why I reckon it works.

Many BCO's are also not aware. BCO's also state that the vent must be at the end of each run, also not true. However the BBA certificate of your chosen AAV valve (e.g. the Durgo AAV BBA cert) is likely to allow the shared use of an open stack between dwellings. It looks like all the AAV manufacturers have copied each others drawings - they all have very similar drawings in their BBA certificates. If you have used the AAV's in the dwellings concerned you should find the BCO can accept a shared open vent.

There are two reasons for venting as far as I'm aware:
A) the release of gasses in the pipes (to prevent pressure build up, explosive gases, harmful gasses).
B) to cope with flood situations in downstream sewers (aka surcharging) - as the water backs up into 'your' local branch (one or more houses) you must allow the air to escape.