With the draft BS 9991 calling for pressurisation systems on single stair buildings taller than 18 metres, we felt it was important to look into whether these systems are the best way to go if you were to develop a building, or if you should implement two stairs, protected by a mechanical smoke ventilation system. Read on for us to take you through some key areas of consideration in order to weigh up which system is best for you.

Section 1: Cost

Cost is an undeniably important factor when it comes to picking a necessary smoke ventilation system and is often the most important factor in working out whether a project is viable or not. Sertus have researched this, working out that the supply of equipment for pressurisation systems costs around 20-40% more than that of a mechanical smoke ventilation system for the same building. Therefore in terms of equipment cost, it’s more expensive. However, the real costs when it comes to pressurisation systems are the hidden costs, due to them taking up a lot more roof and floor space which you’ll find out more about as you read on. To conclude on cost, if you were going on this variable alone, mechanical smoke ventilation systems are the more suitable choice.

Section 2: Design time

Design time is another key element to discuss. It’s well known that pressurisation systems take a lot of time and effort to design. At an early stage, a specialist, like ourselves here at Sertus, would need to be consulted to work through a series of extremely complex calculations, in order to work out what shaft sizes and which fan sets are needed. This would then need to be reviewed at every stage of the project design. When it comes to designing a pressurisation system, positives are present. If a pressurisation system was designed to BS EN 12101-6, this would be a prescriptive system, meaning a CFD model on the system does not need to be carried out to prove that it works as the correct British Standard has been followed. This would however need to be carried out on a mechanical smoke ventilation system, which is serving the two staircases. Due to this fact alone, the pressurisation system would be the stronger choice.

Section 3: Roof Space

Roof space is another key consideration. Sertus always recommends you put the pressurisation fan-sets on the ground floor if possible, however ground floor space always comes at a premium, and most times they will end up on the roof. This then causes a challenge due to the smoke rising from air release paths, resulting in a high risk of smoke re-entry. To get around this issue, two inlet ducts going into the pressurisation fans would be needed so that one inlet duct can be shut if smoke enters it.

Then the quantity of fans would need to be considered, following BS EN 12101-6 to the letter, Sertus recommends having one fan-set per pressurised zone. For a standard system this would be three fan-sets, one for the stair, one for the lift and one for the lift lobby. As soon as there’s a corridor travel distance of more than 15 metres, the draft BS 9991 says we need to have a separate smoke ventilation system for the corridor, which will likely require an extractor fan, leaving the possibility that you could end up having four fan-sets for one stair core. With these facts taken into consideration, having two stairs is more suitable when it comes to space on the roof.

Section 4: Shaft Size

What size should the shaft be? 0.6M² shafts are almost always used in mechanical smoke ventilation systems. With a pressurisation system, the shaft size can vary between 0.6M² and 2M² depending on the size and leakage of the area that is to be pressurised. This additional shaft size can be challenging when trying to make it work with the building layout. From the spatial perspective it makes more sense to go with a second stair at around 30-40 storeys. Another point awarded to two stairs here.

Section 5: Air Movement

In most high-rise residential buildings, the mechanical smoke ventilation system is used to keep the air moving in the corridor to avoid hot and unpleasant smelling air stagnating in the corridor. It’s very difficult to do the same thing with a standard pressurisation system due to a variety of reasons, ones that will be deciphered in a future post. Two stair is the top choice here.

Section 6: Commissioning

Commissioning is hugely important when you make your decision. Commissioning a mechanical smoke ventilation system with a two-stair system involves running the fans and checking the flow rates along with the system's cause and effects. With Sertus’ pressurisation systems, the artificial intelligence makes commissioning the system very simple and it’s a question of calibrating the door system. However there is a large risk with commissioning a pressurisation system, and that risk is that the leakage in the building due to poor construction is more than that designed, which means that when commissioning the system, the BS EN 12101-6 criteria can’t be met. We don’t believe either system is superior on this point.

Section 7: Effectiveness / safety

Lastly, effectiveness and safety need to be taken into account when it comes to deciding between the two systems. It’s difficult to achieve an exact comparison on the two options as they work so they work very differently. Mechanical smoke ventilation systems work by controlling the amount of smoke in the corridor so that no smoke enters the stair, whereas a pressurisation system works by pushing air into the stair to ensure that smoke can’t physically enter the stair. This means pressurisation systems are effective at protecting the stair, but not as strong at clearing smoke in the corridor. In buildings more than 18m tall it is essential that the stair is kept clear of smoke. In our opinion, a pressurisation system is more reliable and effective than having two adjacent stairs protected by an MSVS system. 

With there being a lot to analyse when it comes to these two systems, we believe two staircases protected by a mechanical smoke ventilation system is a stronger choice in most scenarios. However, each project is different and if you’d like an opinion on your specific project, please don’t hesitate to contact us and speak to our friendly team who will be happy to talk you through the options available. If you’d like to continue learning about smoke ventilation, you’ll find some helpful resources on our blog and YouTube channel.