Downstairs WC smells terrible!

[lazyakita]

We moved in about ten years ago and the downstairs WC was old fashioned but functional. A year or two later we updated it with a cheap, modern low flush replacement. Since then, we have been plagued by on and off sewage smells from the toilet! It seems to be worse at night and when it rains, although it doesn't always do it and we can go for weeks without issues. I have had three plumbers look at it, all of whom were stumped and can't find any obvious issues. One replaced a seal in the waste pipe and insisted that would fix it; it didn't. I've asked if a vent could be blocked somewhere but they all say it's unlikely and don't investigate further. We are now having that room fully renovated and the fitter said it could be something to do with the angle the waste pipe is fitted at (or downstairs is slightly lower than the pavement outside) and thinks replacing the toilet and checking the waste pipe will be the solution. Does anyone have any advice? I keep our home really clean and am starting to find this very stressful, as silly as that might sound!

Just a bump in case anyone has any ideas. Thank you :)

Have you got a dual flush toilet? If so, the 'small' flush generally isn't powerful enough to clear the pan and all the pipes - means the bowl is just cleared to the first junction/manhole cover. I'm sure plumbers will have looked but you might have a blockage of... stuff at the first manhole away from the loo. A good poke around/hose would clear it. If not, there could be a blockage nearby, again, caused by the small flush. If you do have a dual flush, i would always recommend using the big flush, unless the bowl is just liquid!

Thanks so much, TwoPlugs. Yes, it is a dual flush, but we don't use the small flush as the button is really tough to press, so we always use the main flush. Interestingly there was a drain blockage a couple of weeks ago that caused our neighbour's toilet to overflow and our water level was too high. Called out emergency drain guy and he shifted a blockage a couple of houses down, but a few days later the smell returned. But it's gone now and hasn't been present for a few days. It is baffling. I will have someone check our manhole in case the problem is there. Thank you again, it's driving me up the wall and I want to fix it before we sell.

I should add that the cistern is tiny (it's a really small toilet!) so the flush has never seemed very powerful to me. I wonder if that is the problem.

I'm not a plumber but I have a little experience of this. Keep in mind for you to get this smell, its most likely some gas from the sewer will be excaping back into your room.

Does the water level in your bowl ever go down or disappear after its been flushed? Maybe when you flush another toilet or run a tap you can see the level move? This would indicate a blocked vent.

Do you know if you have an internal or external vent? If its internal then it will most likely be something called an air admittance value (also known as a durgo valve.). I have seen one of these fail and that certainly will produce smells. See this link and determine which you have:- job-prices.co.uk/durgo-valve/

Do you have an external overflow pipe coming from the toilet? I think most toilets these days have an internal one with just overflows into the bowl, but you can have an external one and this can let gas back in if not installed correctly. If you're unsure take the lid off and take a couple of pictures.

Thank you, TobyHouseMan! The problem tends to get worse in inclement weather when I use the sink in the house bathroom (which is situated above the downstairs WC). However, I just tried turning on sink & bath taps and couldn't see any movement in the level of the downstairs WC now. Very occasionally the water level will go very low down after a flush but it's usually if too much TP was used!

I don't know if I have an internal or external vent, but we do have a three storey house with Velux windows fitted which suggests it might be internal. I've attached a photo of the roof as it looks like a capped valve there to me? Just behind the boiler flue.

I also added a photo of the inside of the cistern as again I'm not sure about the overflow. There doesn't seem to be an external one. Thank you so much for this, being believed that it might be a vent problem is restoring my sanity a bit

I would love to know the answer to this
We have it in an upstairs bathroom and there is no rhyme or reason as to when and why it happens

Thanks for the info. It's not easy to diagnose this exactly, sorry. Someone with more practical experience than me might be more help. It does look like you have an external vent in your roof.

If you do notice that the level in the toilet after a flush is sometimes low then this is worth investigating. When you say low, I assume you mean considerably lower than normal?

What this means is when you have flushed, the exiting waste is actually creating a suction behind it which draws the new water away. No matter how much TP you use this should not really happen. One of the things the vent does is provide a source of air behind the exiting waste so you avoid a suction drawing the new water away.

So when you have a suction is can draw water out of the toilet bowl. Now if there is not enough water left to provide a seal with the drain then there is a means for the sewer gas to flow back up and enter the room. Have a look at the first image and it might explain it some more. The second result of this suction is that it could also pull out the water in the trap for the sink and thus allow gases to flow back.

Have a look at the second image. This shows a correct type of installation. If the waste flowing from the toilet does create a vacuum going to the sink then air can enter the system from the 'branch vent' and it won't pull the water out from the sink -u-bend (which is stopping gases entering the room through the sink plug).

It may well be that you don't have the equivilant of this branch vent which most likely lead to smells.

If this is the case it can be fixed in a number of ways. One option is to fit an air admittance value as in the third image. This acts as a one way value - it will allow air to go into the system when a suction is generated but not allow it back up.

I cannot solve your problem as there are too many unknowns, but at least now you may have an idea of why this is happening. Knowledge is power as they say.

@TobyHouseMan Thank you so much for this, I can't tell you how helpful it is! I'm going to have a long chat with the fitters when they start work to see if we can solve this once and for all. Thank you again, I am extremely grateful. If we do get it fixed I'll update just in case anyone else has a similar issue in future :)