The Natural Philosopher wrote:
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?

No.

Durgos work as anti=siphoning systems. But a positive pressure relief
system is also needed. There must be at least one open vent somewhere.

However you can simply run a pipe into the loft space and take it where
you want..if the loft is cold..I THINK its OK just to exhaust there. Or
run a pipe to a gable.

It's a chalet bungalow so no loft to speak of.

Would the open drain outside into which the kitchen sink, and downstairs
bathroom bath and basin feed, not act as a positive pressure vent? I
don't know if it has an integral trap or not.

There are also two open rainwater drains where rainwater downpipes feed
into the soil pipe (not allowed on new build, I know) although I believe
these have traps in them.

The septic tank itself has vents also (round open 4" clay pipe recessed
into the ground which leads down into it).

--

For mail, replace null with net.