n the event of a fire, the ability to control and limit the spread of smoke is vital. A well- designed smoke control system can save lives by keeping escape and access routes free from smoke and assist fire fighters with entry.


In recent years, dual purpose units that operate smoke extracon fans via inverter drives to control speeds for dayto day running, have grown in popularity. However, using an inverter with a fan while it’s in fire mode requires special consideraon. Simon Plummer, naonal fire safety manager at FläktGroup, examines the opons available to ensure safety and compliance

Traditionally, separate smoke extract fans are installed alongside day-to-day air treatment fans. The former would only be used in the event of an emergency. However, given that smoke extract fans can represent a sizeable expenditure for property owners, a greater return on investment can be attained if fans can operate in more than just emergency mode.

Dual purpose

So, in recent years, there has been a greater prevalence of dual-purpose fan units, where smoke extract fans are used with inverters (also known as variable speed drives - VSD) to vary speeds for day-to-day running. On an everyday basis they exhaust air from a space to allow for fresh air or treated air to be provided, maintaining the indoor air quality, and provide smoke extraction in emergencies. These units have grown in popularity for a number of reasons, not least of all the reduced installation and maintenance costs. Operating a fan through a VSD offers reduced energy use, especially in areas where occupancy levels vary, and it allows for minute control of the air exhaust to balance the supply air and need within the building.

However, a number of issues can arise when using smoke extract fans with inverters both for day-to-day ventilation and in fire emergencies. The specification of the required equipment is rarely clear-cut, and the challenge is to specify a design which complies with fire regulations.

Direct online operation

Legislation and design guidance of products and systems related to the control of fans used in mechanical extraction systems have undergone various updates over the past few years. In 2015 a key change was made to BS EN12101 part three, which specifies the products characteristics of powered smoke and heat control

ventilators (fans). An important part of this update is the ruling that the use of inverters in emergency mode is precluded unless they have been fire tested with the fans, or are equipped with additional voltage waveform filters and the fan motors derated by 20 per cent.

To avoid non-compliance with this standard, a system can be designed to bypass the inverter when an inverter driven fan switches to smoke mode. It is then operated directly on-line in the event of fire. However, bypassing the VSD during a fire can force the fan to run at full speed regardless of the conditions. If the fan is used within a pressurisation system, doing this could cause system component damage by over-pressurising them.

Worst still, over-pressurising stairwell pressurisation systems may make it impossible for occupants to open escape route doors. Therefore, by not using a VSD, it becomes much more difficult for a system designer to achieve the desired level of smoke control and implement extraction strategies safely.

Changing requirements

On 8 April 2017, BS EN12101 part three was changed to include the option to use frequency converters during a fire event. However, to fully comply, the fan and variable speed drive combination still needs to be third party tested and certified. If not adhered to, the high temperature certification for the fans will be invalidated. Where fans and VSDs have not been tested and certified as a packaged solution, the fan motor must be de- rated, i.e. oversized by 20 per cent, and Sinusoidal or du/dt voltage waveform filters must be applied between the inverter and fan. This reduces the over- heating effect of the inverter on the motor and avoids early failure of the fan during fire mode.

However, this is a costly option. Voltage waveform filters are very expensive, often costing more than the VSD. A larger panel enclosure is also needed to house the filters inside a remotely mounted control panel along with the VSDs, and extra wiring would be needed. In addition, a bigger fan costs more, not to mention higher amp ratings and the impact that this would have on wiring and associated control equipment ratings.

Certified and safe

uExtract fans can be used with variable speed drives to vary speeds for daytoday running


Using an axial fan which can be speed controlled with an inverter for normal ventilation and smoke extraction offers designers more solution possibilities. However, the fan and VSD combination needs to be third party tested and certified, and Fläkt Woods, together with Danfoss Drives, are the first to offer a fully certified EN12101-3: 2015 packaged HT fan and VSD solution in the UK and Europe. In order to pass these tests, a number of considerations has to be satisfied for operation in high temperature environments. Individual components, such as the motor impeller, must be made of materials that can withstand stress limits of up to 300-degrees C. It is important that there is a sufficient gap between the impeller tip and the casing because under high temperatures, this

FIRE & SAFETY Withstanding the heat

uCar park fire test

component can expand. Motor specifications are also critical. Bearings need to have the correct gaps for heat expansion and the appropriate lubrication for fluctuating conditions. Apart from the ability to minutely vary fan speeds in line with a smoke control strategy, incorporating a certified VSD dual fan package also boosts reliability. A dual-mode fan which also provides normal ventilation is effectively being ‘run tested’ every day. In contrast, dedicated fire mode fans and associated equipment would only typically be tested at monthly, six monthly or even yearly intervals. If fan or motor bearings are not turned regularly, they can degrade (flat spot) and then prematurely fail, especially when stressed.

With additional control flexibility, a dual system offers a practical way of achieving desired smoke control strategies. Zonal control and reversing extraction flows become possible without the need for additional equipment such as dampers and contactors.


Dual purpose fans that are operated via a VSD provide a wide range of benefits during both day-to-day operations and in the event of a fire. Energy efficiency can be maximised under normal circumstances with the ability to carefully control rates of smoke extraction in fire mode. By choosing a package that is independently tested and certified, specifiers can ensure compliance and help protect occupants as well as firefighters from the dangers of smoke caused by a fire. Read the latest at:

uFläktGroup’s JTv Low Profile Car Park Jet Thrust Fan range includes a unidireconal oering, which features a low profile, space saving casing and an integrated mounng bracket, making installaon easy

Apart from the ability to minutely vary fan speeds in line with a smoke control strategy, incorporang a cerfied VSD dual fan package also boosts reliability

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