We have 2 WCs upstairs connected in the roof space to a common stack venting through the roof and running vertically downstairs leading to a manhole. The downstairs wc is joined into this common stack. One upstairs WC's waste pipe runs a metre or so horizontally to another stack running downstairs to a separate manhole and up to join the common stack in the roof. The other upstairs WC's waste pipe runs a metre or so horizontally before joining the common stack. At that same point another horizontal pipe runs between the joists from the common stack and venting through the outer wall. Why should that vent pipe be needed? The reason for the question is that we had planned an house extension. Where this horizontal vent pipe now goes through the outside wall there would be an RSJ, and the pipe can't be diverted. I thought it could be fitted with a Durgo valve coming up under the bath but it looks like regulations bar this. Could I cap it off? All the pipes are 110mm except the common stack which becomes 140mm. Can anyone help please?
hi difficult to picture the layout,? basically, if the stack is vented, jobs ok, its always better to vent than use aav . the vent has two jobs, one is to allow air in, and stop loss of traps, and let air/gases out to vent the sewer pipe. diagram is good? regards peter
Yes, difficult to visualise, you may want to get a plumber to have a look at it (don't rely on your builder) to ensure your system remains effectively vented. Having your system vented is essential as Kiaora says, note that an AAV will only let air into the system, nothing out. They rely on a diaphragm to achieve this which inevitably gets stuck at some point - hence the need for access to maintain them.
I probably gave too much detail. Sorry. What I said about the downstairs WC and the other upstairs one was just to paint the picture, but they were not the reason for the question. That was why there should be a horizontal vent pipe coming off a standard stack already vented through the roof. I thought it might have been a recognised method of doing something and someone might have come across it before. The fact that it runs between the joists just beside the WC outlet seemed odd. Although the stack would need to be blocked for it to fill up and flow out of that horizontal vent pipe it just seemed to be an unnecessary risk. You're right. I should ask a plumber!
Have you any idea if the 110mm horizontal pipe between the joist is actually a part of the soil/vent system, after all you say the pipe vents out the wall. could there be an extract fan from a bathroom connected to said 110mm pipe which is not uncommon. You should be very sure to check, if possible, what said pipe is being used for.
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