How to calculate Façade Vent Areas

By Sam Gocher
Tuesday 29th August, 20232 minute to read

Welcome to another blog from Sertus. In this edition we’re going to be explaining how to calculate Façade Vent Areas. We’re going to be discussing Free Area & Aerodynamic Free Area. This can be a tough challenge to put across via text, so if at any point you become lost, please do watch our video on the topic, which can be found here.

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

In order to calculate the free area of say a Façade Louvred Vent (click here to see what it looks like), you have to take the total unobstructed cross sectional area. The way this is done is, once the vent is open, you find out how big the open gaps are between the blades. You have to measure from the point of the open blade to the open top corner above it, and the same for the section above. Refer to the above video for a simulation of which areas to measure. You then add the measured areas together to determine the free area.

To work out aerodynamic free area on the louvred vent – you take the geometric area, which is the entire hole, not taking the blades into account, as if they’re not there. You then multiply this by the coefficient of discharge. This is something which needs to be tested for. The product will be put into a wind tunnel where it will be worked out how efficient the vent is with the blades and any other obstructions.

Now, how to calculate the free area of a Façade Single Leaf event (click here to see what one looks like). You take the dimension from the inside left and right of the product, and multiply it by the dimension of the top and bottom of the product. The next measurement you take is from the window frame to the first outward point of the opened leaf, which you then multiply by the width of the product. Whichever of the two products is smaller, we use that measurement to determine the free area. It’s important to note we have to take motors into account, the measurements of these have to be deducted so we’re giving it its true free area.

For aerodynamic free area, it’s very similar in the sense that it’s the entire open hole of the opened product, multiplied by the coefficient of discharge.

We hope you’ve found this blog insightful, and as mentioned please do watch the linked video if you’re struggling to visualise! If you have any questions about our wide range of products, please do contact us. Thanks for reading.

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