Nasty smell downstairs bathroom

Poppy1312

We have a mid terrace house with a septic tank.
We seem to regularly have an accrid stale cabbage water smell in the downstairs bathroom when we use the ensuite shower room upstairs.
The ensuite drains through the wall to a vent pipe, the waste goes down into the ground under the kitchen and living room to the septic tank at the other side of the house.
The down stairs bathroom sink and bath waste go through the wall to an open outside drain and then underground into the tank. The toilet waste goes into the floor and out to the tank.
The smell only happens when we use the ensuite shower, it doesn't do it when we use the ensuite toilet and even then not everytime we use the shower although it's getting more regular.
Everything drains freely and when the smell happens in the bathroom it can't be smelt outside.

Does anyone have any ideas why it would be happening? We've had a plumber in but sods law it didn't do it when he was here!

twopenny

It sounds as though something in the movement of the water is causing the trap to empty, that should have a little water in it to stop smells.
Used to get this on an old boat I used to travel on was going over choppy water. The shower trap would empty and the bathroom smell.put water in and it went.
When you next get the smell try pouring a little water slowly into the bath and sink and see if that stops the smell.
If it does then try each on its own and see if it's just one of them.
Wonder if something in the plumbing is creating a vacuum and emptying a trap.

princeofpounds

Yes very likely to be an issue with the traps (typically a U-bend basically, but other types exist).

First thing to check - are they there? 
Do they have water in them, sufficient to seal the pipe? Even when the shower upstairs is running? Are they clean of debris?
If you have traps with mechanisms (for example some shower traps are raised and lowered by springs) are they working?

Also worth inspecting the drain outside, make sure it is empty and free-flowing, no sitting water if the pipe has 'bellied', no backing up. Water might be pulled out of the traps by the vacuum effect described, or some foul water might be flowing back in through the trap if blocked at the other end. That's less likely as you'd normally notice some backing-up, but if it does happen the downstairs is the first place you see it.

Poppy1312

My first thought was the same - but the traps appear full of water and the downstairs bath and sink go into an open to the air drain with a grill. i.e. you can see the waste pipe coming through the wall and there is an air gap between the end of the pipe and where the water runs down the drain. There's also no smell at all outside when you stick your nose close to the drain mesh cover.
I then thought it might be the downstairs toilet, but why wouldn't it then smell when the upstairs toilet is flushed, only when the shower is used which you would think would cause less of a siphon effect if that was the the problem......
Its very frustrating!