Also called siphon, anti-siphon or vacuum breaker valve. This valve allows air to escape the boiler as the water heats up, hissing and then closing off when the boiler water reaches it's operating temperature, sealed by the steam pressure created. The barbed portion measures 8 mm O.D. and can be used with silicone tubing to collect and direct condensation to the drain.
Gasket to seal against the boiler not included.