Dodgy push-fit or siphon risk?

[hightails]

Guys,
I am a reasonably competent DIYer, many years general experience, a bit on plumbing but not a lot. Currently gutting and completely relaying out my upstairs bathroom.
Bath will be 4m from stack, so I will be running waste in 50mm into a new boss on the stack, and teeing in basin half way along the run and using an anti-vac trap or AAV. Solvent weld throughout. Hoping that will be OK.
My problem is with the 40mm shower waste which is next to the stack. I have two options

• Run it into an existing 30 year old push-fit boss, which looks in reasonable nick apart from a greasy residue around the old push-fit seal – not sure if this old plumbers grease or an oozing joint. This would give the shower a dedicated access to the stack, but I am concerned about that greasy residue and long term reliability. And I would need to connect push fit pipe to solvent pipe with a compression fitting. All this then buried under floor beneath the shower. For obvious reasons I would prefer a solvent weld solution.
• Second option is to tee it as a third fitting into the new 50mm run (solvent weld). But with the bath furthest from stack and this tee nearest stack will I be risking siphoning of the shower seal? There’s nowhere obvious to fit an AAV. Would it help if I minimise the fall past the tee (18mm/m)? Or use 50mm as far up the shower line as possible?

Which option would you go for? Any other advice?

 

[solutions]

Run them all separate back to the stack use a new clamp on boss if you can or if there is nothing further up the stack maybe cut in a floor level manifold to take al 3 seperate pipe runs. You might not just suck the shower trap water but blow it out.

 

[hightails]

Thanks for fast reply. Unfortunately, both those options give me access problems.

I was trying not to go into too much detail, but perhaps pics will help. Not sure if I've uploaded properly.
- The soil pipe is in a narrow recess in a cavity wall. I don't think there's enough width for a manifold.
- No room for an extra boss either. As you can see soil is above floor. The 40mm referred to above is in the void (top one). There's also a 32mm boss in the void, which I didn't mention before, which I plan to block off. The new 50mm boss will be below the ceiling in the downstairs loo, with 50mm pipe coming up through ceiling approx 1 metre from stack

The reason for blocking the 32mm off rather than using it for the basin is that I need the 32mm pipe out of the way to run the new 50mm pipe. Also the pipe run from stack to basin will be 2.5m plus vertical drop, which is too long for 32mm. Also it's push-fit too, so same issues as the 40mm.

 

[solutions]

What is below and above the room the pics are of?

 

[solutions]

Is that an external cavity wall the stack is set in to?

 

[Unregistered]

Above the room is a sloping roof and sloping ceiling (ie no workable loft space).

Below the room at the stack end is the downstairs loo, which extends 1.2m towards the camera in pic 1. Hence the plan to put the 50mm boss into the stack in the downstairs loo, ie just below the 32mm boss visible in the pic. The 50mm pipe will then come up through the ceiling somewhere within that 1.2m and continue in the floor-ceiling void towards the bath which will be behind the camera position.

The cavity wall is not external. Strange, I know. Other side of the cavity wall is the upstairs corridor. Can explain more if you want to know.

 

[lame plumber]

run shower into 50mm and use a hepvo trap simple no risk of pulling the trap seal as it doesnt use water.

 

[hightails]

Lame : thanks for the idea, which sounds like the ideal solution. But don't some people say that hepvo's shouldn't be used on shower trays because they block when installed horizontally? Or have you found different?
Solutions : Just noticed my last post at #6 shows up as Unregistered - I must have forgotten to log in. It is me!

 

[hightails]

Guys,
This is a bit of an off the wall idea. But what if I run the shower waste into the new 50mm as a new branch (option 2 in original post), and then counter the tendency to siphon by running a vent pipe from the shower line to the stack via the old pushfit boss.

Please see attached sketch.
View attachment drawing.pdf
I may have to run the vent in 32mm to get the pipework to fit in the void. Also, there will be only 200-250mm between the vent boss and the waste boss. Will that matter?

 

[solutions]

Guys,
This is a bit of an off the wall idea. But what if I run the shower waste into the new 50mm as a new branch (option 2 in original post), and then counter the tendency to siphon by running a vent pipe from the shower line to the stack via the old pushfit boss.

Please see attached sketch.
View attachment 22710
I may have to run the vent in 32mm to get the pipework to fit in the void. Also, there will be only 200-250mm between the vent boss and the waste boss. Will that matter?

That may work but I couldnt say for sure.
Could you cut out the sectiin between the joists with the push fit bosses below the toilet branch and fit a 110 branch at that height instead then run 110 for a M or so between the joists then boss everything independantly in to it.

 

[hightails]

Thanks again for the reply.

I am nervous of doing that much surgery on the stack in that corner. If you look at pic 2 you will see it's a busy corner. The copper pipes would need moving, and are notched into 7 or 8 joists along the room. Below ceiling level they also obstruct the stack pipe for cutting. Then there's that bunch of cables ... I think it's important us DIY guys know when we've reached our limits, and I think that's beyond mine.

Returning to my vent idea, it must surely relieve the pressure enough to prevent siphoning. I was more concerned about whether venting to the stack would be seen for some reason as bad practice. I have tried googling but can't find anything. Do you know if it's against regs?

 

[Sparkgap]

Lame : thanks for the idea, which sounds like the ideal solution. But don't some people say that hepvo's shouldn't be used on shower trays because they block when installed horizontally? Or have you found different?
Solutions : Just noticed my last post at #6 shows up as Unregistered - I must have forgotten to log in. It is me!

The Hep blurb includes it as suitable for showers. We've used them loads of times for boiler condense and not had any issues with blockage or smells coming back. Sounds like your best bet.

 

[hightails]

Guys, thanks for all your input. I thought you might like to know what I have eventually gone with.

Given the space and access I have available and the some of the comments from you guys I have dropped my own idea of a vent pipe and gone instead with a waterless Hepvo type trap for the shower, as suggested by lame plumber and sparkgap. I wasn’t keen on these originally because people say they block, but my thinking is that it will be a lot easier to unblock one of these if I have problems than it would be to correct a dodgy vent pipe installation.

Hepvo imply their trap never blocks, but that’s not what some people say. So I will probably go with the Wirquin one available from Screwfix, because that is built in to the waste fitting and can be unblocked from inside the shower tray rather than by removing the fitting from underneath.

Thanks again for helping me work this through.