The Vetus anti-syphon device type AIRVENTV has a pressure valve which opens to prevent water syphoning back into the boat.
The stepped hose tails can be cut to enable the device to be connected to hoses of internal diameter 13mm (½"), 19mm (¾"), 25mm (1") or 32mm (1¼").
This can be installed onto a mounting bracket to enable it to be fitted clear of engine room sound insulation.
