Small boilers with atmospheric burners rely on a "natural" source of air supply in the boiler room. It is important that the air supply for this type of burner NOT be obstructed.
Boilers with forced- or induced-draft combustion, that use a fan to supply combustion air, require proper adjustment based on the volume of air that the fan can supply. If after proper adjustment, covers are removed, seals go bad, dampers are propped open, or in some other way extra air is allowed to enter the burner, there is a loss of efficiency. The extra air and the moisture that it contains must be heated to the temperature of combustion and is then sent up the chimney/flue.