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How to fix sewer smells from S trap toilet?

Hi.

I’ve just moved into a rental unit. I’m finding the toilet has an occasional sever smell. I do clean it and it settled for a little while and then starts again.

It’s not all day.

I was trying to look up online and did some trouble spotting after the PM has said ‘nothing is wrong, it’s pretty much a new toilet - 3 years old’.

Online it suggests it could be the S trap and how they’re not often used in Australia anymore. (This is not a new build, possibly 20 years old).

It’s recommended as an ‘easy’ fix to install an anti siphonic trap - is this an easy thing to do or will a plumber be needed? I can only see online that they’re used under sinks etc

Any advice will be helpful. Thank you!

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MitchellMc

Welcome to the Bunnings Workshop community @DIYnewbie1111. It's wonderful to have you join us, and many thanks for your question about fixing smells from a toilet.

Before moving on to solutions, it's best to identify why this issue is occurring. An S-trap in a toilet works by using a water seal to block sewer gases from coming back up into the bathroom. When you flush, waste and water flow through the trap and into the plumbing system, while a portion of water remains in the bend of the S-shape. This standing water acts as a barrier, preventing unpleasant smells from escaping.

Unless the water level in the bowl is too low, the trap should effectively block any odours. If you're noticing smells, check whether the water is filling up to the correct level after each flush. If it's too low, sewer gases can bypass the water barrier and enter the room. This could be due to a blockage, a siphoning issue, or the toilet's flush valve might not be releasing enough water. Can you take a picture of the bowl and upload it, as it would be good to see how much water is in the bowl and whether it is adequate? Let me know if you need a hand uploading images.

Another issue could be the seal between the S bend and the floor. A gasket links the two. If this gasket fails or is not installed correctly, it can lead to leaks and smells. A more in-depth issue is if the building does not have adequate venting to the sewer system. If the system is not vented outside the building, gases can build up and bubble back into the room, periodically emitting smells. But, I'd troubleshoot other issues before considering that.

I'd also ask you to go around and check all other drains to see if the smell could be emanating from there. Check the sink drain, floor drain, and shower drain. They are also connected to the sewer, and if their traps are not functioning correctly, they could be letting smells out. You'll also find that hair and debris caught in these drains can have quite a potent smell when they break down and can be confused with a sewer smell.

Please let me know how you go and if you have any questions.

Mitchell

DIYnewbie1111

@MitchellMc

thank you very much for your detailed response.
I haven’t noticed any odours coming from other drain areas. The toilet room is seperate to the bathroom, next door is the shower and I don’t have issues on shower drain, basin or bathtub.
when I clean the bowl and add solution, the water level does drop until I flush it. Not sure that’s helpful

I did see online ti add water after a flush to possibly help which I will try today.

Thank you.
will upload a photo of the bowl. I hope it’s clear to see the level.

MitchellMc

Hi @DIYnewbie1111,

With a water level at the height shown in your photo, there is no way for smells to enter the room. The water is providing a seal. However, as mentioned earlier, if it's a venting issue, air bubbles could be forcing their way through the S bend and water into the room.

I'd like to hear more about "when I clean the bowl and add solution, the water level does drop until I flush it.". Whether you add water or solutions to the bowl, that should not cause the water level to drop, unless you are physically pumping the water out of the S bend through agitation with a scrubbing brush.

"I did see online ti add water after a flush to possibly help which I will try today." There is no need to add water after a flush if the water is ending up at the height shown in your image. If the water is below the line I've marked, then the airway is open in the S bend and smells can enter. If when you flush, water is not filled to above the line below, then there might be an issue with the volume of water being released when flushed.

Mitchell

DIYnewbie1111

@MitchellMc

Thanks again!

How would I know if it is a venting issue? Is this something easy to detect apart from odours?

With the cleaning - you may be correct in that I’m possibly pushing the water with brushing. I apply the solution and use the brush to scrub all around and as far as I can reach at the bottom of the bowl.

Could it be an issue with the main water line? Or the water shut off valve?

MitchellMc

"Could it be an issue with the main water line? Or the water shut off valve?" I don't think so @DIYnewbie1111.

A venting issue could be hard to detect. If it's just a few bubbles now and then, you'd only notice them as a foul odour in the room. Next time you smell it, check the bowl to ensure the water is at the level in your picture above. If it's low and under the red line, and you can see a gap, that would be something to go on. We can then try to diagnose why the water has disappeared. Perhaps it was sucked out due to a pressure difference in the rest of the building.

Can you please check around the back of the cistern where it connects to the floor? Look for signs of moisture and do a sniff test. If moisture is present or it smells in that location, it could indicate a leak between the S bend and the plumbing work.

Mitchell

DIYnewbie1111

@MitchellMc Hi again.

thanks for the info above. I’ve been looking for leaks or odours around the bottom of the bowl (outside) but it doesn’t look to be anything around or any odour.

i wanted to reach out because as I was cleaning today, I put in baking soda and vinegar, I have done that before- and the water lowers. I’ve attached a photo to see if anything may come to mind why it’s happening.

thanm you.

MitchellMc

Hi @DIYnewbie1111,

I don't think it has anything to do with the issue at hand. I would imagine the mixture is frothing out the back of the S bend and taking some water with it. Or, if you've given it a good scrub, as we mentioned before, the agitation has pushed water from the bowl out the S bend.

Mitchell

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