Drain stack vent - to be or not to be?

[jimmypg]

I have a problem with smells. I do not have mains drainage, so use a private sewage treatment plant. Since occupying the house I have noticed an intermittent, unpleasant (sewage-y) smell in the downstairs cloakroom and the adjacent (platerboard wall) utility room. Checking the gully, I see that output from the sinks in both rooms and the WC come from the same drain outlet, so there is a common feedback route for smells from the sewer.

Doing a quick survey I could not see a drain vent pipe on that side of the house, so there is no outlet to the air for smells. However, I notice that there is a short length of drainpipe in the cupboard housing the pressurised hotwater cylinder. This is adjacent to the upstairs bathroom WC. The pipe is capped at a height of about 1 metre. There is a boxed-in section of about the same height beside the downstairs WC, and that might contain something similar.

Question: Have I got some sort of short, closed sewage stacks? Is this a common practice? Or should I really have a stack that opens through the roof in the usual way.

To see if the smell was feeding back up the sewage pipes, I temporarily blocked the outlet in the gulley (carefully NOT using the sinks or WC since!!!). The smell has gone away. So I think I know the source, but I don't know how it is feeding back into the rooms. All the same, would a stack solve the smell problem? Should there have been a stack all along?

Thanks for your comments.

 

[ivixor]

So you've identified that the smell is coming from the gully?

Can you post a picture of the gully?

It's possible that the p-trap in the gully has failed somehow (cracked/leaking), allowing sewer gas/smell to escape. You should see water filling the gully as you look down into it.

 

[Hugh Jaleak]

Your description would lead me to believe you have Air Admittance Valves on the end of the stack(s), acceptable practice under certain circumstances but the system usually requires an open vented stack at the head of the drainage system.

AAV's are a one way valve, designed to allow air in when the system is under a negative pressure, but remain closed to keep foul air out at all other times. It is possible an AAV may have failed to close properly, hence the smell but if there isn't a vent elsewhere on the system at roof level it may be wise to investigate feasibility of installing one to allow the system to breathe.