Grunff wrote:

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.



That becuase thay have specail vents to prevent pressure build up.

See your building regs.


Apart from that they are pressure tested to make sure they ARE airtight.


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

Please expand.