Ventilation is used for two reasons in terms of central heating systems.
On open flued systems the air coming through the vent is for combustion purposes.
Compartment ventilation is used for cooling.
Unless stated otherwise by the manufacturer an open flued appliance upto 7kw (gross or net) does not need any purpose built ventilation (execpt in northern ireland).
Natural ventilation or "adventitious ventilation" as its know is enough. This is the air that enters the home through every conceivable crack or hole.
Anything in excess of 7kw (gross or net) will require 5cm² per kw (net)
Compartment ventilation for open-flued appliances