Smoke control systems are vital to keeping buildings and occupants safe. However, unfortunately there are also occasionally times when things can go wrong and the systems fail. In this blog, we aim to cover some of the most common issues and how they can affect the overall performance of smoke shaft systems.
Depressurization of the fire-fighting lobby
When designing a mechanical shaft system, a major issue to contend with is avoiding excessive depressurization of the firefighting lobby. When we've got a source of inlet air, we will want a flow rate through that lobby of typically 2 - 4m3/s depending on the particular design. However, if, when all the doors are closed, the flow rate stays at this rate without providing some provision for inlet, then there will be much more than the 50-55 Pascals of depressurization that we are permitted under BS EN 12101-6. Therefore, there always has to be some way of achieving either a means of inlet or a reduction in flow.
To achieve this, you can use two methods. By using a pressure differential sensor and an inverter on the fans, you can reduce the fan flow rate when the doors are closed. This matches the sort of performance that would be expected from a natural shaft.
The other option is to reverse hang the stair door and put an adjustable door closer on it. This allows the door to be pulled slightly open to provide relief and maintain the acceptable negative pressure. This system is cheaper but has a disadvantage as the door is then opening in the wrong direction for escape - depending on the geometry of the building, that may or may not be acceptable.
Building design challenges: large doors to the stairs and accommodation
Another issue that can cause problems with smoke shaft systems is if there are large doors leading to the stairs and the accommodation - the larger the doors are, the harder it is to keep them closed and to avoid excess door opening forces. Therefore, if the doors are larger, a lower pressure generally has to be established within the lobby in order to ensure that doors are not pulled open or made too hard to open.
Additionally, if the doors are ceiling height rather than having a down stand above them, this could cause problems. If smoke is flowing along underneath the ceiling, then that smoke will find it easier to get into the stair if there is no down stand when the door is open since the ceiling jet can just push right into the stair. Therefore, it is advisable not to use ceiling height doors into the stair when a smoke shaft system is being used. Note, there is nothing in any Standard or Regulation to say it cannot be used, but it is a recommendation that we frequently make in these instances.
Similarly, it is a good idea not to put the stair door directly opposite the door from an apartment or from an office. When that door is opened, a jet of air is being created which is more likely to push smoke into the stair.
Extending down to basement level
Another issue that sometimes arises with smoke shaft systems is if they extend down to basement level. In these instances, there is quite often a door between the ground floor level. What that means is that we have got a permanent source of air inlet for the upper stair through the vent at the head of the stair but we have got no permanent source of air inlet to the lower stair level.
This can be overcome by putting a fire damper next to the door between the basement and the upper ground stairs. More commonly, if the stairs need to be protected only for Fire Service use, it is often expected that the Fire Service will open that door if they need to get down to the basement. This will then provide an automatic source of air inlet, overcoming the problem.
The above are just some of the most common issues that can affect smoke shaft systems. Every building’s requirements will be different and issues can arise at all stages of a project. To get expert advice and guidance from design to installation and even the maintenance of your smoke control systems, talk to Colt.
Conor Logan is Associate Technical Director of Colt UK, Smoke and Climate Control Division. Conor designs innovative smoke control and HVAC systems and was Chairman of the Smoke Control Association for over 9 years.