Window air-conditioning units are often used for cooling only. In such situations, the outside air vent should be covered in the winter to prevent infiltration through the unit. Otherwise, the window unit becomes a full opening to the outside.
This applies to air-handling units in large buildings as well. They get outside air through louvered wall openings with dampers that are supposed to close airtight when sensing controls do not call for any fresh air. These louvers are gasketed and should shut completely when the louver operator is in the closed position. This air seal can be verified periodically to limit unnecessary outside air intake. At the same time, the louvers should be tested to be sure they close entirely when the pneumatic or electronic controls so require.
There are many other openings through the building envelope that are in seasonal or occasional use. These include exhaust vents and fans, dryer vents, shop vacuum exhausts, kitchen vent hoods, and laboratory fume hoods. These devices are often installed to vent through a permanent opening. They have backdraft dampers that close automatically to prevent reverse flow through the unit, from the conditioned space outward. Many manufacturers market retrofit kits that can be installed to incorporate a damper into existing units. These can be real money savers, especially in buildings with high stack effect.