Bad smells - quick plumbing question

[Adam.]

For context: OH house and we are under offer and moving

Downstairs loo which flushes out of side wall. Occassionally we get bad smells coming up into the room (once or twice a week).

This is the waste pipework on the exterior wall. From L to R - shower, loo and sink. Sink waste connects to loo waste!



I had thought loo waste needed to be connected to a stack which vented at roof height? And sink waste would be separate?

Presumably the issue here is a loo flush pushes trapped air through the U bend in the sink as it cant vent anywhere else?

Please confirm

[CharlesElliott]

Looks like it. You could extend the soil stack upwards and add an air admittance valve. Or you can get an anti-syphon trap for the sink which has its own mini AAV built into it.

OR you could modify the sink drain to feed into the same place as the shower drain, so it isn't connected to the toilet.

[Sheepshanks]

It's usually more of a pull issue than a push - when the toilet is flushed the water running through the soil pipe pulls the water out of the sink trap. In practice you wash your hands and refill the trap.

It's more of an issue for showers connected to the same line, especially if they're little used - we had to get a different kind of trap for ours. And we already had an AAV.

[andy43]

LL answer : drill a 5mm hole in the top of that stack to balance the air pressures out

[Adam.]

Sheepshanks said:

It's usually more of a pull issue than a push - when the toilet is flushed the water running through the soil pipe pulls the water out of the sink trap. In practice you wash your hands and refill the trap.

that makes sense thanks, and yes I run the sink tap when washing hands

given the context I wont be paying for an extended soil stack or diverting pipes, just wanted to confirm my understanding

thanks all

[Adam.]

actually for future reference on the next property

is it against current regulations or just bad practice to:

a) have a capped low soil stack like that reather than properly extended and vented?

b) connect sink or shower to a soil stack?

[ashenfie]

Can’t tell from the photo, but the pipe should have a 1 in 40 gradient. Looks like it’s running level?

[Adam.]

ashenfie said:

Can t tell from the photo, but the pipe should have a 1 in 40 gradient. Looks like it s running level?

loo waste is on a gradient not level

[Panamax]

I thought there's a one-way air valve that can/should be fitted on top of that short stack.

[Panamax]

Yup, Air Admittance Valve, check out this link,
https://community.screwfix.com/threads/smelly-air-...

[P13cur]

Your waste pipe needs a vent ... remove the cap add a length of 110mm soil pipe take it higher than the pan at least .... you don't need a AAV as its fitted externally

[Sheepshanks]

P13cur said:

Your waste pipe needs a vent ...

It seems to be either a black art - or game of chance. I tried to read the regs but they're very convoluted. They do seem to say that you shouldn't need a vent for a downstairs toilet.

Our original downstairs toilet went into clay pipe in the floor and then to the main sewer. No vent. It did whiff a bit but neighbours said the seal stops being gas-tight over time so we replaced the toilet and it was fine for years.

Then we added a downstairs bedroom and we converted that bathroom into an en-suite. Builders said drains would be fine as they were. Their plumber said no chance, and wouldn't even hear of a stubstack, he wanted a full height (through the ceiling) stack with an AAV in the roof space. Yet we still had issues with the shower trap emptying and smelling, until we changed to a waterless trap.

[119]

If that’s the only sink there just either use the sink when you use the loo or just turn the tap for a few seconds to refill the sink trap after flushing.

[Craikeybaby]

CharlesElliott said:

Looks like it. You could extend the soil stack upwards and add an air admittance valve. Or you can get an anti-syphon trap for the sink which has its own mini AAV built into it.

OR you could modify the sink drain to feed into the same place as the shower drain, so it isn't connected to the toilet.

Hmmm. I wonder if this is the issue we have got in our main bathroom. There is occasionally a pong coming from the shower, and I noticed that it had a strange plug hole. All the plumbing is concealed though. There does not seem to be a similar issue with the bath or sink, and the shower is the furthest point in the room away from the soil stack. But given the other bodgery I have seen here nothing would surprise me any more.

[Simon_GH]

If the shower isn’t used the the trap can dry out and let evil smells into the house. It took is a while to realise this in a house we rented for a few months.

[Craikeybaby]

It is the bathroom that the boys use, and they seem to do anything they can to avoid showering... Might just need to run it myself occasionally, but I will try that next time it smells in there.

[Simon_GH]

We used to pour a pint of water in every couple of weeks and that sorted it. It was simply the trap drying out so there was no block between drain and house.

[Adam.]

119 said:

just turn the tap for a few seconds to refill the sink trap after flushing.

That’s what I do, just wanted to confirm my understanding of the problem. If we were staying I would do a cheap fix