What is an AOV rooflight?

What the three letters actually describe

The vent is the opening. Automatic means it operates without anyone deciding to open it, on a signal from a detector, a panel or a manual call point. The rooflight is simply the form the vent takes when it sits in a roof rather than a wall or a shaft head.

Mechanically it is a rooflight with a powered actuator, a control line back to a panel, and a power supply with battery backup so it still works when the mains has gone. The opening angle is larger than a comfort vent would need, because the figure that matters is the free area the opening presents to rising smoke, not how nice the draught is.

The two jobs it is asked to do

The first is keeping an escape route usable. In a residential block or an office with a single stair, a vent at the head of the stairwell clears smoke out of the shaft so people can get down it and so the fire service can get up it. This is the most common AOV in Oxford, and it is why so many converted buildings have one lonely opening unit at the top of the stair core.

The second is smoke and heat control in a larger volume: a warehouse, an atrium, a shopping arcade. Here the vents are working with the geometry of the space to hold a smoke layer above head height for long enough for the building to empty. That is a design exercise with calculations behind it, and the vent positions and areas come out of the design rather than being chosen for daylight.

How it differs from a rooflight you open for air

A comfort vent opens perhaps 200mm on a chain actuator, closes on a rain sensor, and runs off ordinary mains power. If it fails, the room is stuffy. An AOV opens to a specified angle to deliver a specified free area, has to open against wind and possibly snow load, has to open when the mains supply has failed, and has to be proved to do so on a schedule.

Everything downstream follows from that. The actuator is heavier duty. The wiring is fire-rated cable on a monitored circuit. The controller has a battery. The unit will not simply close because it is raining, because a rain sensor that overrides a fire signal would be lethal. And the whole assembly is a certified product tested as a system, not a rooflight with a motor added afterwards.

The parts sitting behind the glass

A working AOV installation is usually five things. The vent unit itself, on its kerb. One or more actuators, commonly electric spindle or chain type, sometimes pneumatic on older systems. A control panel with a battery, which may serve several vents. Fire-rated cabling between them. And the call points or the interface to the building’s fire alarm that tells it to go.

Add to that a way of resetting it, because after a test or an activation somebody has to close it again, and on a stair head that means either a switch at ground level or a key at the panel. If the reset arrangement is awkward, the system stops being tested, which is how a large number of AOVs quietly stop working.

Where you meet them around Oxford

Overwhelmingly at the head of common stairwells: purpose-built flats, converted townhouses in the central conservation areas turned into apartments, student accommodation, and the upper floors of commercial buildings with a single stair. You also find them in the deeper plan office and laboratory buildings around the science parks, and over the covered circulation in retail.

On a converted period building the vent is often the only new thing on a roof otherwise unchanged for a century, which makes its appearance from the street a genuine issue. In a conservation area or on a listed building the visible profile, the finish and the position all become part of the consent, and the fire requirement and the heritage requirement have to be reconciled rather than one simply overruling the other.

Running the same unit for everyday ventilation

Many AOV units can also be opened on a normal switch for comfort ventilation, which is genuinely useful at the top of a warm stairwell in July. The important detail is that the fire function always overrides the comfort function, and the comfort control is wired so it can never prevent the vent opening on alarm.

Where a building wants both, say so at design stage rather than adding a switch later. Retro-fitting comfort control to a life safety circuit is not a small job and it has to leave the fire function provably untouched. Our page on opening commercial rooflights for ventilation deals with the everyday case.

Our part of an AOV installation, and the parts that are not ours

We install and replace the rooflight and we build and detail the roof opening it sits in: the kerb, the upstand height, the flashing or membrane termination, the insulation and vapour control at the perimeter, and the weathertightness of the whole junction. That is the part that decides whether the roof leaks, and it carries our ten-year workmanship guarantee.

We do not design fire strategies, we do not size vents, we do not commission fire alarm systems and we do not issue certification for a life safety installation. Those belong to your fire engineer, your alarm contractor and your electrical contractor, and on any AOV job we expect to be working alongside them with the vent specification handed to us rather than invented by us. If you approach us for an AOV without that specification in place, we will tell you to get it first. Our page on smoke ventilation rooflights and AOVs sets out how the packages fit together.

Replacing an AOV that has reached the end of its life

Older units fail in predictable ways: the actuator seizes, the polycarbonate dome goes brittle and chalky, the seal hardens and the unit no longer closes flat, or the whole thing has been painted shut by a previous decorating contractor. The replacement is a like-for-like or better unit on the existing kerb where the kerb is sound, or a new kerb where it is not.

Two things have to be settled before ordering. Whether the new unit’s free area at least matches what the fire strategy requires, which is a question for whoever holds that strategy rather than for us. And whether the control equipment is being renewed at the same time, because a new vent on an obsolete panel is a short-lived arrangement. Call 01865 704245 or email info@heritageskylightsoxford.co.uk with the vent size, the roof covering and who holds your fire strategy, and we will tell you honestly what part of the job is ours.