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Commercial extractor fans: types, sizing and costs

Updated 23 October 2026 · SEO Dons Editorial

What types of commercial extractor fan are there?

Regulation 327/2011, the ecodesign rule for fans, defines four fan types by how air passes through the impeller.

TypeHow the Regulation describes it
AxialMoves the air along the line of the impeller's axis
CentrifugalAir enters along the axis and leaves at right angles to it
Mixed flowThe air path sits between the centrifugal and axial types
Cross flowAir crosses the impeller at right angles to its axis, in and out at the edge

In a building the format matters as much as the type. Approved Document L Volume 2 distinguishes local units, which serve a single room and need no ducting, from zonal systems, which serve a group of rooms and require ducting. Systemair, a fan manufacturer, describes a duct fan as built into the ventilation duct and says that placing the fan in the duct minimises noise in the room. It describes roof fans as placed on the roof and especially used in industrial ventilation for removing gas, vapour or air.

How much extract airflow does a space need?

Start from the room, not the catalogue. For offices, Approved Document F Volume 2, the Building Regulations ventilation guidance for England, sets intermittent extract rates of 15 litres per second per shower or bath and 6 litres per second per WC pan or urinal. It sets 20 litres per second per machine for rooms with printers or photocopiers in substantial use, and 15 litres per second where a food and drink area is only for a microwave and drinks. Commercial kitchens follow separate guidance: see the kitchen extraction guide.

A hypothetical toilet block shows the sum.

ItemRateNumberExtract
WC pans and urinals6 l/s each848 l/s
Showers15 l/s each230 l/s
Total78 l/s (0.078 m³/s)

Controls matter as much as the number. Approved Document F says sanitary extract should be capable of continuous operation if required, and that in a room with no openable window it should run while the room is occupied and for at least a quarter of an hour once people have gone. Its ventilation strategy relies on extract from rooms that release moisture or pollutants, so these are removed before they spread through the building, and the air taken out has to be replaced. Whole-building supply for offices is 10 litres per second per person or 1 litre per second per square metre, whichever gives the higher total.

How is an extractor fan sized from airflow and pressure?

Two numbers set the duty: the airflow and the pressure the system puts against it. Annex II of Regulation 327/2011 defines fan efficiency as gas power divided by electrical input power, where gas power is volume flow rate multiplied by the pressure difference across the fan. Rearranged, electrical input is airflow multiplied by pressure, divided by overall efficiency.

Take a hypothetical fan moving 0.5 cubic metres per second against 300 Pa. The gas power is 150 W. The overall efficiency here is assumed, because a datasheet states the real figure.

Overall efficiency (assumed)Electrical inputEnergy over 2,600 hours
40 per cent375 W975 kWh
50 per cent300 W780 kWh
60 per cent250 W650 kWh

The pressure is the sum of the losses through ductwork, bends, grilles, filters, dampers and discharge. Approved Document F asks for ductwork designed to minimise overall pressure losses, which keeps the pressure, and the motor, down. It also says fan units should be sized so that fans in normal background mode are not overly noisy, which might mean not running near maximum capacity. Systemair's selector asks for the airflow, power and external pressure, the same inputs.

What do the fan efficiency rules require?

Regulation 327/2011 covers fans with a motor input power between 125 W and 500 kW that are axial, centrifugal, cross flow or mixed flow. The version on legislation.gov.uk is the UK text. From 1 January 2015 all fans must meet the second-tier target in Annex I, Table 2. Fans built only for short-duty emergency use in fire safety, for potentially explosive atmospheres, or for gas above 100 degrees C are outside it. Dual-use fans for ventilation and emergency use have their Table 2 grades reduced by 5 per cent.

For buying, the Regulation's product information list is the useful part. The technical documentation should show whether the efficiency calculation assumed a variable speed drive, the rated motor power input, flow rate and pressure at optimum efficiency, and the speed in rotations per minute at that point. Approved Document L Volume 2 adds that general air distribution fans rated above 1100 W should have variable speed drives, that specific fan power at 25 per cent flow should be no greater than at full flow, and that design-flow specific fan power should not exceed Table 6.9, where a zonal extract system with a remote fan is 0.5 watts per litre per second.

What about controls, EC motors and noise?

Systemair says a duct fan is most often controlled by a timer, humidistat or thermostat, and says its duct fans use highly efficient EC motors. The Regulation sets efficiency, not the motor type; the EC fans retrofit guide covers that choice. For noise, Approved Document F asks for ducts to be sized and joined correctly and for equipment to be selected to minimise noise. The Regulation's information list gives efficiency, flow, pressure and speed but no sound figure, so ask for sound data separately.

What drives the cost, and what is outside this page?

This page gives no prices because they come from quotes. The cost drivers are the airflow and pressure, which fix the motor size; the fan type and format; the ductwork and discharge route; silencers, dampers and controls; the replacement air path; and commissioning. Approved Document F says mechanical ventilation must be commissioned and a notice given to the building control body. Running cost is input power multiplied by hours, as the 780 kWh example shows. The commercial ventilation cost page explains what moves a ventilation quote, and the HVAC dampers guide covers the dampers at the fan.

A quote that can be compared line by line should show:

  1. the design airflow and the system pressure it was sized against;
  2. the datasheet's overall efficiency, with its measurement and efficiency categories;
  3. whether a variable speed drive or EC motor is included;
  4. sound data, requested separately;
  5. the controls, including any run-on timer, and the replacement air path;
  6. commissioning and the records handed over.

Two topics sit elsewhere. HSE's workplace ventilation guidance does not cover removing harmful substances such as welding fumes and process dust, and HSE publishes separate guidance on local exhaust ventilation. Kitchen canopies and grease extract have their own guide. For the duties behind general ventilation, read commercial ventilation requirements. You can send an enquiry through the form on this site.

Sources

Common questions

Do ventilation fans really work?

An extract fan works when it is sized for the room and the replaced air can get in. Approved Document F relies on extract from rooms that release moisture or pollutants, to stop them spreading. HSE warns that desk or ceiling fans will not improve fresh air in a poorly ventilated area.

What size extractor fan does a commercial toilet need?

Approved Document F Volume 2 sets 6 litres per second per WC pan or urinal and 15 litres per second per shower or bath for offices. Add the units served: eight pans and urinals is 48 litres per second. The fan must then deliver that airflow against the pressure of its ductwork.

How long should a toilet extract fan keep running?

For a room with no openable window, Approved Document F says the extract should run while the room is occupied and for at least 15 minutes after people leave. It also says sanitary extract ventilators should be able to run continuously if required.

Does the fan efficiency regulation cover smoke extract fans?

Regulation 327/2011 excludes fans built only for short-duty emergency use in fire safety. Dual-use fans for ventilation and emergency use are covered, with their Table 2 efficiency grades reduced by 5 per cent.

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