Most of the time, exhaust fans are designed to pull air out of a room, duct or enclosure and discharge it to the outside. They do not normally push air in. The whole purpose of an exhaust fan is to remove stale, hot, dusty or contaminated air and create a slight negative pressure so that fresh air is drawn in from other openings such as doors, windows, louvers or intake fans.
In a typical installation, the exhaust fan’s blades are oriented so that air flows from the room or duct into the fan inlet, through the impeller and then out through the discharge opening to the outdoors. The fan does not know “in” or “out” by itself; the direction depends on how it is mounted and which way the impeller turns. For an exhaust fan, the motor rotation and blade angle are chosen so that the fan develops pressure in the direction leading away from the space being ventilated.
In buildings and houses, bathroom and kitchen exhaust fans work exactly this way: they extract air loaded with moisture, odours or fumes and expel it outside through a wall cap or roof vent. Fresh replacement air enters the room indirectly from adjacent spaces or from outdoors through slightly open windows or infiltration. In factories and workshops, large wall or roof exhaust fans remove hot air, welding fumes or process emissions and let cooler outside air flow in through intake louvers on the opposite side of the building.
In industrial plants and mines, the same principle applies on a larger scale. Exhaust fans and mine exhaust airways pull contaminated air out of headings, haulage drifts and process areas. Separate intake fans or natural openings supply fresh air. By controlling the balance between exhaust and supply, engineers can guide airflow along specific routes and keep harmful gases away from occupied zones.
There are some special fans that are reversible, meaning they can be run in either direction to exhaust or supply air, but these are specially designed and usually used in tunnels or emergency smoke control systems. Standard exhaust fans are not intended to be reversed or used as supply fans without careful checking of blade geometry and performance.
In summary, an exhaust fan’s normal job is to push air out of the building or system, creating negative pressure so that cleaner air flows in from elsewhere. It is an air-removal device, not a supply fan, even though both are built from similar components.
