Fan selection starts with a number the fan supplier cannot calculate for you: the resistance of your duct system. Send an airflow requirement without it and you will get a quotation for a fan that moves the right volume against the wrong pressure — which in practice means it moves considerably less than the right volume.
Establishing the two numbers that matter
Airflow comes from the application. In HVAC it usually derives from air changes per hour: room volume multiplied by the required change rate. A 400 m³ space at 8 air changes per hour needs 3,200 m³/h. Process ventilation, capture hoods and dust extraction each have their own basis, generally face velocity multiplied by open area.
System resistance is the total static pressure the fan must overcome, and it is the sum of every loss in the path: straight duct friction, bends, transitions, dampers, filters, coils, silencers and terminal fittings.
Two things about resistance are worth knowing before you start. Filters are quoted both clean and dirty, and you must design on the dirty figure — a filter at final pressure drop can be two to three times its clean value, and a system designed on clean numbers loses airflow steadily as filters load. And bends and transitions often contribute more than the straight duct, particularly in compact plant rooms with short runs and many fittings.
Once calculated, add a margin — commonly 10–15% — for construction differences from drawing. Do not add 50% "to be safe". An oversized fan is not a neutral error, as the next section explains.
Reading the fan curve
A fan curve plots static pressure against airflow. The fan does not have one output; it has a locus of possible operating points, and where it actually lands is set by the system.
Your system also has a curve. System resistance rises roughly with the square of flow — double the airflow and you need about four times the pressure. The operating point is the intersection of the two curves. That intersection is the only place the fan can run.
This is why a fan rated "5,000 m³/h" will not deliver 5,000 m³/h into your system unless your system resistance at that flow matches the fan's pressure at that flow. The rating is a point on a curve, not a promise.
Manufacturers overlay efficiency contours and power lines on the curve. The target is to land near peak efficiency, which typically sits at 50–65% of the fan's free-delivery flow — noticeably to the right of where buyers instinctively choose.
Why oversizing is the expensive mistake
An oversized fan operates far left on its curve, in the region of low flow and high pressure. Three things follow.
Efficiency collapses. Running well left of the best-efficiency point can mean 40–50% efficiency where 75–80% was available, and that difference is paid every hour the fan runs for its whole service life.
On forward-curved and some backward-curved impellers, the far-left region is unstable. The airflow separates from the blades and reattaches cyclically — stall, and in ducted systems surge. It produces pulsing flow, audible noise variation and mechanical fatigue on bearings and blades. Fans have failed early from nothing but sustained operation in this region.
And you pay for the capacity twice: once in the capital cost of a larger fan and motor, and again in energy for the life of the installation.
If uncertain about system resistance, do not oversize the fan. Select correctly and add a variable speed drive, which lets you match actual conditions after commissioning.
Temperature, density and altitude
Fan curves are published at standard air density, conventionally 1.2 kg/m³ at 20 °C at sea level. A fan is a volume machine — it moves the same volumetric flow regardless of density — but the pressure it develops and the power it draws scale directly with density.
Hot air is less dense, so a fan handling 200 °C air develops substantially less static pressure than its rated curve suggests. Correct the required pressure to the actual operating density before selecting, or the fan will underperform in service while meeting its published specification exactly.
The reverse case catches people during commissioning: a fan destined for hot service, started cold, draws considerably more power than its design point because cold air is denser. Motors have been overloaded on start-up in high-temperature applications for precisely this reason. Check the motor against the cold-start condition, not only the design condition.
Altitude reduces density too — roughly 12% at 1,000 m, 22% at 2,000 m.
Motor and drive
Size the motor on absorbed shaft power at the operating point, plus margin. Confirm supply voltage and frequency, because 50 Hz and 60 Hz supply changes fan speed and therefore everything else — a fan built for 50 Hz run at 60 Hz turns 20% faster, moves 20% more air, develops 44% more pressure and absorbs 73% more power. That last figure is how motors get burnt out by a supply assumption nobody checked.
Direct drive is simpler, more efficient and has no belts to maintain, but fixes fan speed to motor speed. Belt drive allows speed selection by pulley ratio, which is valuable when the exact duty is uncertain, at the cost of a few percent efficiency and ongoing belt maintenance.
What to send for a quotation
Airflow in m³/h or CFM. Static pressure in Pa or inches water gauge, stated as total system resistance. Air temperature and any dust or moisture content. Supply voltage, phase and frequency. Installation orientation and discharge direction. Noise limit if any, with the measurement distance. And any hazardous-area classification.
Send us these and we will return a curve with the operating point marked and the absorbed power at that point — so you can see where on the curve your system actually lands, rather than a catalogue number.