What is an AOV rooflight?

Smoke is what kills people in most building fires, not flame. It fills a room from the ceiling downwards, it is hot, and it hides the way out. An AOV works on a simple piece of physics: hot smoke rises, so if you give it an opening at the highest point of the space it will pour out of its own accord and pull cooler, cleaner air in lower down. Put that opening in the roof, over a stairwell or a corridor, and you hold the smoke layer higher for longer and keep the escape route usable.

The word automatic is the important part. A manual rooflight needs someone to reach it and wind it open, which is no use when a landing is already filling with smoke. An AOV opens on its own the moment smoke is detected, or when the fire alarm panel tells it to, with no person in the loop. That is the difference between a normal opening rooflight and an AOV: the same aperture in the roof, but wired into a life-safety system and expected to work in the worst conditions the building will ever see.

Behind the glazing sits an electric actuator, a motor that drives the vent open. It is held on a control system, and there are two common ways it gets its instruction to open.

• Local smoke detection. A detector on the ceiling of the stair or lobby senses smoke and signals a dedicated AOV control panel, which powers the actuator open. This is the usual arrangement for a residential stairwell where the vent protects the escape route directly above it.
• Fire alarm interface. In a larger or more complex building the AOV is driven by the main fire detection and alarm system, so the vent forms one output of a wider strategy that may also close doors, start fans and release the smoke shaft dampers.

The control panel carries a battery backup so the vent still opens if mains power has failed, which in a fire it often has. Most systems also give the fire and rescue service a manual override switch at ground level, so a crew can open or close the vent by hand as they fight the fire and clear the smoke afterwards. On a day-to-day basis many AOVs double as ordinary ventilation, opening on a wall switch or a timer to air the space, then reverting to their life-safety duty the instant the alarm sounds.

AOVs are not a nice-to-have on most commercial and multi-occupancy buildings, they are how the building meets the fire strategy signed off at design stage. In England and Wales the relevant guidance is Approved Document B, which covers means of escape and smoke control, supported by the British Standards, chiefly BS 9999 for the general design of buildings and BS 9991 for residential ones. The detailed engineering of smoke and heat exhaust sits under BS 7346 and the European standard EN 12101, which is the standard an AOV rooflight is tested and certified to as a smoke ventilator.

In practice the common cases are a stairwell in a block of flats, where a vent at the head of the stair clears smoke that has leaked in from a flat below, and a top-floor corridor or lobby serving several units. Offices, schools, care settings and warehouses use them over stair cores and over large open volumes such as atria. Because the vent is part of a certified life-safety system, it is specified, installed and commissioned to that standard, and it is tested and maintained on a schedule afterwards rather than fitted and forgotten.

An AOV is still a hole of glass in a roof, so everything that governs an ordinary rooflight applies to it as well: it lets in daylight, and it lets in solar heat. That second point matters more than people expect, and it is where the direction the roof faces comes back into play.

A stair core or an atrium with an AOV at its head is often a tall, enclosed volume with little air movement, exactly the shape that traps heat. Glaze the top of it facing south, as many rooflights over south Essex extensions and cores end up doing, and on a clear day you are pouring solar energy into a space that has no easy way to shed it until the vent happens to open. The fix is the same one that governs any south-facing glazing: specify the glass by its G-value, the share of the sun’s heat the glazing lets through, so a solar-control unit at around 0.3 keeps the summer load down while a clear unit near 0.6 would let roughly twice as much heat in. Glare off an estuary-facing roof is the other half of it, and it is answered the same way, with the coating rather than an afterthought blind. Getting the vent right for smoke and getting the glass right for sun are two separate specifications on the same unit, and a good installer holds both.

The comfort ventilation an AOV provides day to day helps here too. A vent that opens on a warm afternoon lets the hottest air out at the top of the space, which is the most effective place to remove it, so a dual-purpose unit earns its keep between fires as well as during one.

If you are responsible for a commercial or multi-occupancy building, start from the fire strategy rather than the rooflight catalogue: the strategy tells you where a vent is required, what free area it has to provide and which standard it is certified to, and the rooflight is then selected to meet that. If you are replacing an existing AOV or adding one as part of a wider roof job, treat it as both a life-safety component and a piece of glazing, and settle the smoke performance and the G-value in the same conversation.

Our commercial skylight installation work covers exactly this kind of unit, and there is more on getting the heat side right in our guide to energy-efficient skylight installation. If you would like us to look at a specific roof, tell us the building, the orientation and the opening you have in mind. Ring 01702 898232 or request a quote, and we will work back from what the space actually needs.