External Pipe - what purpose and is replacement necessary?

[andyrobinson]

During November we have to replace a corroded cast soil pipe on the exterior wall of the 1st and 2nd floors of our 1930’s building. Fine.
However, it also has a smaller diameter pipe running beside it up the full height.
It appears out of the wall, level with the first floor toilet, runs up the wall, does not vent to the atmosphere, before joining into the soil pipe at second floor level. The soil pipe itself vents higher up above the flat roof.

• What function does the narrower pipe fulfil?
• Is it necessary to replace it as well as the soil pipe?
• Or just remove it or close it off?

Thanks for any input.
Rgds, Andy

 

[Last Plumber]

It looks like the original vent off the WC. You won't need it. Just replace the soil stack and vent it as is, 3' above any opening into the building.

 

[andyrobinson]

It looks like the original vent off the WC. You won't need it. Just replace the soil stack and vent it as is, 3' above any opening into the building.

Thanks for speedy reply Last Plumber. First time I’ve used this forum.

 

[snowhead]

It's a dual vent system used in commercial buildings or large houses where many sanitary appliances are fitted to one soil pipe and the end of the pipe is vented with a smaller pipe back into the stack.
They tend to go in just above the highest junction, not almost at the roof as yours does.
There was almost certainly a reason it was done that way.

There's a few variations, one example below borrowed from H Group Services, but probably not their picture.

 

[andyrobinson]

It's a dual vent system used in commercial buildings or large houses where many sanitary appliances are fitted to one soil pipe and the end of the pipe is vented with a smaller pipe back into the stack.
They tend to go in just above the highest junction, not almost at the roof as yours does.
There was almost certainly a reason it was done that way.

There's a few variations, one example below borrowed from H Group Services, but probably not their picture.


View attachment 85459

Thanks Snowhead for your time, explanation, and diagram to illustrate even though only 2 basins and 2 WCs are connected in our case. Going the extra mile.

 

[Last Plumber]

Snowhead is quite right with the drawing and comments. I was looking at the exit position of the vent and the single outlet (4") I can see at high level as well as the single outlet (4") at low level nearer the vent pipe. From this I made the assumption that you had a WC inside upstairs with a separate bath/shower waste as well as a separate basin waste into that hopper and one WC downstairs with a wash basin and a kitchen in the next room with wastes into a gully.

Obviously I haven't seen the internal plumbing so I do not know what is inside or where the vent connects. I think it would be unusual to have 4" lead or cast inside a domestic property, connecting a row of WC's. They would normally exit behind the WC and be joined together externally as on the drawing Snowhead posted.

What do you have inside that is connected to the 4" soil at high level and at low level?

Sometimes these vent pipes were installed unnecessarily but it is best to check. Apologies for being presumptuous.