How do automatic air vents function?

Automatic air vents function by using an internal disc that swells with the presence of water. This mechanism is designed to allow air to escape from the system while preventing water from leaking out. When air becomes trapped in a hydronic heating system, it can create issues such as reduced efficiency and uneven heating. The swelling of the internal disc in the automatic air vent creates a seal that lets any built-up air escape while maintaining a barrier against water, ensuring the proper operation of the heating system. This automatic process aids in the maintenance of the system without the need for manual intervention, improving efficiency and reliability in air management.

The other options describe actions or features that are not characteristic of automatic air vents. For example, releasing air from the boiler automatically is a function of these vents but explained in a way that doesn't clarify the mechanism in play. Manually opening vents to relieve pressure is not automatic, and prevention of water leakage relates more to the overall system design rather than the specific function of automatic air vents.