Questions and answers from Paul Compton's pressurisation webinar

Posted by Paul Compton on 23/04/13 10:11

First of all a big thank you to those who attended my webinar last Friday on 'Pressurisation Systems in residential and commercial buildings'. If you missed it or would like to watch it again a recorded version is available here. Many questions were asked after the presentation and I decided to post the most interesting ones up here for everybody’s benefit.

Could a "pressure relief damper" or "air release" be fitted to an external window/fixed pane of glass at the end of a corridor?

The pressure relief damper, if we are talking about pressure relief in a pressurised space, is normally fitted in the stair, not the corridor, because the stair is where we are wanting to relieve pressure. When we talk about the accommodation air release, then if the corridor is not pressurised, then yes, the air release can replace an external window. Obviously it needs to be fully automatically opening, so it would need to be a window with a motorised actuator, or it would need to be a specific ventilator, but yes this can be done.

We’ve come across projects without ductwork with the air going straight into the stair - any issues there?

In a small building that is fine. If you have only got a building which is 11m or less, then one inlet to the stair (so effectively no ductwork) is perfectly permissible. If you’ve got a building which is taller than that, then it comes down to whether you can make it work and achieve pressure differences between the maximum and the minimum, i.e. between 55 and 45 Pa, between the top and at the bottom of the stair. If you can achieve that, then this would normally be acceptable. 

Obviously the requirement to have a grille every three storeys as set out in the standard is intended to be conservative and safe. So, if you choose to put all the air in at the top, then that will normally be accepted if you can achieve the pressures that you need to.

Is there a minimum amount of sample rooms need pressure testing, or do all rooms require testing?

Normally if we have a very tall building that has a lot of similar storeys, then we wouldn’t necessarily test every one of them. We would normally agree with client/building control, whoever is interested, that we would do an initial test on a proportion of those rooms, perhaps  four or five of them, on four or five levels, and if the results on those are all satisfactory, then we would assume that the others are probably ok as well. If those results aren’t satisfactory, then we would have to go through and test every storey.

What is the process for deciding if we need a pressurisation system in the first place?

There is a requirement in the Building Regulations that you need ventilation to residential escape stairs and to commercial building fire fighting stairs. Whether you need pressurisation is basically entirely up to you unless there is a very specific requirement for pressurisation in BS9999 or BS5588 Part 5, and that generally is for tall buildings for deep basements or for if you are putting a system in, in order to avoid discounting the stairs. 

If you do not have one of those conditions, then it is your choice whether you use pressurisation, or a shaft system or an AOV system, and generally if there is no pressing requirement for you to use pressurisation, then I tend to suggest that you use one of the other approaches if you can, simply because they are cheaper. The other reason that you may need to use pressurisation is that it may be a requirement from Building Control or the Fire Service where the building itself is not Building Regulations compliant and therefore Building Control have asked for a pressurisation system as a compensating factor.

Can you now tell us about the doors that are kept open when you are doing the testing?

Where you are doing a stairwell pressurisation system to Class A, B, C, D or E then you have to keep the doors open in accordance with the standard, so if you know the class then the standard will tell you which doors have to be open for that particular system. If you are doing pressurisation or de-pressurisation of other spaces then the standard doesn’t very specifically tell you which doors need to be open, that’s a design decision. But, for Class A to Class E systems, you simply follow the standard to decide which doors are going to be open.

How are EN requirements different than NFPA/IBC requirements?

Here we are talking about the difference between the European standards and the American NFPA standards.  Basically there are an awful lot of detailed differences but the main difference is that under the American NFPA standard, there is no velocity requirement in a door open condition. Instead the standard requires you to maintain the pressure, even with doors open. So it requires the designer to make a decision as to how many doors are going to be open, and then ensure that they can maintain the pressure within the stair, even with all of those doors open. That tends to require a higher airflow rate particularly if you have decided you are going to have a lot of doors open. 

The other difference is that in the EN standard we have a fixed pressure requirement of 50 pascals; in the NFPA standard the pressure differential required varies depending upon the height of the rooms and is generally a little bit lower that the EN. So if you are designing to NFPA you will have a very different system to if you design to the European standard.

We regularly design buildings with mechanical ventilation but use depressurisation as opposed to positive pressurisation. Are there many instances where you would see a benefit of using positive pressurisation over depressurisation?

So I presume there we are talking about mechanical shaft systems which, although they are not generally classed as depressurisation systems to EN12101, do have the effect of depressurising the corridor. Those systems generally are used because they are less complex and less expensive than a pressurisation system. 

The pressurisation system will generally give slightly better conditions within a building than a mechanical shaft depressurisation type system, so yes there is a benefit but of course, under the regulations, unless you have these particular times when pressurisation is absolutely required, nobody really wants to pay for the additional benefit of a pressurisation system. So people would tend to use these mechanical shaft systems simply because they are cheaper and because they meet the regulations.

Can one fit a pressure relief damper on a window in a stair?

The pressure relief damper can replace a window, certainly. If you look at a pressure relief damper it’s a louvred damper assembly, its not normally weather proof on its own, so if you are going to replace a window with it, normally you would have the damper and then you would put some kind of weather proofing ventilator behind it, but certainly you can replace a window although you wouldn’t actually use the window.

What effect would the fitting of smoke seals have on doors in the lobby/corridor?

Generally when we have a pressurisation system we wouldn’t fit smoke seals to the doors between the various pressurised spaces or between the pressurised space and the accommodation. The reason for that is basically that a bit of leakage there is generally beneficial in that it limits or minimises the size of the pressure relief damper that we need or the variation that we are going to need on an inverter system. So no we wouldn’t normally fit smoke seals on any of the doors within the pressurised spaces.

Is the noise of a system a problem in relation to a voice comms system say from a disabled persons refuge?

The answer to that is, yes it can be. If you have that sort of situation, then normally there will be a noise spec or a volume spec for the voice comms system, and from that you will be able to set a maximum noise level within the stair. Once you have that maximum noise level required within the stair then its quite possible to calculate the noise level from the fan system and fit an attenuator if required. On smaller systems it’s not normally a problem, but as the systems get bigger and noisier then yes it can be a problem, particularly on the upper storeys close to the fan system.

Paul ComptonPaul is a Technical Director for Colt, experienced in smoke control, HVAC, solar shading and louvre systems.

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Topics: Webinar, Pressurisation, CPD