Bathroom Wall Air Vent allows Sewage odours to enter

On occasions I have experienced very strong unpleasant sewage smells in each of my bathrooms which appear to be entering through the air vents in the wall of the bathroom itself.  I think wind conditions outside and whether the patio doors are open/closed etc may have some effect on the problem.  The sole purpose of the wall vent presumably is to exhaust warm damp air from the bathroom after a shower etc, and I expect that the vents connect to a main pipe rising up through the block to the roof area.  However it is strange that they seem to operate in reverse sometimes allowing a "backdraught" of sewage type gases to enter the bathroom.  I am situated at semisotano level and do not know whether that has anything to do with it.

Quite often the problem will disappear after an hour or so.  Nevertheless I am very keen to find a solution to this and would appreciate any information or helpful suggestions for dealing with the problem.

Hi Gericom

A very unpleasant problem and one which others have had to deal with. I do, however, believe you are wrong about the source. The air vents do indeed run vertically to the roof but even if someone was quite unwell in a neighbouring apartment it is unlikely that you would experiance the strong smells you describe. 

The sewage system is the more likely source. The water in the toilets and in the traps below the metal discs on the bathroom floors should provide an effective barrier to odours. In some apartments a bad seal or missing seal below the toilets where they connect to the pipe has been the cause of the problem. In others the trap which protects the bath and sink and is located below the metal disc on the bathroom floor has not been properly fitted and does not provide a complete seal. Either of these problems can be fixed from within your apartment by a competent plumber.

In view of the number of people who have this problem (and you are right the problem is made worse when the wind blows in 1 dirrection) Ana and I considered fitting a U trap at the end of the sewer pipe in each block before it joined the main sewer. However, the plumber we consulted did not think this would solve the problem - pointing out that the horizontal pipes in the garage area did not have a sufficient fall to empty properly and that the contents of these pipes would continue to smell even if we fitted a U bend before they joined the main sewer and as the gas would be trapped in the pipe it might even make the problem worse!

I hope no one has been eating while they read this - Gericom if you need any further info I will be happy to talk to you.

Thank you David for your comprehensive reply.  

I do agree that the sewage system is the likely source of the bad odours I describe, however the means of entry of the odours into the apartment is what is puzzling me.  I did conduct "nose sniffing " tests at the washbasin, bidet and shower/bath water outlets and the floor trap you mention a few times and could not detect any odour.  On the other hand I also did the same at the wall air vents and was convinced that I detected the odour at that point.  

Nevertheless I will follow your advice and get all the seals that you mention checked by a competent person when I am next out.

If I find a definitive solution I will be happy to share it with all...

Until the next bulletin...

Just back after a stay in Don Juan and unfortunately the problem of odours in the bathroom was present again on and off. I have looked at the traps and seals in the past, especially around the base of the toilet and the one below the metal disc in the floor and I have been unable to detect any  problem with them.  As an experiment I sealed the metal disc on the floor with duct tape all around for a week but this made no difference to the problem.

I am fairly certain that the odour is coming in from the grill opening in the wall and I am now going to install extractor fans fitted with backdraught preventers where the vents are in the hope that this will cure the problem.  I presume that the vent stack pipe rises up through the building with no openings other than the bathroom vents at each level but I wonder if it terminates at roof level right beside the sewer vent pipe which could allow gas from the sewer vent to travel back down the fresh air vent pipe under certain wind conditions.  I have known this to happen with two chimneys terminated side by side where smoke from one entered the other in certain circumstances.  The solution I think was to raise one chimney pot some distance above the other.

I do not believe that the odour is coming via the vent pipe from any other bathroom.  The strange thing is the "quality" (if this is the correct word) of the smell is more that of "p**s" than "s**t"..... sorry about the use of such words, if translation is need please get back to me

Any comments or suggestions are welcome.

Hi Gericom,

We are in the process of having some work done in our apartment and as a result I have discovered something I didn't know when I last posted on this subject - the ventilation shafts also house the vertical sewer pipes into which the waste from neighbours above is discharged. If any of the joints in this pipe are not properly sealed this could be the cause of your rather unpleasant problem.

I think your plan to instal extractor fans with backdraft preventors is at least worth a try as it is not going to be easy for the community to access the pipe to find and fix the problem!

Thanks for that info David.

I had been thinking of two independent vertical pipes rising up through the shaft to the roof, one carrying the sewer gases and the other the air vented from bathrooms.  However, from what you say it seems that there may be a sole pipe carrying the sewer gases and that the bathroom vents are simply openings in the walls leading into an open ventilation shaft.  Am I correct in thinking that this is the arrangement?

Hi Gericom

I was somewhat surprised when I saw it but yes that is precisely the way it is in our block so I expect it will be the same in yours! The vent cover is only stuck on with silicon. If you remove it you will find a rough hole punched into the open shaft. Shine a torch in and you will see the pipe with a branch coming into it from above your ceiling (from the neighbour's bathroom immediately above).