Buying guide
How to choose a ventilation fan box
Choosing a ventilation fan box comes down to establishing two quantities and checking that they intersect in the right place on the fan curve: the air volume flow rate to be extracted or supplied, in m³/h, and the available static pressure, in pascals (Pa), which the unit must generate to overcome the resistance of the ductwork, the filters and the terminals. Small units typically cover 300 m³/h, while large roof-mounted units reach around 5,000 m³/h and beyond. Read catalogues carefully: the headline maximum airflow is often quoted at a reference pressure drop of 100 Pa, not at the real duty point of your ductwork.
Step 1 — calculate the airflow you need
The airflow follows from how the space is used, not from the size of the unit. Three methods coexist, and you take the most demanding of the three.
| Application | Indicative airflow | Usual unit type |
|---|---|---|
| Washrooms / small rooms | 300 – 800 m³/h | Compact extract fan box |
| Offices, retail | 1,000 – 5,000 m³/h | Insulated double-skin fan box |
| Workshop, production hall | 5,000 – 20,000 m³/h | Fan box or roof-mounted extract unit |
| Smoke extraction | Per sizing calculation | EN 12101-3 rated smoke extraction unit |
- Per occupant: in commercial buildings, design guidance works to around 25 m³/h of fresh air per occupant in an office, rising to 30 m³/h as soon as light physical activity is involved (values from the European standards NF EN 16798-1 / NF X 35-102, widely used in Moroccan design practice).
- By air changes per hour: room volume × number of air changes per hour, suited to workshops and plant rooms.
- By appliance footprint: in a commercial kitchen, the airflow is calculated over the cooking appliances (see our dedicated page).
Step 2 — estimate the available static pressure
Static pressure is the 'force' the unit has available to push air through the system. It has to cover the sum of the pressure drops: straight duct runs, bends, dampers, filters, grilles and terminals, attenuators, fire dampers. A correctly sized unit places its duty point in the high-efficiency region of its airflow/pressure curve.
A modest safety margin is healthy, but heavy oversizing is paid for in noise, energy consumption and an unbalanced system. Undersize it, on the other hand, and it will never hold the stated airflow once the filters load up.
Step 3 — choose the type of unit
- Extract fan box: the most common, removing stale air from a space or a duct network.
- Supply fan box: brings in fresh air, usually filtered and sometimes thermally treated.
- Insulated double-skin fan box: enhanced acoustic and thermal insulation, the choice wherever the unit sits close to occupied spaces.
- Roof-mounted extract unit: vertical discharge at roof level, widely used in industry and commercial kitchens.
- Smoke extraction unit: specific construction and motor, with a temperature rating declared to EN 12101-3.
The criteria that make the difference in service
- Acoustics: sound power level at discharge and breakout; allow for an attenuator if the unit serves offices or a public-access building.
- Motor type: an EC (electronically commutated) motor allows fine speed control and lower consumption than an AC motor driven by an inverter.
- Integrated filtration: check the filter class the unit accepts and the space available to change it (see our filter class guide).
- Maintenance access: inspection door, motor withdrawal, clear height around the unit.
- Operating environment: dirty, humid or corrosive air calls for a surface treatment or a different material.
The three most common mistakes
- Choosing on headline catalogue airflow alone, without checking the static pressure actually available at that airflow.
- Ignoring the pressure drop of loaded filters: the duty point moves throughout the filter's service life.
- Fitting a comfort ventilation unit where a rated smoke extraction unit is required — the two are not interchangeable.
Frequently asked questions
What airflow does a ventilation fan box need?
The airflow is calculated from how the space is used: per occupant (around 25 m³/h per person in an office, 30 m³/h where light physical activity is involved), by air changes per hour for workshops and plant rooms, or by appliance footprint in a commercial kitchen. You take the most demanding figure. In practice, a compact unit covers 300 to 800 m³/h, a commercial unit 1,000 to 5,000 m³/h and an industrial roof unit beyond that.
What does the static pressure of a ventilation fan box mean?
It is the pressure, in pascals, the fan has to generate to overcome the resistance of the system: ducts, bends, dampers, filters, grilles, attenuators. Maximum airflows quoted in catalogues often correspond to a reference pressure drop of 100 Pa; if your system is more resistant, the airflow you actually get will be lower.
Should I allow a safety margin on the airflow?
A modest margin is advisable, to absorb filter loading and calculation uncertainty, but heavy oversizing is counter-productive: it generates noise, increases consumption and unbalances the system. Good practice is to place the duty point in the best-efficiency region of the fan curve and to fit speed control.
Can a standard ventilation fan box be used for smoke extraction?
No. A smoke extraction unit must keep performing in hot smoke, with a temperature rating declared to EN 12101-3, a suitable motor and suitable materials, and a backed-up electrical supply. A comfort ventilation unit meets none of those requirements.
Further reading
Our range of extract and supply fan boxes.
Read more Sizing a ventilation duct networkAir velocities, duct sizes and pressure drops.
Read more Ventilation serviceAirflow studies, installation and maintenance.
Read more Air filter classes G4, F7, HEPAChoosing the filtration built into your unit.
Read more