"John Carlyle-Clarke" wrote in message
...
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!

When on a septic you need an open vent on the main stack that draws air in
and out and releases methane gas. On some legs off durgos can be fitted.
Look into HepVo traps, by Hepworth, which are air admittance valves.