HI John

John Carlyle-Clarke wrote:
We have a septic tank in the garden and an underground soilpipe that
feeds into it. The downstairs toilet drops into it via 110mm plastic
pipe, which has a vent up through the roof. The downstairs bath and
sink use standard 2" waste which feed into an open ground level drain
outside.

We added later an upstairs shower room with toilet and sink, which has a
smaller (2" maybe) air admittance valve in a cupboard behind it.

We are now redoing the downstairs bathroom and want to remove the vent
and replace it with an air valve inside the WC wall unit.

(1) Is it OK not to have an outside vent in the system?
(2) How low can the valve be in the WC unit? I saw something about it
being higher than the highest trap in sinks etc, but I assume this
refers to items on the same pipe segment.

Any advice much appreciated!

The gentleman who built this house cunningly extended the top of the
soil pipe into the gap between the roof tiles and the plasterboarded
ceiling in the upstairs bathroom..

Not a good - plan - nasty smells in the house when the wind's in a
certain direction.

More out of hope than anything else, I added a 'durgo'-type valve on the
open end of the stack - but (of course!) it's fine at allowing air
_into_ the stack when the toilet flushes or the bath empties, but we
still get pressure biuld-up & foul air coming back up from the septic
tank and passing round the various water traps - so nasty smells again...

Yesterday I purchased the necessary bits to connect a 2" pipe into the
top of the existing stack, and run outside the house and up the gable
end - thus (hopefully) allowing the noxious fumes to find their way out
of the system and away... (Because of the way the 4" pipe has been taken
into the house through the soffits you can't just extend the stack up in
4").

If it wasn't snowing at the moment I'd be out there fitting it all
together... perhaps tomorrow....

Doncha just wish people'd do things right the first time !?

Adrian