At our recent webinar on smoke shafts v pressurisation, 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.
On the use of the stair door for inlet air for the mechanical extract system, how do you typically deal with opening that door? Is it powered open? Is it only open when used by occupants? Also, for the life of the building and for this system, has the day-to-day use and wear and tear on a mechanism on that door been considered?
I think you are referring to the system where you can reverse hang the door for the stair into the lobby to provide pressure control and inlet to the mechanical shaft system. In this instance an absolutely standard stair fire door is used with a commercially available door closer. The closer needs to be adjustable so we can set the pressure regime on commissioning, but it would be rated for closing many thousands of times.
In principle in any building fire doors should be maintained as part of meeting the requirements of the RRO, so the door should be looked after and should operate properly over the lifetime of the system. These doors are more important than others in the building though, as they are not only fire doors but also part of the smoke ventilation system.
Can a window in the accommodation be used as accommodation air release? Would there be any special requirements for it (e.g. should it be an automatically opening window, etc.)?
Yes there would be some special requirements. The system must be automatically opening, since the accommodation air release must open when pressurisation system starts. You can use a window but it would need to be actuated so as to open when the pressurisation system operates.
We are planning on using a mechanical system as an alternative solution on a 42 storey building (approx. 125m in height), and are most likely going to have a mid-point exhaust. is this something that you have seen previously?
Yes. A mid-point exhaust, though not common, is fine and has some advantages. We can make the shaft smaller because we are only pulling air 60m rather than 120m so we could accept a higher pressure drop per metre, thus we can design a smaller shaft. However, the smoke is buoyant and will want to rise so we would need to make an allowance in the fan sizing.
Please give us some information on the siting of the system controls and the positioning of manual overrides.
Where we have fans, then ideally the main control panels (the big panels doing the switching) are mounted close to them. So, given the fans are on the roof, the panels would be sited for example in a top floor plant room or on the roof in a weatherproof enclosure. Panels can also go into the stairs if they are not too big and if there is no danger of them being vandalised, as the stairs are a protected area. For a means of escape stair or a fire-fighting stair the manual controls would be mounted at the fire-fighters’ entrance.
Is there a higher probability of functional failure with a mechanical or pressurisation system than a natural system?
Yes – there is, as you have more components (the fans and controls), but there is a low risk. The reason why is because there will always be duplicate systems. We have duty and standby fans, each with its own contactor and inverter, so the systems are separated, as well as standby (i.e. primary and secondary) power. Everything is done to make such a system as reliable as possible. Ultimately any mechanical system has a higher risk of failure than a natural system, even though it is still a very low risk.
In buildings where the building quality is poor, have you seen instances where other services have passed through the mechanical shaft, and how much effect on the commissioning has this had?
It is not recommended to put any mechanical services into the mechanical shaft or into a pressurisation duct, though it is tempting as there are always pressures on space in risers, and here there is an empty shaft going from the top to the bottom of the building. BS 9991 now specifically states that no services except those related to the shaft itself should pass through the smoke shaft.
If the shaft is not well enough sealed, the effect is that there may be not enough air flow at the extract point in the lobby. When we are doing a design we do make an allowance for some leakage through the vents on other floors and we accept that the shaft won’t be 100% sealed, and we give a sealing spec. If there are big holes in the shaft this can mess things up. However we are only concerned about the quality of build of actual shaft which is being used for ventilation rather than that of the entire FF or MOE stair core.
Is there a requirement for servicing and checking of the system for pressurisation?
Yes. Any life safety system should be tested once a week, the cause and effect and zonal controls should be checked once every 3 months and the system should be serviced once a year. EN 12101-6 requires a full commissioning test every year, to check pressures and open doors velocities to check that the building has not been changed or deteriorated. This aspect can be easily forgotten!
What is the design temperature of the exhaust fan?
It is almost always F300 to EN 12101-3, that is to say 300C for 1 hour.
Whitepaper: Which is best for protecting escape routes: a smoke shaft system or a pressurisation system?
How do you decide whether to use smoke shafts or pressurisation? The decision is influenced by legislation and Standards, building configuration, budget and space requirements. There is no universal “right” choice, but there’s certainly a best choice for each individual building. Download the whitepaper to find out more.