An automatic opening vent, or AOV, is a rooflight that opens on a smoke detection signal to clear smoke from an escape route so people can get out. It is life-safety equipment, not a comfort rooflight, and its specification is set by the building’s fire strategy, not by us and not by a builder’s preference. We install AOVs to the free area and control arrangement a fire engineer or the fire strategy has specified, we work alongside the alarm contractor and Building Control, and we commission and prove the unit on completion. This page explains how an AOV differs from an ordinary electric rooflight, what dictates its specification, and why it must be designed by someone qualified to do so.
An AOV is designed, not chosen from a brochure. The size, the free area, the control logic and the trigger arrangement come from the fire strategy for your building. We install to that design. We do not set the life-safety requirement ourselves, and no installer should. If your project needs an AOV, it needs a fire strategy that specifies one, and that is the document everything follows.
Where AOVs are required
Two situations bring an AOV into a project most often.
Loft conversions to a third storey
When a house goes from two storeys to three, which is exactly what a loft conversion does, Building Regulations Part B generally requires a protected stairway from the top floor to a final exit. That protection is built from fire doors, a mains-linked alarm system and, at the head of the stairwell, an AOV that opens automatically to vent smoke from the escape route. If your loft conversion drawings do not mention an AOV and you are going to three storeys, ask the question at design stage. Retrofitting one after the plasterboard is up is a genuinely miserable job.
Commercial and communal buildings
In commercial premises and communal residential buildings, AOVs vent stairwells, corridors and lobbies as part of the smoke control strategy. The requirements are more involved and are set by the fire engineer’s design and the relevant standards. Our commercial smoke vent compliance page covers that side, and the commercial skylight installation service page covers the wider commercial work.
What separates an AOV from a normal electric rooflight
An AOV looks like an opening rooflight and is a different animal underneath. The differences are all about working reliably in a fire.
| Aspect | Comfort electric rooflight | AOV smoke vent |
|---|---|---|
| Fail-safe behaviour | Fails closed on power loss | Fails open on power loss or signal loss |
| Trigger | Handset, wall switch, rain sensor | Smoke detection signal, automatic |
| Control | Standard receiver | Dedicated control panel with battery backup |
| Backup power | Optional | Standard, commonly 72 hours standby |
| Manual override | Handset or wall switch | Break-glass or key switch at ground level for the fire service |
| Opening size | Whatever suits ventilation | A specified geometric free area, often 1m² |
| Actuator | Usually chain | Usually hinge or spindle, for the larger opening |
| Commissioning | Set and test the unit | Proved to open on a detection signal with the alarm contractor present |
Fail-safe to open
This is the crucial inversion. A comfort rooflight fails closed, so it stays weathertight when the power goes. An AOV fails open, because in a fire that has taken out the power the vent must still clear the escape route. It is the opposite behaviour, and it is correct for life safety.
A dedicated control panel with battery backup
An AOV runs off its own control panel, not a domestic receiver. The panel carries battery backup, commonly sized for 72 hours of standby, so the system works in a fire that has cut the mains. The panel interfaces with the fire alarm so smoke detection triggers the vent automatically, and it provides the manual override.
A specified geometric free area
This is where people are caught out. An AOV is specified by its free area, the measured area of the actual opening, not the size of the rooflight. A common figure for a stairwell AOV is 1 square metre of free area. A 1.2 by 1.2 metre rooflight does not give 1 square metre of free area, because the sash only opens so far. Reaching the specified free area is why AOVs use hinge or spindle actuators that swing the sash to a large angle, rather than the chain actuators that suit comfort units.
Commissioning and handover
An AOV is not finished when it is fitted. It has to be proved to work, and that is a scheduled part of the job.
- Mechanical checks. The unit is set, weathered and its full travel checked so it reaches the specified free area cleanly.
- Electrical connection. The control panel is wired and the fire alarm interface connected, work that involves the alarm contractor.
- Detection test. The vent is proved to open on a genuine smoke detection signal, with the alarm contractor present, not just on the manual override.
- Override test. The ground-level break-glass or key switch is proved to open the vent for the fire service.
- Battery and fail-safe test. The standby supply is verified and the fail-safe-open behaviour confirmed.
- Documentation. The commissioning record goes to you and to Building Control alongside the fire alarm certification.
Weathering an AOV, which is still a rooflight
For all its life-safety electronics, an AOV is still a hole in a roof that has to keep water out, and the weathering is exactly where our trade sits. On a flat roof the unit goes on an insulated upstand built to a minimum 150mm above the finished roof, 200mm on exposed coastal sites, with the covering dressed up the full height and over the top. On a pitched roof the flashing kit is fitted and the tiles or slates re-hung with the laps mechanically terminated. A hinge actuator that swings the sash to a large angle puts more wind load on the frame than a small comfort vent, so the fixings and the upstand detail matter more, not less. On the coast that means 316L stainless throughout and control gear sited inside the warm envelope rather than a cold void where it would condense.
Maintenance and periodic testing
An AOV is life-safety equipment, so it is not fit-and-forget. The building’s responsible person is expected to keep it maintained and tested so it works when it is needed, and the regime is set by the relevant standards for smoke control rather than by us. In practice that means the vent is function-tested regularly, the battery backup is checked, and the actuator and seals are inspected, alongside the fire alarm testing it is interfaced with. We hand over the commissioning record and the operating information so whoever manages the building knows what has to be checked and how often. On a domestic loft conversion the householder holds that responsibility; on a commercial or communal building it sits with the duty holder, and the commercial smoke vent compliance page covers that side.
An AOV is not a comfort vent you can also use to air the room. It is sized and controlled for smoke clearance on the fire strategy, and its everyday use is limited by that design. If you want a rooflight over the landing that you also open on a warm evening, that is a separate comfort unit, specified alongside the AOV rather than instead of it. Mixing the two duties into one unit is not how the fire strategy is met.
Lead times and where it sits in the build
AOV units with a control panel and fire alarm interface run longer than comfort units, typically five to eight weeks from order, and the installation is three to four days plus the alarm contractor’s testing on top. It has to be planned into the conversion from the start, not bolted on at the end. The wider sequencing, and the actuator choice that gives the free area, is covered on the main electric skylight installation page. For the fail-safe behaviour in a power cut, see the electric rooflight in a power cut answer, and for costs the AOV cost answer breaks the figure down.
Domestic loft conversion against commercial smoke control
The words AOV and smoke vent cover two related jobs of very different scale, and it helps to know which you are in.
On a domestic loft conversion, the AOV is usually a single unit at the head of the stairwell, triggered by the mains-linked smoke alarm system that the conversion needs anyway, with a control panel, battery backup and a ground-level override. It is a defined, contained part of achieving the protected stairway that Part B calls for when the house goes to three storeys. The fire strategy for the conversion specifies it, and it is a manageable addition when it is planned in from the drawings.
Commercial and communal buildings are a bigger undertaking. Smoke control there can involve multiple vents across stairwells, corridors and lobbies, larger control systems, and design to the smoke-control standards by a fire engineer, with more demanding commissioning and a formal maintenance regime under a named duty holder. The principle is the same, a vent that opens to clear an escape route, but the scale, the controls and the compliance load are greater. That side is covered on the commercial smoke vent compliance page and the wider commercial skylight installation service. In both cases the rule holds: the design comes from the fire strategy, and we install to it rather than setting the life-safety requirement ourselves.
Common questions
Does my loft conversion need an AOV?
If it takes the house from two storeys to three, very likely yes. Going to a third storey generally triggers a Part B requirement for a protected stairway, and an AOV at the head of the stairwell is a standard part of achieving that alongside fire doors and a linked alarm. But the requirement comes from the fire strategy for your specific conversion, not from us. Ask the question at drawing stage, because retrofitting an AOV after the plasterboard is on is a serious job.
Who decides the size of the AOV?
The fire strategy does. An AOV is specified by its geometric free area, the measured area of the actual opening, commonly 1 square metre for a stairwell, and that figure comes from the fire engineer or the fire strategy for the building. We install to it. A rooflight of a given size does not automatically give that free area, because the sash only opens so far, which is why AOVs use hinge actuators to reach a large opening angle.
What happens to an AOV in a power cut?
It opens. An AOV is fail-safe to open, the opposite of a comfort rooflight, because in a fire that has cut the power the vent must still clear the escape route. Its control panel carries battery backup, commonly 72 hours of standby, so the system stays live through a mains failure. That fail-safe behaviour is a defining feature of smoke ventilation and is proved during commissioning.
Get a fixed price for an AOV installation
Send us the fire strategy or the specified free area and control arrangement for your conversion or building, along with the roof type and where the stairwell sits. We install to that design, work alongside your alarm contractor and Building Control, and give you one written price covering the unit, the control panel and commissioning, fixed after survey with a 10-year workmanship guarantee.
Request a fixed quote for AOV smoke vent rooflight installation across Ipswich, Suffolk and north Essex.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.