I have a downstairs toilet and upstairs toilet ,one above the other..
The soil/stench pipe connects upwards to above the Roof heaves and downward into the sewer... Coupled into the sewer also is a Gully trap drain to take sink and bath / shower wastes and from time to time , from that drain gully come a god awful smell of typical drain sewer stink.. ( Stench is the Word !)
I do not know how it ever get through the drain gully u bend water barrier , but it does.. Obviously there comes a build up of sewer gas/smell and I though it was supposed to be drawn up the soil stack and dissipated into the air above the heaves level not to come up into this drain..
Can anyone tell me if they have experienced this type of action before and how can I go about curing this smelly situation which drifts into my house...
I did think that maybe the stack may not be drawing it away and that when any waste water runs into the drain part and disturbs the natural gully barrier it is then when gas escapes Could my stack be blocked some where do you think ? Or might there be an extractor whirly fan that one might fit within the stench vent pipe to pull the foul air away from coming up the drain ?
Any help would be appreciated thanks

A gully Trap Drain, by definition, is loexhausting ide the house. Are you referring to a floor drain inside the house?
" I though it,(the smell) was supposed to be drawn up the soil stack and dissipated into the air above the heaves level not to come up into this drain."

Vents don't work that way. Vents are not for exhausting anything. Their function is to pull air into the system to relieve the vacuum caused be water draining down the pipes.
Are you on city sewer or a septic tank? Does water back up into the drain every once in a while? Waiting on your answer, Tom

Yes It is on a City Sewer system and in the UK... No the Drain is not inside the house it is outside at the side of the house to catch bath, shower and sink waste and runs into the same sewer... It has a gully water trap like a larger U bend, similar to that is within most sinks , for the very purpose of placing a water barrier to prevent smells coming back from the drains and sewers also similar to the U bend in our toilet systems..
There are no blockages at all to the inspection chamber at the end of the property and flows perfectly... I have lifted the lid slightly on Inspection Chamber Hoping it will allow any smell/Gases to vent at the chamber rather than being pushed through and back into the drain which is nearer the house living areas... So I am going to see if this helps Thanks for trying

Have you removed the grate and cleaned the crud that's built up above the trap out? Lavatory and bath wastes contain grease and hair that smells bad when exposed to air. Clean the basin above the trap with bleach and see if that doesn't help. Good luck, Tom