Why you cannot select an air handling unit on airflow alone

A building needs 2000 m³/h. The catalogue lists a unit with a maximum airflow of 2000 m³/h. On paper, the problem seems solved, but in reality selection has not even begun. The key question is: against what resistance must the unit deliver this airflow?

Airflow and pressure must be selected together

The fan interacts with ductwork, filters, heat exchangers, dampers and diffusers. The operating point occurs where the pressure generated by the fan matches the resistance of the duct network at a given airflow.

For one illustrative unit, 2000 m³/h may be the maximum airflow at almost no external resistance. A second delivers the same 2000 m³/h at 400 Pa. If the duct network requires 400 Pa, these units are not equivalent.

An assumption cannot make up for insufficient pressure

For a simplified duct network with fixed geometry, resistance can be expressed as Δp = kL². If resistance is 400 Pa at 2000 m³/h, then at 1600 m³/h it will be approximately:

400 × (1600 / 2000)² = 256 Pa

A unit with insufficient pressure capability may run without a fault but deliver less air: the duct network and fan find a different equilibrium point. Determining it precisely requires the performance curve for the specific fan.

External pressure and fan pressure are not the same

The unit already contains filters, a heating coil, a cooling coil and a heat recovery exchanger. Some of the pressure is used within the casing; the remainder is available to the external duct network. The selection sheet must show which components are included, so their resistance is not counted twice and an external duct filter is not overlooked.

Check more than just clean-filter conditions

As filters become dirty, their resistance increases. At a constant fan speed, airflow may fall. Constant-airflow controls will increase speed only if there is sufficient margin in speed, power, noise and the permissible operating range.

Before approval, obtain a selection sheet showing supply and extract airflow rates, external pressure, power at the operating point, filter condition and acoustic data. Maintenance access to filters and heat exchangers is equally important.

Correct selection starts not with ‘how many cubic metres this model delivers’, but with ‘what operating point it can achieve in my duct network’.