What is a smoke shaft?

‘Smoke shaft’ is the common term for ventilation systems in the lobbies of tall buildings, used to maintain tenable conditions in the common escape routes in the event of a fire in the building.

They are essentially a simple ventilation system designed to extract any smoke leaking into a common lobby to protect the escape stairs. Typically, a vertical builder’s work duct rising through the building would be used to extract smoke from the lobbies. Each lobby would have a damper connected to the builder’s work duct.

 For natural shafts, the head of the shaft is terminated with an automatic opening ventilator (AOV). Mechanical shafts use extract fans, mounted on the roof and connected to the builder’s work duct with sheet metal ducting.

How does a smoke shaft system work?

For natural shafts, the head of the shaft is terminated with an automatic opening ventilator (AOV). Mechanical shafts use extract fans, mounted on the roof and connected to the builder’s work duct with sheet metal ducting.

An AOV would be mounted at the top of the stairwell and the complete system would be controlled by an addressable control system that provides automatic operation of the ventilation by interface with the fire alarm system or smoke detectors.

A provisional design can be achieved in minutes by selecting modular components:

  • Builder’s work shaft
  • Lobby vent
  • Roof extract unit
  • Control system

So, how does this eliminate smoke from a lobby?

This video demonstrates:

  1. Smoke detector detects smoke which initiates the smoke shaft

  2. Lobby vent to the smoke extract shaft opens

  3. Folding arm actuator opens the door to the stairwell

  4. Stairwell vent opens to allow fresh air to enter

  5. On the roof, the fans in the smoke extract unit begin to remove smoke via the smoke extract shaft

  6. Fresh air is drawn into the stairwell

  7. Smoke in the lobby is removed via the smoke extract shaft

  8. The system becomes ready for firefighters

 Find out more about how smoke shafts work

What extract rate should I use for a mechanical shaft system?

The primary purpose of a mechanical smoke shaft is to maintain a clear escape route for occupants of the building in the event of a fire in an apartment – this is known as ‘Means of escape mode’. For tall buildings there is also a requirement to maintain safe conditions for the fire brigade to enter the building to fight the fire, commonly known as ‘Firefighting mode’.

The common method of deciding on the smoke extract rate is by the constructing a computational fluid dynamic (CFD) model of the building and using a set of standard parameters such as the size and heat output of the fire to run a simulation to determine if the conditions within the building are acceptable.

What size should my mechanical smoke shaft be?

When you have decided on your flow/extract rate, the next step is to decide on the size of the builder’s work shaft that is needed to transport the smoke to the external atmosphere.

Once again, a set of industry standards have evolved and the most commonly used shaft area is 0.6m2 which caters for both the ‘means of escape’ and ‘firefighting duty’ with a maximum duct velocity of 10m/s which is not unreasonable for smoke extract purposes.

Find out more about sizes of mechanical smoke shafts

What size fan do I need for a mechanical smoke shaft?

After deciding the flow/extract rate and the size of the mechanical smoke shaft you’ll now have all the variables required to select the fan needed to extract smoke from the fire zone. It is usual to provide run and standby fans which are typically rated at 300 0C for 1 hour. They tend to be mounted in series on the roof with flexible connections and anti-vibration mounts along with back draught dampers to prevent cold air being drawn into the building. A duct resistance should be completed for the installation once the design has been fixed.

The extract shaft or duct should meet the requirements for fire resistance for a period at least equal to the highest period of fire resistance through which the ductwork passes, when tested and classified in accordance with prEN 13501-3. In practice this will usually mean a minimum of 1 hour fire resistance.

Acceptance Testing for Smoke Shaft Systems

Mechanical smoke shaft systems are commonly installed as part of a fire engineered approach to safety in buildings and the presentation by contractors and acceptance of these by Building Control Officers is a key event in the handover process of construction projects. These are routinely conducted in an ad hoc manner with little structure, evidence of commissioning, or record of performance data. It is common for demonstrations to be repeated several times for to a variety of reasons including system non performance, absence of key personnel and incomplete interfaces with other systems.

A short course for contractors, approving authorities, and fire fighters held at the SCS Group’s purpose built training facility presents a model for a smooth handover for all parties concerned. The training runs through the commissioning procedure and key data that should be recorded and sets out a demonstration procedure that covers all of the vital functions of the system in an efficient and robust manner.

Read more in our blog

Electrical contractor self-installs mechanical smoke shaft system

Wheeler Electrical won the electrical contract for the new student accommodation at Camberwell College of Arts, which included installing an 11-storey mechanical smoke shaft system. With support from Easivent, Wheeler Electrical decided to self-install the system using Easivent’s modular smoke shaft system.

Read our case study

Mechanical Smoke Shaft system allows extended travel at Sheffield halls of residence

Pearl Works is a modern, attractive student accommodation building on campus at Sheffield Hallam University. Managed by Omnia Space, the site has room for 57 student apartments and a Tesco retail unit across four stories.

In accordance with Building Regulations Approved Document B, the building required an appropriate smoke control system to reduce the risk to residents in the case of a fire, enabling safer escape routes by directing smoke away.

Read our case study