The latest innovation for the flushing of a toilet is called an air valve or to give it its full name, an air admittance valve. There is a good chance that one is fitted in your loo. It replaces the siphoning mechanism that was a regular feature for many years.

This air admittance valve is very similar a really big non-return valve.

Basic Operation.
The air admittance valve allows air to enter into a system through its body, but will block any foul odours from returning back into the immediate area of the WC and (or bathroom). This means that because in the drain there is a negative pressure, the valve will open up instead of a suction to move the water and waste. These valves can be fitted by Enfield Plumbers.

The (air) admittance valve can be found in many sizes and sometimes this valve can be used to terminate the main waste drainage stack. The usual employment of this valve is where there are two soil stacks in the same building or lots of buildings are closely situated, so this type of valve really stops the need to run the high point of a soil discharge stack out of the roof. This will also remove the need for tiling etc. This also forms some protection for the building against rainwater ingress. The air admittance valve really is best installed higher than the critical level (also called the spill level) of any appliance. If this is not carried out and a block occurs it will back up with water and remove the valves watertight capacity. An Enfield Plumber EN2 is familiar with this type of valve.

If the valve is insulated it may be installed in quite exposed locations, like a loft space, because the condensation inside the pipes will freeze.