John Carlyle-Clarke wrote:
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.

It must have a trap by law, so no, it wouldn't.



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.


IF you are flooding a septic tank with rainwater, you have a problem.
Thy cant be expected to not dump fairly raw sewage out of the outflow
when it rains..


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

Fine, if the soil pipes aren't blocked..this is an issue..positive build
up of explosive methane. As well as whiffs at ground level.

The whole point is to NOT have concentrated bubbles of methane and
smells bubbling up from traps and ground level vents, where flames may
be reasonably expected to ignite them.


Methane is slightly lighter than air at normal temps and pressures,
therefore high level venting 'sucks' the methane out of the drains and
disperses it reasonably harmlessly above habitable levels.

In this context your septic tank vents are actually more air INTAKES
than exhausts..

Look nothing says you HAVE to do things by the book especially if the
BCO is not involved, but its worth understanding the rationale behind
the regulations, so you can make an informed decision.

I am not a fan of over regulation. But most of the regs are sane
sensible cheap ways to avoid unecessary risks.





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