foul smell in bathroom

periodically we experience a foul smell in the bathroom (not just after a recent visit!) I suspect the soil connection to the wc pan the current connection appears to be clay (?) wrapped in some kind of bandage which dissappears horizontally into the wall. The exterior stack is cast iron. the clay pipe is obviously very old and does not even have a circular cross section any more (but there are no signs of a leak this side of the wall) Would a new plastic pipe with rubber flange simply push-fit into the cleaned-up cast iron? Could the smell be caused by a blockage in the top of the soil stack (birds nest etc)?
The house is a two storey stone building of 100+ years
Many thanks, Ian Perthshire
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ianw wrote:

What is the level of the vent?
On an old property too, there may be an open junction just outside the loo where other plumbagoes.
Perhaps a picture of the outside with the offensive window exed would help. If it is odour coming from the top of the stack, the weather is likely to be similar in all cases where the niff comes in.
If it was a leaking joint then the stink would be deadly and permanent and you'd soon notice where it was.
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Any chance that you have a separate shower cubicle, which you are using in preference to the bath?
In this hot weather especially our bath U-bend then dries up, giving us a direct nasal passage to any nasties in the outside drain.
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Tony Williams.

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Does anything else like a basin or bath go into the same stack? If so the water in the trap in these can dry out this weather and allow smells through.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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ianw wrote:

I suspect that one or more U-bend traps is being sucked dry. Possibly in periods of high wind.
Sniff at the plugholes next time it happens
It could be simply bad design or a blocked vent.
It may be possible to install a Durgo type air admittance valve into the pipework if it is bad design.
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Thanks for the posts.
After reading your comments I had another look at the layout: The top of the soil stack is about 3m above the wc connection however about 18 months ago I fitted an extractor fan. Due to coombed ceilings I chose a 12V model about 1.6m from floor level above the wc. However this is only approx 1.5m from the head of the stack. The date of installation roughly corresponds with the appearance of the pong. Our shower is over bath so, along with the presence of a teenage daughter, there is little chance of traps being dry. I will disable the fan, seal with cling film for a few weeks and see if the smell returns. Thanks again, Ian
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ianw wrote:

That makes sense.
You can fit a fan with electrically operated shutters, or add a bit to the soil vent top.
Or arrange the vent to be further away..its only there to prevent pressure/vacuum build up so it can be pushed horizontally in the loft for miles..mine is..
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