Roof Vent Areas

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

Something we get asked a lot is for an explanation of smoke vent ‘areas’ - and which ones matter.

In this post we’re looking at roof-mounted ventilators, here's a link to another post on facade ventilators.

First up is Geometric Free Area. The reason I’m starting with this is to debunk a myth, there’s actually no such thing as Geometric Free Area. But there is such a thing as Geometric Area.

Geometric Area is, quite literally, the hole in the roof. There’s no need to worry about any product at this stage, just simply the fact that there’s an opening. You’ll see why this matters in a moment.

What I’m now going to explain is the Coefficient of Discharge (Cv), now, stick with me here, I know it sounds complex but there’s no need to be daunted. For the most part, all you need to know is that the Cv is something that manufacturers like us pay lots of money to have complex tests carried out and it tells us how efficient our smoke vents are. You can find out the Cv of a vent by looking at the product’s Declaration of Performance which is available from the manufacturer.

The next value we need to talk about is Aerodynamic Free Area. This is the Geometric Area, (the hole in the roof), multiplied by the Cv, the efficiency of the vent.

So, you ask, why does the Aerodynamic Free Area matter? It matters as it’s the most accurate way of looking at the performance of a vent. We as manufacturers have to state on the product's CE label what the Aerodynamic Free Area of our vents are.

However, you also need to be interested in the Free Area. The Free Area is a way of describing how well air can travel through a vent for the purposes of natural ventilation. It’s essentially a non-scientific way of calculating the efficiency of the vent.

Approved Document B states:

The total unobstructed cross-sectional area, measured in the plane where the area is at a minimum and at right angles to direction of airflow.

So we take the vent, we figure out its size and we deduct the area taken up by the motor (and blades if present) and that’s the vent’s Free Area. Now, whilst Free Area is pretty non-scientific, in that it doesn’t take into account cross winds and splayed curbs et cetera, it’s the number that matters in building regulations such as Approved Document B. Building regulations state, for example, that you must have a vent with a Free Area of one square metre at the head of a stair.

However, the industry is campaigning for the Aerodynamic Free Area to be the ‘number that matters’ as it’s a scientifically tested value that can be relied upon and that all products are tested to - not a somewhat woolly number, like Free Area.

Because of this misalignment, it’s important to correlate the Approved Document B Free Areas with a CE marked ventilator by approximating to an Aerodynamic Free Area.

That would mean that where a ventilator CE marked to EN 12101-2 is used in accordance with Approved Document B 5a, then the declared Aerodynamic Free Area of the vent must be no less than 1.0m2 Aerodynamic Free Area for a 1.5m2 Free Area requirement and a 0.7m2 Aerodynamic Free Area for a 1.0m2 Free Area requirement.

So, to recap:

  • Geometric Free Area - doesn’t exist

  • Geometric Area - the hole in the roof, with no products involved.

  • Coefficient of Discharge - the tested efficiency of a smoke vent

  • Aerodynamic Free Area - the Geometric Area multiplied by the Coefficient of Discharge.

  • Free Area - the size of the vent (at the smallest point) minus the area of any obstructions such as motors or blades.

Which ones matter when you’re specifying or procuring for a project?

Free Area and Aerodynamic Free Area.

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