At our recent webinar on the general principles of smoke control, I received some excellent questions during the Q&A section. Here you can see my answers to these questions, slightly edited for clarity.
There is also a recording of the webinar available.
Are there any restrictions on location of smoke outlets?
It is important that smoke ventilators do not make smoke impinge on any buildings in the vicinity, so they should be situated well away from adjacent walls or roofs. Also by positioning any exhaust vents from inlet vents at least around 5-7m apart, recirculation of any hot smoky air back into the building will be avoided.
How can you ensure that fresh inlet air won’t make a fire bigger?
If there is a fire in a small internal space, you close doors and windows and starve the fire of air. By contrast, with a big building you cannot do this as there is so much air available. The amount of air is not the most important consideration; it’s far more important not to cause excessive air velocities local to the fire, and we would recommend that these are reduced to around 1 m/s, possibly by oversizing the inlet ventilators.
Please tell us more about how pressurisation systems are tested and by whom.
The minimum and maximum pressure differences, which are nominally 50 Pa for the stair and 45 Pa for lobby +/- a variation of 5 Pa, need to be measured. This is usually conducted by the supplier using an electronic manometer. Guidance states that performance tests should be carried out annually. Normally the ambient temperature makes no big difference to the tests, but if it is gusty there can be unsteady readings, so avoid such conditions. [We cover pressurisation in greater depth elsewhere on the blog].
What is the minimum height of a multiple residential building in the UK which requires a smoke vent system?
All such buildings higher than 4.5m need one.
Please clarify why the BS 5588 Standard can still be referred to even though it has been withdrawn.
It has indeed been withdrawn and been replaced by BS 9999 & 9991. But since Approved Document B still refers to it, in practice you can use any of these three Standards.
What are the UK requirements as regards secondary power supplies?
All mechanical ventilators need them and failsafe natural ventilators do not need them. Also there is no legal requirement to monitor ventilator status.
In the UK are smoke ventilation schemes in corridors regarded as providing smoke control or smoke clearance?
It depends on the approach taken. A mechanical system is a fire engineered system and designed to provide smoke control. By contrast a natural system simply provides a proscribed area so as to conform to a regulation.
What is an impulse fan?
It’s a small fan mounted under the ceiling of a car park to ensure that the smoke and air flow through the complete car park. [We cover car park ventilation in greater depth elsewhere on the blog – we will also be holding a webinar on this subject on 19th June].
With mechanical car park ventilation systems, what maximum velocity would you recommend to pass through inlet louvres?
There is no official guidance in the UK. We would recommend in principle to keep it down to 2 m/s for day to day or 4-5 m/s for fire use.
What is the minimum air change rate if a CO detection system is used to control a mechanical car park ventilation system?
This will depend on form of control. We recommend that if there is any movement of cars, there is some background ventilation, otherwise we won’t see any risk if the system is switched off altogether. There is no official guidance on this and it depends on the project.
In the UK are there any Building Regulations requiring the use of attenuators?
Such requirements do not exist in the Building Regulations relating to fire, since they are only concerned about the ability of the system to operate in a fire and are not concerned about a temporary nuisance when a smoke ventilation system operates in an emergency. In practice they are only fitted because of testing or if the equipment is dual purpose for smoke control and day to day ventilation.
Does EN 12101 proscribe a 30 minutes fire resistance rating for natural ventilators in their closed state?
No, it only covers how well such ventilators perform in practice. Infact many ventilators are not even fire rated.
Paul Compton is Technical Director for Colt, experienced in smoke control, HVAC, solar shading and louvre systems.