What is the purpose of this pipe?

[PipeDownSoldier]

I'm about to undertake my first partial bathroom renovation and I'm trying to see if I can tidy up some of the old pipework. This is a first floor bathroom. Directly under this bathroom on the ground floor, there is a utility room with sink and washing machine in it.

I have a downward sloping upvc pipe (I will refer to this as PIPEXYZ for the rest of this post) running from the soil stack. It runs behind the toilet and sink and towards the bath tub. PIPEXYZ does not tee off to the sink waste or toilet. The soil stack continues all the way down to the room below, and also upwards, which I'm guessing vents bad smells up onto the roof.

At the bath tub, PIPEXYZ connects to a 3-way pipe which connects to the trap and a downwards waste. The trap under the bath tub has a u-bend (although it looks like a shallow u-bend).

What is the purpose of PIPEXYZ under the bath tube connecting to the soil stack? Can I safely remove it? Is it a pipe to vent smells from a drain somewhere? If so, what is the point of the u-bend under the bath tub?

Here's some photos and a diagram (unfortunately I can't seem to embed a video so if anyone knows how to do this let me know).
Photo 1: PIPEXYZ
Photo 2: diagram of pipe layout
Photo 3: where PIPEXYZ connects to the bathtub trap
Photo 3: where PIPEXYZ connects to the soilstack (note this also has another unknown pipe that tees off PIPEXYZ and heads down vertically).

I'm a bit of a noob so please go easy on me....

 

[pete01]

I'm no plumber but I think that's a waste pipe.

Edit....sorry you knew that I didnt read all your post

 

[Deluks]

Some kind of weird overflow contraption?

 

[PipeDownSoldier]

Some kind of weird overflow contraption?

Sadly not - the toilet overflows into the bowl and the sink overflows into the sink waste pipe (which is not connected to the pipe I want to remove)

 

[CountryFan]

Do you know how it vents outside?

 

[PipeDownSoldier]

Do you know how it vents outside?

I'll double-check tomorrow, but I seem to remember the soil pipe connects to a vent stack through the roof tiles, with a vent terminal.

The reason why I'm confused is that I thought the whole point of a u-bend on pipework is to act a barrier to the gases in the sewage/soil pipes. If that's the case, and given my bath has a u-bend trap, why would I need to vent that pipework? Is it to facilitate water flow?

 

[CountryFan]

The reason why I'm confused is that I thought the whole point of a u-bend on pipework is to act a barrier to the gases in the sewage/soil pipes. If that's the case, and given my bath has a u-bend trap, why would I need to vent that pipework? Is it to facilitate water flow?

Yes that is the point of a u bend, the reason I asked is because that could be acting as a vent pipe. Seems a bit odd, and the fact it runs down to the bath seems odd too.

 

[Hugh Jaleak]

Think you really need to try and trace where it goes, it's a very odd set up and I'm not quite sure what whoever installed it was trying to achieve.

 

[Madrab]

Possibly a supplementary vent for the bath waste run?? Is there anything wet above that waste pipe coming out of the stack?

 

[Nige F]

Think you really need to try and trace where it goes, it's a very odd set up and I'm not quite sure what whoever installed it was trying to achieve.

some sort of misguided vent system

 

[PipeDownSoldier]

Possibly a supplementary vent for the bath waste run?? Is there anything wet above that waste pipe coming out of the stack?

Nope, nothing above that waste pipe connected to the soil stack.

 

[PipeDownSoldier]

Yes that is the point of a u bend, the reason I asked is because that could be acting as a vent pipe. Seems a bit odd, and the fact it runs down to the bath seems odd too.

I checked and as I suspected, just a regular vent stack that comes out of the roof tiles.

 

[Madrab]

OK - I'd bet on it being a secondary vent setup, primarily to vent the bath run, especially if the bath waste run is vertical and on the longer side, it'll stop the bath trap from being pulled/self syphoned.

 

[Justin Passing]

It's absolutely standard, and vital, in multi storey buildings to have vent pipes so a "plug" of water going down a pipe doesn't pull the traps which are connected to it.
eg:

Yours is probably a situation similar.

 

[PipeDownSoldier]

It's absolutely standard, and vital, in multi storey buildings to have vent pipes so a "plug" of water going down a pipe doesn't pull the traps which are connected to it.
eg:
View attachment 394915
Yours is probably a situation similar.

Thank you, and the diagram was super helpful. If I replace the bath trap with an anti-siphon one, do you think I could safely remove that secondary vent? Or would I be better off leaving it?