What do the WL, SL and T classes on a smoke vent mean?

They are environmental classes declared under BS EN 12101-2. WL is the wind load class, expressed in pascals or N/m2, proving the ventilator stays stable and still operates under wind pressure. SL is the snow load class, proving it can open with snow sitting on it. T is the low ambient temperature class, written as a minimum temperature such as T(-05) or T(-15), proving the actuator still drives the vent open in the cold. All three should be named in the specification, not left as a generic reference to the standard.

Can a rain sensor stop a smoke vent opening in a fire?

It must not. Rain and wind sensors belong to the comfort ventilation control path and should only close vents that are open for everyday ventilation. A fire signal has to take priority over every weather, timer, temperature or user input, so the vent drives to its smoke position and stays there. If a weather input can inhibit or reverse smoke mode, the cause and effect is wrong and the system should not have passed commissioning.

What should be checked on an AOV before winter?

Seals and gaskets for compression set and splits, drainage channels and weep holes for blockage, the roof area and gutters around the vent for leaves and debris, full actuator travel to the fully open and fully closed positions with no binding, lubrication of hinges and linkages strictly to the manufacturer's instructions, panel and battery enclosures for water ingress and condensation, and the operation of any heater kit or thermostat fitted to the assembly.

Why do smoke control panel batteries fail in winter?

Sealed lead-acid batteries deliver less of their rated capacity as temperature falls, and a battery that is already partly aged will show that loss first. A panel sitting in an unheated roof plant room or an external enclosure is exposed to the coldest conditions in the building. Battery condition should be load tested rather than judged from a green LED, and cold locations are where a marginal battery set turns into a failed standby test.

Winter-Proofing Smoke Vents: Pre-Winter Checks

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Publishing Sunday 25 October 2026

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An autumn checklist for AOV roof vents, windows and louvres: the BS EN 12101-2 classes that decide winter performance, and what to inspect before it freezes.

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