Oil- and gas-fired boilers require copious quantities of oxygen to burn the fuel. Mechanical codes require a large enough opening in the walls of the boiler room to supply this air volume, in addition to an opening near floor level for complete ventilation. These are usually louvered wall openings, and they may have become cluttered or partially covered with boxes, cabinets, or supplies. These obstructions should be removed so the process of combustion can occur most efficiently to generate the maximum amount of heat from the fuel.
Another consideration with clutter and supplies in a boiler room is the potential for spontaneous combustion, especially in the presence of oily rags and semi-combustible chemicals.
In older buildings, outside-air openings may not have been provided at all or may be undersized. Consequently, the boiler must draw the oxygen it needs from the occupied space; this will have an adverse effect on that environment. Also, the hot combustion gases may not be completely exhausted to the outside if there is not proper ventilation in the boiler room. A portion of these fumes may even make it into the building itself.
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Excerpt from Energy Conservation Projects.