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!

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