To check if a fan is intake—that is, to find out which side of the fan air is entering—you should use a combination of visual inspection and simple airflow tests, always keeping safety in mind. You never need to touch the rotating parts. Instead, look for markings, feel the airflow from a safe distance or use a light indicator such as paper or smoke.
The first and easiest method is to look for arrows or labels on the fan housing. Many manufacturers print or emboss two small arrows on the frame: one shows the direction of rotation, and the other shows the direction of airflow. The arrow pointing in the direction that air moves identifies the discharge side; the opposite side is the intake. Some fans also have words such as “INTAKE” and “EXHAUST” or “INLET” and “OUTLET” near the openings.
If there are no clear markings, you can use a simple paper or tissue test. With the fan running and guards in place, hold a small strip of light paper or tissue near one side of the fan, without letting it get close to the blades. If the paper is pulled towards the fan, that side is the intake. If it is pushed away, that side is the discharge. Repeat the test on the other side to confirm. This method works well on axial panel fans, cabinet fans and many industrial ventilators.
For larger industrial or mining fans, it may be safer to observe from a distance using smoke or mist. Technicians sometimes use smoke pencils, theatrical smoke or tracer gas in commissioning tests. In normal operation you might see dust or fog patterns that show which way the air is moving. Any visible plume being drawn towards the fan indicates the intake side, while jets moving away from the fan show the discharge direction.
Another clue can come from the shape of the impeller and housing. On axial fans, the intake side often has a bellmouth or smooth inlet cone, while the discharge side may connect directly to ductwork. On centrifugal fans, the circular opening at the centre of the housing is the intake, and the rectangular side opening is the discharge. Understanding these standard geometries helps you identify the intake even before the fan is energised.
When working around large fans in industrial plants or underground mines, always follow lockout–tagout and safety procedures if you need to get close to the equipment. For routine checks of airflow direction, it is best to observe from safe positions or rely on installed indicators and drawings. Once you know which side is intake, you can position filters, silencers, ducts and guards correctly to achieve the intended ventilation performance.
In summary, you can check if a fan is intake by looking for manufacturer markings, using a light paper or smoke test, and understanding the typical geometry of axial and centrifugal fans, all without touching moving parts.
