Why doors are a key part of a smoke control system

By Carlsson Elkins
Wednesday 19th August, 20205 minute to read

When you think of a smoke control system, you may think about mechanical units or vents, but did you know that doors play a significant role in a smoke ventilation system?

The particular door that I’m referring to is the door from a protected corridor into the stair core. If you live in an apartment, that’s the door you take to the stairs every day (unless you use the lift!)

In fact, doors are so important that the SCA Guide has 134 references to doors and Approved Document B has 294 mentions.

The reason a door is a fundamental point of smoke control is that it is the ‘gateway’ between the protected corridor and the protected stair core. In a typical pressure differential smoke control system, air is extracted through a damper in the protected corridor, drawing make-up air down the stairs via a head-of-stair vent and into the corridor via the door. If make-up air is coming from the stair, you can’t have a vent or damper from the corridor to the stairs, you must use the door.

What we’re going to explore today is the concept of a reverse-swing door. A reverse-swing door system is a mechanical extract system where the door to the stair core opens into the corridor, rather than into the direction of travel, into the stair core.

So, why would you use a reverse swing door system?

By having the door opening towards the corridor, air can be drawn down the stair core and into the corridor. It serves two purposes:

Firstly, it keeps the stair core free of smoke very effectively as it’s drawing fresh air down the stairs.

Secondly, it ensures that there’s no pressure build-up in the corridor. This is really important as with a mechanical system you have to ensure that apartment doors open with no greater force than 100N at the door handle (BS 9991:2015 14.2.4) otherwise, you can essentially lock people in their apartment with no way to escape.

It also means that the system can be run at one speed, rather than requiring a Means of Escape and Fire Fighting speed. The system can just run at its maximum speed. Which means that the firefighters do not have to alter the smoke control extract speed from the slower means of escape mode to fire fighting.

You may be wondering what the alternatives are to a reverse-swing door system.

As an alternative, the door is hung to open in the direction of travel and differential pressure sensors are fitted to ensure the corridor doesn’t depressurise to a point that would cause a force greater than 100N at the apartment door handle, usually 50 pascals. If the stair door is open, the system will run at full speed, however, when the stair door closes, the system has to go into set-back mode which reduces the fan speed to make sure the doors can still be opened. Make-up air comes from air leakage, for example, from apartment doors and lift cores. The system must react within three seconds of a change in pressure.

This alternative method is significantly less suitable for the following reasons:

Firstly, the system is much less effective at clearing smoke when running at set-back speed

Secondly, there is much less effective stair core clearance as fresh air isn’t being drawn down the stairs. In this scenario, any smoke that gets into the stair core is likely to stay there until the door is opened again.

Finally, there is a risk of the pressure sensor failing. If that were to happen, you would end up locking people in their apartments.

Your next question may be ‘Why don’t we always use reverse-swing doors then?’

The answer to that is found in Approved Document B: 3.94 The door of any doorway or exit should be hung to open in the direction of escape whenever reasonably practicable. It should always be hung to open in the direction of escape if more than 60 people might be expected to use it during a fire.

As you can see, if there are less than sixty people likely to use the door in the event of a fire, which is usually the case, it is possible to use a reverse-swing door. It is our ethos that the inconvenience of the reverse-swing door is far outweighed by the greater effectiveness of the smoke control system.

There is one other alternative that we can consider and that is what’s known as a ‘floppy door’ or ‘double action door’. Evacuees can exit in the direction of escape and the smoke control system can draw the door open to avoid pressure build-up.

These are what you might consider a win-win situation, however, they aren’t always favoured due to the extra cost that can be involved.

I’m hoping that by now you understand why doors are a key part of a smoke control system and why having a reverse-swing or double action door is a preferred system.

Finally, there are a few other points in relation to doors that it’s important to note:

  • Full height doors are a challenge as there’s no allowance for smoke logging: BS9999:2017 20.3.4: Note: Full height (floor to ceiling) doors are not recommended for the door between the stair and the lobby. This is because of the adverse effect the increased door height can have on the ability of the smoke ventilation system to keep the stair smoke-free.

  • The proximity of the apartment door to the stair door makes a considerable difference to a smoke control system

  • For CFD modelling, our recommended approach is to model fire fighting mode with the stair core door fully open

  • The maintenance of fire doors is an essential part of maintaining a smoke ventilation system.

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