Lawrence Webster Forrest (LWF), Fire Engineering and Fire Risk Management Consultants
Lawrence Webster Forrest (LWF), Fire Engineering and Fire Risk Management Consultants



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Lawrence Webster Forrest
Legion House
Lower Road
Kenley
Surrey
CR8 5NH

Tel: +44 (0)20 8668 8663 Fax: +44 (0)20 8668 8583
E-mail: fire@lwf.co.uk

Fire Resistant Ductwork and Dampers

An important part of a building's infrastructure is the ducting network that runs throughout the premises, it is often invisible, but is a key element to the environmental operations in a building. Ducting may carry fresh air to improve the environmental ambience for the occupants in the building, or may serve a dirtier role in helping to remove smells, dust or toxic gases from a room or piece of equipment. It may also form part of the vital systems to protect occupants from the effects of smoke, or help prevent fire spread. Whatever its role, most ductwork must travel extended distances through a building to reach its destination – usually the open air or treatment plant.

What’s so important about ductwork?

Not much notice is taken of the ductwork in a building – indeed it and its associated dampers are often neglected and forgotten. We can never underestimate the importance of these systems, both in keeping us comfortable, or as far as risking our lives or the integrity of the building to which it is installed.

Buildings are designed with fire resistant enclosures, known as compartments, to contain any fire within one area. Due to its possible routes through a building, ductwork is a key element that can, if not suitably protected, weaken the passive fire protection built into a building through the construction of fire walls and floors / ceilings. Without adequate fire resistance, along with the other services installed such as plumbing, electrical wiring or communication cabling, this can lead to rapid uncontrolled spread of smoke and fire through an entire building, often without the occupants being aware for some time.

Why make ductwork fire resistant?

Fire resistant ductwork and dampers can prevent fire spread from the room of origin, thus protecting occupants, and the fabric of the building.

While specially made fire resistant ductwork can be acquired, there are also different methods of protecting ductwork. It can be enclosed within a fire resisting material, commonly plasterboard or other substance designed specifically for this purpose. Dampers can be fitted within the compartment wall to seal the building structure in the event of a fire.

In the same way that fire doors are provided within compartment walls, dampers are also provided within compartment walls and floors where ducting passes. These allow free access throughout the ducting system normally, but will shut down and prevent the passage of smoke and flames between compartments in the event of a fire. Dampers can be linked to the fire alarm and close when this is triggered, or thermal links may activate when exposed to high temperatures . These are typically set to activate at temperatures of 72°C ± 4°C.

As with fire doors, ductwork and damper fire resistance must match the fire rating of the compartment through which they pass or are installed in.

There may be many reasons why fire resisting ductwork and dampers are installed, for example:

1. To be used for smoke extract in the case of a fire in fire engineered solutions.

This may be the case in basements, car parks or means of escape, to increase the available safe egress time for occupants by helping prevent the build up of toxic gases and heat. The ducting is responsible for removing smoke from an area, which can reach high temperatures very quickly. The system, including any fans, must also be able to withstand these high temperatures long enough that all occupants can safely evacuate.

Where smoke extract is required in basement areas, these areas must also be provided with sprinklers. The ductwork in this case must be able to prevent water ingress as well as high temperatures, and also the possibility of thermal shock from sprinkler systems activating.

2. Kitchen extract systems (non domestic).

These systems are a particular risk, especially if the required maintenance and cleaning regimes are not adhered to. The build up of grease and fat within the ducting, coupled with high temperatures, can self ignite and cause a fire in the ducting itself. It is important that this fire does not break out of the ducting into adjacent rooms or service risers, and that radiant heat is minimised to prevent ignition of a second fire outside the duct.

It must be noted that dampers must not be installed to kitchen extract systems, so further emphasis on the fire resistance of the ducting is made.

3. The ductwork passes through a means of escape.

Where the ductwork must pass through a means of escape, then it must be capable of protecting that escape route from a fire in the adjacent rooms to which it is linked, much like the fire doors to which we are accustomed.

What makes ductwork fire resistant?

Ductwork must pass three key tests before it can conform to British Standards:

  • Stability – the ability to resist collapse when exposed to fire conditions
  • Integrity – the ability to resist penetration from, or the escape of, flames, smoke or sparks
  • Insulation – the ability to prevent radiative spread of fire

Tests are performed on the ducting as a whole, including joints and hangers, so you can be assured that the whole system will perform as required in the event of a fire.

In order to achieve adequate fire resistance, ducting can be treated in various ways. The ducting can be coated, sprayed, enclosed or wrapped in a wide range of proprietary materials available from a large number of manufacturers. As with maintenance of the ductwork itself, the protection supplied must also be maintained and protected from damage when works are carried out.

As can be seen from this bulletin, fire resistant ductwork and dampers can be key to preventing unforeseen fire spread throughout your building. It is therefore imperative that if any works are being undertaken to ductwork within your property that you clearly understand any fire rating or resistance that maybe necessary to that ductwork. 

This bulletin has been written by Karissa Thomas DipHE Fire Safety.

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