Façade Vent Areas

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

Here's a handy guide on how to calculate façade vent areas.

I'm going to keep this as simple as possible - if you want a recap on any of the technical terms, have a quick read of our roof vent areas guide. The main areas we're going to be talking about this in this guide, are Free Area and Aerodynamic Free Area.

We have to look at both of these areas because one of them relates to BS EN 12101-2, and the other one relates to Building Regulations. Specifically, Free Area relates to Building Regulations and the Aerodynamic Free Area relates to BS EN 12101-2.

There are two types of vent that we will be focussing on in this guide - a Façade Louvre Blade and a Façade Single Leaf vent.

Facade Louvre Blade

Mounted in façades and end of corridors, Façade Louvre Blade – Residential (FLB-R) smoke vents exhaust smoke and heat from a building allowing escape routes to be kept clear of smoke and are fully certified to BS EN121012. They may also offer inlet air and comfort ventilation.

Free Area

In order to calculate the Free Area of this vent, we have to take the total unobstructed cross-sectional area and the way we do that is by figuring out how big the gaps between the blades are.

However, we can't just take our measurement from the area within the frame, we have to take it from the end of the blades. This is because the blades aren't quite horizontal, they're at a slight pitch and the result will vary depending on the pitch.

Once we've calculated the areas of the sections, we add them all together and that's our Free Area.


Aerodynamic Free Area (Aa)

Next up is Aerodynamic Free Area. To start with, we must first calculate the Geometric Area. That's the size of the hole in the product without any of the blades:

Now, we have to multiply that by the Coefficient of Discharge. The Coefficient of Discharge isn't something we can just pluck out the air it's something we have to test for.

The product will go into a wind tunnel and calculations will be run to work out how efficient the vent is with the blades and any other obstructions.

So, to find out the 'Aa value', the Aerodynamic Area, we take this Geometric Area (the hole in the frame) and we multiply it by the Coefficient of Discharge which is found out from having the testing carried out: FA x Cv = Aa


Facade Single Leaf

Façade Single Leaf – Residential (FSL-R) smoke vent windows are mounted in façades to provide smoke and heat ventilation, air inlet, comfort ventilation and daylight. They are particularly suited to apartments and commercial buildings at the head of stairs or end of corridors.

They are configured as bottom hung opening out units and incorporate chain or spindle actuators enabling a wide range of size options or opening angles and ventilation areas.

Free Area

When calculating the Free Area of a single leaf vent, you have to make two calculations and choose the one that's the smallest.

First of all, we take the dimension of the frame, however, it's important to note that we must stop at the window sill.

The second measurement we have to take is a measurement between the top of the window frame and the furthest opening out point of the frame of the window, we then multiply that by the width.

Whichever area is the smaller, that's the area we use.

Once we have the smallest figure, we have to take into account any motors and mechanism, thus giving us the true Free Area.


Aerodynamic Free Area

Next up is the Aerodynamic Free Area.

In order to be able to work out that, we take the Geometric Area which, very similar to the Facade Louvre Blade above, is the area of the frame multiplied by the Coefficient of Discharge (the tested efficiency figure.)

So, that's how you measure the Aerodynamic Area and the Free Area For a Facade Louvre Blade and a Facade Single Leaf!

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