I received some excellent questions during the Q&A section during the recent webinar that I presented. Here you can see my answers to these questions, slightly edited for clarity.
The background to the webinar is provided on our recent blog. In this blog we refer to Revision 2 of ‘Guidance on Smoke Control to Common Escape Routes in Apartment Buildings (Flats and Maisonettes)’ from The Smoke Control Association (SCA), which is available for download from www.feta.co.uk/smokecontrol. This is an important revision to the Guidance.
There is also a recording of the webinar available.
If services are designed to maintain the fire resistance of the vent shaft, can services be located within the shaft as long as they do not affect the free area?
I think we have had occasional instances where services have gone through the shaft. The trouble is that if they occur on every level, they can cause quite an impact on resistance, particularly with mechanical systems.
Certainly when we have looked at installing grilles across the shaft, for example for maintenance access, these can add anywhere between 10 and 20 Pascals per level. So if you have got a tall building of 20 storeys for example, then you are looking at adding possibly 2 – 400 Pascals to the resistance of the system, so that can have quite a big impact.
On the other hand there have been some instances where fire-resistant components have been permitted within the shaft. It is necessary to judge the viability of these on a case by case basis, but certainly anything that jeopardises the performance of the system should be avoided. I know of one particular instance where a HTPE rainwater pipe was being run through one of the shafts which had to be changed to cast iron to make sure that it was fire resistant. So there is the possibility of adding services, but this issue has to be dealt with on a case by case basis.
Are there any issues relating to wind forces opposing smoke at the outlets?
If automatic opening vents (AOVs) are used on the outside of the building, they can be affected by wind pressures. This does indeed have an impact on their performance.
If it is a code compliant solution and if the requirements of Approved Document B (ADB) have been met, this is the limit of ADB’s guidance. Please note that ADB sets out that if there is wind pressure then this could compromise that particular aspect of fire safety, but also that the other criteria defined in ADB should still be able to provide a sufficient level of safety.
Clearly this is something that will be avoided with a natural shaft system or with a mechanical shaft system, where a the flow rate and direction of flow from the system can be guaranteed, particularly with mechanical shaft systems.
Should OVs and AOVs really should be bottom hung as shown in the SCA Guidance?
The Guidance sets out that OVs (Opening Vents) and AOVs (Automatic Opening Vents) should be bottom hung, following the diagram in ADB. We have seen quite a lot of instances of people looking to use side hung ventilators as OVs or AOVs.
We have added in the Guidance that we don’t recommend them because we believe that they are fundamentally unsafe. People can fall out of windows at a height unless some measure of protection is installed to prevent this, so they are an additional complication. Certainly the preferred solution is either a bottom hung or a louvred type ventilator.
Do you still need a CFD model to demonstrate compliance?
If you are going for a performance based solution, yes we would recommend that a CFD model should be provided, depending on the extent of the performance requirement. It may be possible that you can provide a zone model, which is a calculation to demonstrate the system performance, in which case it may not be necessary to provide a CFD simulation. So, again, this should be something that should be clarified with the Authority having jurisdiction, i.e. Building Control or the Fire Officer.
Should smoke detectors in common parts of residential buildings only detect, and not have sound as to prevent false alarms?
Yes, we are dealing with a “stay put” policy here, or “defend in place”, depending on how you want to describe it. Certainly we are not looking to provide a common alarm system as the stairs aren’t sized for everyone evacuating at the same time. The compartmentation that is provided in the building by the front door of the flat and by the walls of the flat should be sufficient to keep them safe, while the Fire Brigade are dealing with the fire.
So we are not looking to alert everyone to have to escape: the staircase wouldn’t be sized to accommodate this, so the detection system should be exactly that, simply a detection system to active the smoke control system and perhaps any other devices that are part of the system. So if you have got doors held open by mag locks for ease of movement around the building, or lifts that need to be grounded in the event of a fire, they could be controlled by the smoke detection system, but certainly wouldn’t be looking to get everyone to evacuate by putting sounders on.
Are there any ways of adapting the active components of a smoke venting system to overcome the issue of having full height doors between the stair and the lobby? I am assuming that full height doors are a key architectural objective.
The normal way of overcoming this is to increase the extract rate to try and overcome this issue. However as the extract rate goes up so does the complexity of the solution: potential pressure drops are increased and the resistance of the system will be affected. If the leakage is induced into the system, it may grow considerably, and you would have to do quite an in-depth study on the sensitivity of the system to make sure that the smoke would not spread into the staircase, so unfortunately this isn’t a straightforward answer. I know of instances where full height doors have been used and the system has been able to overcome them but it certainly needs a lot more analysis than a conventional solution.
Have you had experience in the use of a smoke shaft as a dual use for environmental air management of the common corridor?
Yes, this is a solution that we adopt frequently. Our webinar, "Ventilation solutions for overheated corridors in multi-storey residentail buildings" is availble to watch now as a recording and will be held live in April 2016. If you need more information on this, do please email me and I will look to assist you.
What measures or regulations need to be in place when the system fails? Is there a need to evacuate?
I presume you mean a long term failure which could be identified when carrying out maintenance or a similar situation: then, yes you would need to put additional management procedures in place to deal with a long term failure of the system. We try and design in as much resilience as we can into our smoke control systems and invariably we have a stand-by fan and back-up controls to make the system as resilient as possible, but yes, if the system is down for whatever reason, then yes, a management strategy would need to be put in place to compensate the lack of protection. I know that this does happen not just with our systems but with sprinkler systems, for example
Please clarify the extended travel distance
This is 30m, that is from the furthest flat to the stair door, but obviously that needs to be justified by the performance based system.
With respect to the ventilation of lobbies to ancillary accommodation, the Guidance states that a mechanical system can be used as a replacement for the 0.4m2 vent at a rate equivalent to the natural vent. Is there any guidance or recommendation on how this equivalent rate should be established?
This would need to be a comparative study. I think it would certainly be difficult to demonstrate how that rate would be established. I think that the approach that should really be used is something more deterministic, so we would be dealing with a fire size from the ancillary accommodation and demonstrating that the staircase is protected by a mechanical system. It is difficult to know exactly how much performance you are going to get from a 0.4m2 vent, so it would be better to demonstrate that you are actually achieving something worthwhile rather than achieving something that is only as good as a fairly limited performance.
Can you clarify if an AOV opening roof light or AOV window is CE marked, then it would be acceptable for it to have the reduced free opening area you mentioned?
Yes, the reduced area is based only for CE marked ventilators in accordance with EN 12101-2.
What access controls should control panels have, and should they be in public areas?
ADB provides only limited guidance in this area and there is a lot better guidance now in the revised Smoke Control Association document. If such panels are in public areas they should be locked to prevent inadvertent use but it is obviously important to make sure that they are accessible for the Fire Service.
Our own system is key locked (or key enabled), so that the Fire Brigade, once they have the key, can activate the control system or override its performance. So if it has been activated by a smoke detector and they want to change it from one floor to another for smoke clearance purposes or anything like that, then they can activate the key and override the system as they require. If they are in public areas they shouldn’t be easily tampered with by the public.
Taking an existing situation in an older building, if a staircase is forming the inlet shaft then can doors open in opposition to flow into the corridor?
This is a solution that we use quite a lot, particularly with the Colt Shaft Constant solution. Ordinarily we are only evacuating the occupants of the apartment on fire and the number of people who are escaping at any one time is likely to be very low at the time when the Fire Service arrive, so there is a general acceptance that the door could open from the staircase into the corridor if that suits the solution. So it would certainly be acceptable to turn the door round that way and use the staircase as a source of inlet air. It can be demonstrated that the airflow through the stair door is quite often sufficient to keep the stair clear of smoke, and likewise if the system fails (for whatever reason) then the stair door will still close once the mechanical system has shut down.
Conor Logan is a Technical Manager of Colt UK, Smoke and Climate Control Division. Conor designs innovative smoke control and HVAC systems and is also Chairman of the Smoke Control Association.