Swan neck vent

[freedom44]

I am replacing the pipes in the heads (lovely winters occupation!) and also the swan neck. I understand the swan neck is designed to allow air into the system to break any possibility of a siphon. The “duck bill” is a one way valve to let the air in, but not out.
Why then does the old one have a small clear pipe attached which runs to the holding tank vent overboard? Is this a “just in case” modification.
Any help much appreciated

 

[Salty John]

Vented loops on intake and discharge lines are, as you say, designed to prevent water syphoning back into the bowl, if the bowl is below the waterline. The pumps and duckbill valves (joker valves) provide the first line of defence but seals can be worn or broken.
The vented loop will have a spring loaded valve or a small rubber flap to allow air in if the pipe comes under suction. On the discharge side this valve can allow odours into the boat so a small bore pipe can be run to the deck take the smell outside.