Why HVAC Fan Selection Starts with the Duct System

A ducted HVAC system is a resistance network built from straight duct runs, elbows, transitions, dampers, filters, heating and cooling coils, and terminal devices. The fan does not decide how much air moves through that network on its own. Airflow settles at the point where the fan curve and the system resistance curve intersect — the operating point. HVAC fan selection is the task of placing that intersection at the design airflow.

Get it wrong in either direction and the symptoms are predictable. A fan selected well above the system requirement runs further up its curve than necessary, producing excess velocity pressure, more noise and wasted power. A fan selected below the system requirement simply cannot overcome the resistance: airflow falls short, coils lose capacity and terminals are starved.

Duct Static Pressure: The Load the Fan Must Beat

External static pressure is the total pressure the fan must develop to move design airflow through the ductwork and components outside the fan itself. It combines friction losses along ducts with dynamic losses at fittings, and it is calculated for the most unfavourable path in the system — normally the longest run plus the highest-loss components.

System resistance does not rise linearly with flow. It rises roughly with the square of airflow, which is why the system curve is a steepening parabola and why small flow increases near the design point cost disproportionately more static pressure. Two consequences follow:

  • Dirty filters and partly closed dampers shift the system curve upward, pushing the operating point to lower flow. Selection should be checked at both clean and loaded filter conditions.
  • Adding a filter, coil or extra fitting after the fan has been selected can move the operating point off the curve it was chosen for. Re-check the intersection after any system change.

Velocity: Duct Size, Pressure Drop and Noise

Velocity pressure is a useful shortcut when reading duct profiles. At standard air density (about 1.2 kg/m³), velocity pressure is approximately 0.6 × velocity², so 5 m/s corresponds to roughly 15 Pa, 8 m/s to about 38 Pa, 10 m/s to 60 Pa and 12 m/s to about 86 Pa. Because friction and fitting losses scale with velocity pressure, pushing velocity up to shrink duct size raises both pressure drop and regenerated noise.

As general guidance, main HVAC supply ducts are commonly kept in the 6–10 m/s range, branches lower, and noise-sensitive spaces lower still. Lower velocity means larger ducts, but a smaller fan duty and quieter operation.

Axial Fan for Ducting vs Centrifugal Fans

Fan type should follow the shape of the duty, not the other way round.

  • Axial fans suit low-resistance, high-flow duties — long straight duct runs, return-air paths, and ventilation where resistance is moderate. SINDFAN SM axial fans cover impeller diameters from Ø250 mm to Ø3150 mm with efficiencies up to 80%+, and mount directly in or in line with ductwork.
  • Centrifugal fans suit high-static duties: systems with filters, coils, long duct runs or tall risers where resistance is significant.

Axial fans run most stably in the flatter, right-hand part of their curve. If system resistance pushes the operating point toward the stall region, flow can become unstable and noise rises sharply — a sign that a centrifugal unit or a larger impeller is the better answer.

Vibration, Noise and Certified Performance Data

Ducted fans are structurally connected to the building, so rotor balance matters. Balancing to ISO 1940 G4.0 keeps vibration low, reducing structure-borne noise and wear. Where a ducted system serves occupied space, silencers are the usual way to control fan and duct-borne noise.

For design work, ask for performance data measured to AMCA 210 or ISO 5801 rather than indicative catalogue figures, and request the fan curve alongside air power, speed and sound data so it can be plotted against the system curve.

From the room load to the airflow: HVAC fan selection before the duct

Duct sizing is the second half of an HVAC fan selection. The first half is deciding how much air the space actually needs, and that number is not chosen — it is calculated from the cooling load, the permissible temperature difference and, where ventilation is the driver, the required air changes.

  • Cooling load to airflow. A sensible cooling load in kW is turned into a volume flow with Q = P / (rho · cp · dT). With dT as the supply-to-room temperature difference, a small change in that difference moves the airflow a long way: the same room load met with a 10 K difference needs twice the air of a 20 K difference, and both the duct and the fan change size with it.
  • Air changes per hour. For plant rooms, kitchens, car parks and battery rooms the airflow is set by an air change rate rather than by a cooling load. Work out Q = room volume × ACH / 3600 and check which of the two governs — the larger number wins.
  • Duct sizing on friction rate. Once the airflow is fixed, the duct is sized on an equal friction rate, typically 1.0 to 1.5 Pa per metre for low-velocity systems, subject to a velocity limit for noise. Reduce the friction rate and the duct gets bigger but the fan static pressure falls; raise it and the fan pays for it continuously.
  • Add the dynamic losses back. Fittings, dampers, grilles, filters at their dirty condition and the terminal units each carry a loss. The fan duty is the sum, and it has to be quoted on the same basis as the fan curve, using Pt = Ps + Pd.

For VAV systems the selection point is not the design airflow. The fan has to be selected so that the part-load condition, where the boxes throttle and the static-pressure setpoint drops, still sits in a stable part of the curve and clear of the surge or stall region. A fan that is only checked at full flow is being checked at the condition it will rarely see.

Selection Checklist

  • Calculate worst-case duct static pressure, including loaded filters.
  • Choose duct velocities and confirm the resulting losses.
  • Plot the system curve and confirm the operating point sits in the stable region of the fan curve.
  • Check the fan at both clean-filter and dirty-filter conditions.
  • Confirm impeller balance grade, vibration limits and noise treatment.
  • Verify performance is certified to AMCA 210 or ISO 5801.

If you send SINDFAN your duty — design airflow, duct static pressure, air temperature and layout constraints — we can advise on axial or centrifugal selection and the right impeller size. Contact us on WhatsApp at +852 9524 8434 or email info@sind-tech.com.