Why some appliances need a permanent air vent and where the rest of their combustion air comes from.
Every gas appliance that burns its fuel in room air needs a reliable supply of combustion air, and a recurring CCN1 theme is where that air comes from and when a permanent vent is required.
Adventitious air is the small amount of air that leaks naturally into a room through gaps around doors, windows and the building fabric. It is uncontrolled, it varies with how the building is built and maintained, and modern draught-proofing steadily reduces it.
Purpose-provided ventilation is the opposite: a deliberate, permanent air vent sized for the appliance and positioned so it cannot easily be blocked. Because it is intentional and fixed, it can be relied on where adventitious air alone is not enough.
The distinction matters because open-flued and flueless appliances take their combustion air from the room. Above a certain input an open-flued appliance needs purpose-provided air on top of any adventitious leakage, so that it always has enough air to burn cleanly and clear its products.
Room-sealed appliances are different. They draw combustion air from outside and discharge to outside through a sealed system, so they do not depend on room air in the same way and generally do not need a purpose-provided combustion-air vent.
For revision, remember that you cannot count on adventitious air, which is why standards set out purpose-provided ventilation for the appliances that need it. Confirm the thresholds and vent sizes for the specific appliance and room against the current standard.