Questions and answers from Paul Compton's Pressurisation webinar
At Colt’s recent oversubscribed CPD accredited pressurisation webinar, we received some interesting questions in the Q&A session at the end. I’d like to share some of them. They are edited to make the questions clearer and to reduce my ramblings.
Q: Do all buildings that operate a stay put policy have to have a Class A pressurisation systems?
A: No, if you have a residential building then certainly in the UK that would normally have a stay put policy and, if you choose to have pressurisation, then it would be designed as a Class A system. In a residential building in the UK you are permitted AOV or shaft systems instead so the fact that you have a stay put policy in a residential building doesn’t mean that you need pressurisation.
Q: What if you have a residential building that needs simultaneous evacuation; is Class A still suitable or do you need a different Class?
A: In that case, you would normally use a Class C system because that is the system used for simultaneous evacuation. Certainly, in the UK that is very unlikely to happen because all of our residential buildings are built on a stay put design, but if you are somewhere other than the UK and you have a system where everyone would be expected to escape together then you would want a Class C system.
Q: Do all stairs need duct risers? What is the idea behind having a supply grille every three storeys?
A: If we dump all of the pressurisation air for the stairs in one location, then we are likely to get a variation in pressure up the stairs. If it is a 2 or 3 storey building, then the differentiation is going to be small, but if it is a 50 storey building then it is going to be quite a big difference. The idea behind having a grille every 3 storeys is that it is a reasonable compromise. Over 3 storeys there won’t be any significance difference in pressure and in taller buildings it means you have more than one grille, so if you happen to open a door close to a grille then all the air that you are pumping in doesn’t just go out of the door without providing some pressurisation. So it makes the system more robust. The 3 storeys is a guide value for good practice in the standard.
Q: BS EN 12101-6 states that only one leaf of a double door is open for design purposes, how can you ensure that this will happen in practice?
A: You can’t. The principle behind it is that, particularly when you are opening lots of doors, the chances of them all being double doors and with both leaves open at the same time is fairly low, so it is considered to give a reasonable degree of protection if we assume that one leaf of each double door is open. With all of these things, we are trying to balance a robust system with something which is commercially viable.
Q: If there is nowhere in the building for accommodation air release, either through actuated windows or through a shaft, how can we achieve it?
A: You might be able to rely on HVAC system for air release. If you can’t do that, then it is very difficult. We do come across situations quite often where we are looking at a refurbishment and we have got a stair in the middle of the building and there just isn’t anywhere sensible to put the accommodation air release. If you can’t provide accommodation air release, then you can’t actually do a system which complies fully with the EN12101-6 (the British Standard for pressure differential systems) and you then you have to talk to the authorities and agreeing what is a practical solution. My own preference in this situation is to not use pressurisation at all, but just to ventilate the stair, because without the accommodation air release you are not going to get the full benefit of the pressurisation system and you might just as well have a cheaper and simpler through flow ventilation system instead.
Q: Where is the accommodation air release fitted in a residential unit?
A: We try not to put the accommodation air release within the residential unit. We try to put it in within the common corridor because of the difficulties of providing and maintaining accommodation air release inside a residential unit or apartment.
Q: In a large building where there might be a number of pressurised shafts serving a common floor, do all systems start at the same time or are there dedicated smoke detectors for each stair?
A: Generally, we would recommend that you have dedicated smoke detectors for each stair. If the smoke hasn’t reached the door adjoining the stair, then there is no particular need for the pressurisation system to be operating. EN 12101-6 recommends operating the system from a detector close to the door to the stair. If you choose to operate all the systems together from just general floor detection system then there is no technical objection to that.
Q: Can you adapt a pressurisation system for environmental ventilation in common areas of flats with district heating?
A: It depends upon which areas are being pressurised and where the accommodation air release is. It would be possible for example to use the supply plant to blow air into the stair, put in a separate damper to allow air to flow from the stair into the corridor, and then use accommodation air release from the corridor as the exhaust system. So it is possible, it is not commonly done, but it is possible.
Q: Are the fire services made aware of the type of system installed?
A: The fire services should have access to information about what systems are in the building, but it is possible that the firemen coming into the building isn’t aware. However, in terms of the operation of the system, as far as the fire services are concerned it doesn’t make any difference which class the system is. The only difference is that if it is a fire fighting system they will get a higher flow rate. In general they should enter the building via a fire fighting shaft so they are going to get the higher performance class B system unless the building is residential.
Q: What maintenance and testing is needed?
A: Maintenance and testing needs to be done every year. Systems also need simple operational testing at regular intervals to make sure they operate. Guidance in British Standards says that a performance test needs to be done every year. This would be a check on the open door air flows and the pressurisation levels, but not necessarily a full check every year at every level. If there have been no changes to the building and you check 3 or 4 random levels (different ones each year) to make sure that the system is performing correctly you can be reasonably confident that the performance at the other levels should be satisfactory too.
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Published December 2013
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... At Colt’s recent oversubscribed CPD accredited pressurisation webinar, we received some interesting questions in the Q&A session at the end. I’d like to share some of them. ...