The Natural Philosopher wrote:

In essence you need to have a pressure relief system that will carry the
pongs away high up SOMEWHERE in the sewer. This may or may not be shared
between houses, and it may or may not be visible. In my case I have
carried a long exhaust pipe down the length of the roof to exit under
the eaves.

Each toilet then needs some way to ensure that as it flushes, the
suction of the falling turd and water mix does not suck dry any basin
traps etc. That is the purpose of the air admittance valve.

So you need an open exhaust somewhere downstream of every bog to prevent
pressure build up, and an inlet somewhat upstream of it, to stop partial
vacuums.


Are you sure about this? This wasn't my understanding at all. The air
admittance valve/partial vacuum thing, yes, definitely. Pressure build
up? Where is the pressure building up and why? Sewers aren't airtight.

I've seen lots of systems with an air admittance valve and nothing else.

Please expand.

--
Grunff