Venting a shower waste via a WC pan boss.

[Kasheen]

I'm in the middle of installing a slim shower base, I'm at the "test the flow stage".

I have my shower with 90mm fast flow waste connected to 40mm. About 30cm from the shower trap I've installed an AAV on the run which continues to a swept 90 before meeting a ridiculous (imo) 40mm down pipe which is not vented at the top (seems to be a common theme in this house, not something I've installed).

I have almost literally zero flow from the fast flow waste, a trickle at best, tray fills immediately.

I swapped the cheap fast flow waste that came with the tray for a mcalpine fast flow, exactly the same problem, zero flow.

When I remove the strainer and mushroom top thus having no water seal the waste trap will happily drain my garden hose which supplies 10l/m (yes that's less than a shower but the hose is a long run hence low flow and the easiest constant water supply I can test with at the moment).

I've also noticed that when I take the cap off the AAV leaving open pipe work further down the line the trap will flow like it should.

I've come to the conclusion that there must be some kind of back pressure because or the trap is airlocking its self as the 40mm waste isn't vented in both directions (only against vacuum by my aav). The 40mm down pipe drops into the drain through the floor, there are 110mm stacks on the house vented to atmosphere that go to this drain also.

In an attempt to easily vent this 40mm to atmosphere is it allowable to use a boss from a pan connector which is on an atmosphere vented stack?

I realise naturally that you'd want the 40mm just to drop water straight into the 110mm stack but the layout won't allow it.

I'm guessing the answer to this question will be "not allowable" as I've never seen a set up like this.

I've seen diagrams where a pipe could be taken off the horizontal of the 40mm and put into the 110mm stack vertical section higher than where the pan connector meets it - so I'm wondering if this method which could be contained inside the bathroom would be functionally equivalent and allowed?

Thanks for any advice, or perhaps laughter.

 

[muggles]

How long is the run and how much fall do you have on it?

 

[Kasheen]

The 40mm has a drop of 40mm - 50mm over a length of 1.2metres into a 92.5 degree (swept 90?) corner and continues for another 1.2metres with around around 40mm drop on that section.

 

[Nige F]

If you can join "no vent" to "open air vent" at a high level as per your drawing you will have a setup that pre dates the Single Stack system - and works effectively. No need for blue AAV or green pipework

 

[Kasheen]

If you can join "no vent" to "open air vent" at a high level as per your drawing you will have a setup that pre dates the Single Stack system - and works effectively. No need for blue AAV or green pipework

Thanks for the reply Nige, unfortunately the top of those two pipes are about 9 metres up, the 40mm is inside the building and the 110mm outside. Your suggestion would have been an ideal solution to vent the whole 40mm pipe and I've seen diagrams showing that can be done.

I had hoped that installing the green pipe to vent this appliance (shower) would be a solution to my problem, but I'm not sure if I'm allowed to vent via another discharge branch even if I connect the pipework in a way that the 110mm branch couldn't discharge into the 40mm's vent pipe.

Seems like the building standards say a vent pipe like the green one must start no further than 750mm from the appliance and be on a constant incline until it meets a vented stack - this interests me because surely the stack could then discharge into the vent pipe of the appliance and makes no sense to me.

I've called a plumber to have a look, I suspect I have a bit of non-standard plumbing going on here when it comes to the unvented 40mm vertical section, I'm also confused how I have positive pressure despite a well vented drain. Appreciate any more ideas when it comes to venting this shower if anyone has any.

Thanks again Nige.

 

[Hugh Jaleak]

Is there any way you can take the shower waste out through the wall and join into the stack? Alternatively, upgrade the vertical section internally to 50mm? There's an issue elsewhere I'd think if you're getting positive pressure at the shower, Once into the vertical section the pipe should cope handsomely with the flow.

 

[Kasheen]

I might be able to upgrade the bottom of the 40mm to 50mm where my horizontal run from the shower meets it. The 40mm vertical seems to be replacing (sleeved inside) an old cast iron of about 75mm (just eyeballing this measurement at the moment), I don't know who decided that was a good idea but it drops into a concrete floor and what I assumed would be the stump of the 75mm.

I'm thinking where the 40mm vertical drops into the floor that it doesn't just go through a bend and into the main drain but rather into some kind of trapped bend or trapped gully or something hence there is maybe pressure infront of the flow of water from the shower. I'm also guessing this old cast iron 75mm that the 40mm runs inside of was vented somewhere but whoever has made this modification has deemed fit to remove the vent.

I'm going to explore options with a plumber of just taking the shower waste directly out the wall into a 110mm stack, but I think I'm too low to the ground (maybe I could reduce the drop) and there's other couplings to the stack in the way such as where the WC enters the stack.

Also from my earlier post where I was saying I saw a picture of a constantly rising vent pipe for a branch meeting a stack and I was unsure of how the stack didn't flow into it - I think that's because the vent pipe is meant to meet the venting section of the stack, not the discharge section. Just thought I should clarify that incase anyone reads my post in the future.

 

[Hugh Jaleak]

If the underground section of the stack is accessible, the waste can be connected underground without an issue.