A very popular question to Diydoctor is " How can I stop my basin/shower gurgling when I pull the plug out?". To solve this problem its a good idea to know what causes the problem.
The water leaves the basin and begins its run into the trap underneath. This trap has water in it which stops smells coming back up through the pipes into the room through the plughole. When the water from the basin passes through the trap, it begins its run to the main waste pipe and out into the external manholes and main sewers. Some of this journey is made through (inside the building) vertical pipes and sometimes through pipes which have rather more than the 1 in 40 slope which is recommended for foul water drainage. This encourages the water to run very quickly and sometimes this causes a vacuum behind the flow. This vacuum sucks at the water which remains in the trap. Most often this "sucking" causes the water in the trap to bubble and gurgle - hence the gurgling noise. Sometimes the water is sucked out of the trap completely and this is when foul gasses can get back into the room.
Fitting an air inlet valve is easy. The pipe from the trap can be cut (as near to the trap as possible) and a Tee piece inserted. The pipes in the image above have been solvent welded together put with pipes that push-fit together the principle is exactly the same. The air inlet valve is then fixed to the top of a vertical length of pipe from the Tee piece.
When the valve is fitted the water runs away as before but instead of the vacuum sucking water from the trap it just sucks air which is admitted through the valves. This allows the water to run away noiselessly from basin, bath and shower.
If you have little space under your sink or basin, then fitting an anti-vac trap could be the answer to this problem.
See our major project on air admittance valves for details of larger valves for toilets.