Toilet Crossflow

plumbing, drainage and waste water questions here please

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SimBed
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Toilet Crossflow

Post by SimBed » Wed Jun 13, 2018 10:02 am

I have two toilets (one in a bathroom and one in an ensuite, one room along) which share the same soil waste and functioned harmoniously together for many years. Toilet B, which is 'downstream' of Toilet A has recently started to receive a a little of the waste from Toilet A, when Toilet A is flushed. Although this has only recently become noticeable (Toilet B was previously used and flushed regularly, but less so recently, so the build up has become obvious) I suspect the issue arose a year or so ago when I replaced the soil pan connector on Toilet B. I didn't knowingly adjust the relative positioning of the toilet to the pipes, just removed the toilet, replaced the soil pan connector and put back in place. The toilets flush well, there is no blockage.

What might have happened to cause this crossflow? And how can I correct it? If there is no simple solution, I thought I could mitigate the problem by fitting an anti-rodent barrier. Does that seem a sensible plan?

Thank you
Daniel
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Re: Toilet Crossflow

Post by chrrris » Wed Jun 13, 2018 2:42 pm

:welcomeuhm:

Based on what you've said so far, I'm going to go for a broken or blocked durgo or vent, incorrect fall on the soil pipe, or a blockage further downstream of the both loos, in that order of likelihood.

Can you post some pics of the whole stack please?
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Re: Toilet Crossflow

Post by Bob225 » Wed Jun 13, 2018 3:48 pm

+1 on the air vent, the grease in them dries out over time they either stick open or closed
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Re: Toilet Crossflow

Post by wine~o » Wed Jun 13, 2018 6:13 pm

:welcomeuhm:

1st check I'd make is to flush loo A. (without anything in it) after about a 1/4 or 1/2 a second the water level will rise by about an inch or maybe 2, If it rises any higher then you have a partial blockage.

Same with loo B.

EDIT otherwise as per Chrrrises post
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Re: Toilet Crossflow

Post by SimBed » Thu Jun 14, 2018 12:53 pm

Thanks for your input.

To reiterate:
1. there is no blockage
2. the fall on the soil pipe has not changed in 20 years and for 19 of the years it all worked fine
So I cannot see that either of these are the cause.

Nobody has suggested I may have caused it by inadvertently lowering the height of the toilet when replacing the pan connector last year. Is this because you are all too diplomatic or because this is unlikely to be the cause?

This leaves the air vent suggestion. I'm not familiar with air vents. Could you help me understand why this might cause the problem please? I have posted some pictures (low quality I'm afraid) to give some more context. These are from archive - i can provide improved pictures of anything that might be relevant. I should point out that it is a 3-storey house without easy access to the roof.

I'd also welcome your comments on whether the anti-rodent barrier might be the simplest cure if replacing the air vent goes beyond my skill set...

Thank you

Daniel
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Toilet B.jpg
Toilet B - stack is visible left and connects to Toilet A in room right of this
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soil stack vertical.jpg
Toilet B is on floor above directly above this shower
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Soil Stack attic.jpg
Is this the air vent?
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Re: Toilet Crossflow

Post by wine~o » Thu Jun 14, 2018 4:43 pm

Better pic of the waste connections behind the offending loo please. Also (if you can) a pic of the air vent itself. (on the roof, I'd expect that to be fully open to air)
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Re: Toilet Crossflow

Post by chrrris » Thu Jun 14, 2018 11:44 pm

Yes, what wine~o said about a pic of the top of the stack is vital (hence my asking for pictures of the entire stack).

Another reason I wanted pictures of the entire stack was to establish how the second, most downstream loo was connected to the stack (your pictures don't show this). But yes, depending on how the loo is connected to the waste, if you don't have a positive fall from the loo to the waste, then of course sewage will enter the toilet. Sh*t runs downhill.

Forget your anti-rodent barrier. That is for stopping rodents entering a drain, not a method for bodging your broken drainage installation.
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Re: Toilet Crossflow

Post by SimBed » Fri Jun 15, 2018 11:32 am

I've added some additional pictures.
I was able to remove the vent and inspect it. It seems to be in good condition and clear.
The 'horizontal' soil stack runs at approximately 1 degree downhill (in the direction of the vertical stack)
Hope this gives a clearer idea of what's going on.
Thanks
Daniel
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Re: Toilet Crossflow

Post by wine~o » Fri Jun 15, 2018 6:32 pm

All looks good in terms of falls etc. Can't quite work out where loo A is in relation to these pics and how air-gets-behind-water from loo A ??

EDIT. Must be an AAV somewhere to the right of these pics ?
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Re: Toilet Crossflow

Post by chrrris » Fri Jun 15, 2018 9:46 pm

I would first try temporarily disconnecting the roof vent completely as it looks it's on a push fit -- if that stops the problem happening then you know it's the vent that's the cause.

The layout isn't clear... does the waste from the other toilet run from the right of the toilet in the picture, across that tee at the back of the pictured toilet? It's impossible to tell accurately from a picture, but that horizontal run should be a good 1:40 drop -- it doesn't look like that to me, if the batten above it is horizontal then it looks like there's no drop, or potentially even a slight fall away from vertical stack, but like I say it's tricky to tell from a picture.

Quite a lot of guesswork there about the layout, etc. But if I've guessed right, the solution would be to remove that tee where the horizontal run meets the vertical, cut enough off the end of the pipe to ensure a 1:40 drop, and replace the tee (with a bit of luck, you'll be able to pull the top part of the vertical run down to meet the lower tee, assuming it's not connected to anything above, otherwise cut out some of the vertical pipe and use a slip coupling, so that the amount you've removed from below the tee, is added above it, IYSWIM).

If you install a rodent trap on a section of pipework that isn't draining properly, then all the poo and bog roll will sit and accumulate around the back of the trap, dry out, and stop the flap opening - it'll block in no time.
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Re: Toilet Crossflow

Post by SimBed » Sat Jun 16, 2018 12:21 pm

Thanks for sticking with me.

Update:
I have just tested having removed the black conduit, which is pushfit as chrrris suggested (I'm unsure if this is a vent itself or a conduit connected to a vent in the roof). With the top of the stack in the position shown in the new picture attached, the crossflow seems to have been resolved. I will try again later to be sure. Any ways of testing this that don't involve a hearty breakfast?!

I assume leaving the end of the stack hanging by a piece of old rope would be considered a bit of a bodge! What would the next step be? I'm unsure, what is at the end of the thin opening in the attic roof.

Clarification:
Toilet A is in the room to the right of the bathroom shown. Waste from toilet A runs from the right of the toilet in the picture. There is no air entry point on the right hand side.
Air entry is via the vertical stack (left of picture in the photos shown) which goes up to the attic and out through the roof (via the black conduit shown).
At the very exit from the roof, I cannot tell what is there. The best I've done so far is take a long-ranged photo of the roof (previously attached)...I don't know if this gives any clues.
The phorographs may give a false perspective of angles, as I've measured the fall on the 'horizontal' stack at about 1 degree (which translates to about 1:50). Less steep than chrrris recommends but is unchanged from the time when all was working.
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Re: Toilet Crossflow

Post by chrrris » Sat Jun 16, 2018 3:20 pm

SimBed wrote: I have just tested having removed the black conduit, which is pushfit as chrrris suggested (I'm unsure if this is a vent itself or a conduit connected to a vent in the roof). With the top of the stack in the position shown in the new picture attached, the crossflow seems to have been resolved. I will try again later to be sure. Any ways of testing this that don't involve a hearty breakfast?!
:lol: Pour something brightly coloured down the loo before you flush it - food colouring or blackcurrent cordial or something. The black flexible connector in your loft space connects to the back of a tile vent in the roof. It's that tile vent that you need to remove and clean -- probably just blocked up with dirt or bird poo or the remnants of a bird's nest or something. There are loads of different types of those vents and I honestly don't know how to get at your particular one, but it should be relatively obvious if you undo the flexi and have a poke about. Hopefully you'll be able to unblock it fully from inside the loft rather than having to get up on the roof.

Don't leave the vent disconnected for any length of time as, basically, smelly sewer gases will be escaping into your roof space.

The 1 degree fall isn't ideal, but as long as there is a fall and as long as it hasn't moved or sagged anywhere since it was all working, then it all points towards the vent. Which is probably a good thing, as that's the easiest thing to fix. :thumbleft:
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Re: Toilet Crossflow

Post by wine~o » Sat Jun 16, 2018 5:53 pm

I'm sure Chrrris is correct,but still can't work out the layout from loo a and b and where the air comes in any chance of a diagram ?
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Re: Toilet Crossflow

Post by SimBed » Mon Jun 18, 2018 8:35 am

'm very pleased to report, this has been resolved. I did not need to remove the tile vent itself. Removing the connector, poking around from below and giving everything a good dusting and replacing seems to have been enough.

Many thanks for the helpful input, particularly chrrris for sagely steering me away from the anti-rodent device!

I've posted a crude pic of the layout as it was requested (perhaps I should have done that at the start...)

Thanks Daniel
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Re: Toilet Crossflow

Post by wine~o » Mon Jun 18, 2018 5:35 pm

I'm sure Chriiss will be along later to put me right if I'm wrong, I'd have expected that there would be an aav to the right of toilet A.

Air behind water?
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