Smoke ventilation rooflights and AOVs

What an AOV has to do that an opening rooflight does not

A comfort rooflight opens when somebody presses a button and it is nobody’s problem if it does not. An AOV opens when a detector triggers, or when a firefighter operates a control, and its failure to open is a failure of the building’s fire strategy.

That difference produces requirements a normal unit has none of: an actuator sized to open against wind load, a power supply that survives loss of mains, a control panel that monitors the circuit and reports a fault, a defined opening time, and a tested performance under the conditions of a fire. It also produces an ongoing duty, because a life safety system has to be tested and serviced rather than fitted and forgotten.

Aerodynamic free area against the hole in the roof

The most misread figure in the whole subject. Geometric free area is simply the size of the opening created when the vent opens: length times width of the clear gap, in square metres. Aerodynamic free area is that figure multiplied by a coefficient of discharge derived from wind tunnel testing, and it accounts for the fact that air does not flow through an opening as efficiently as the geometry suggests.

The coefficient is typically somewhere between 0.5 and 0.7 depending on the vent geometry, so a unit with a geometric free area of 1.5 square metres might deliver an aerodynamic free area of around 1.0. If a fire strategy calls for 1 square metre aerodynamic and somebody supplies a unit with 1 square metre geometric, the building is short by a third and nobody notices until commissioning.

The one square metre stairwell figure and where it comes from

The commonly quoted requirement for a vent at the head of a common stairway is one square metre free area, and it appears in guidance for residential blocks and in many commercial fire strategies for stairs serving upper storeys.

Two cautions. First, check whether the figure quoted in your strategy is geometric or aerodynamic, because the required unit size differs substantially. Second, one square metre is a minimum for a particular arrangement, not a universal answer. A stair of unusual geometry, a stair also serving a basement, or a building relying on the stair for firefighting access may need considerably more, and that is the fire engineer’s call.

BS EN 12101-2 and the classes on the label

Natural smoke and heat exhaust ventilators are covered by BS EN 12101-2, and a compliant product carries a declaration of performance with a string of classified properties. They are worth reading rather than skimming.

A specification that names only the aperture size and the words “AOV to BS EN 12101-2” is incomplete. The classes are what make the product suitable for the building.

Approved Document B and who decides the duty

The requirement for smoke ventilation comes from the fire strategy for the building, which is derived either from the guidance in Approved Document B or from a fire engineered approach set out by a fire engineer. It is not derived from a rooflight catalogue and it is not our decision.

Common triggers are a common stair serving flats above a certain height, a firefighting shaft, a basement requiring smoke clearance, and a protected corridor arrangement in a commercial building. What we need at enquiry is the strategy or the relevant extract, naming the location, the required free area and whether it is geometric or aerodynamic. Everything else follows from that.

Natural venting against mechanical shafts

Natural smoke ventilation relies on buoyancy: hot smoke rises and leaves through an opening at high level, with replacement air entering low down. It is simple, needs no running power once open, and works well where the geometry allows a clear vertical path.

Mechanical smoke shafts use fans and are specified where natural venting cannot achieve the performance, typically in tall buildings, deep plans or where the shaft has to be small. Where a building uses a mechanical system, the roof component is usually a terminal rather than an AOV, and it is a specialist system contractor’s package. We install the rooflight and AOV elements of natural schemes; we do not design or install mechanical smoke systems.

Actuators, panels and the standby supply

The mechanism is normally a 24 volt electric chain or linear actuator, sized on the sash weight and the wind load, wired back to a dedicated control panel. Pneumatic and gas strut systems exist and are now uncommon in new work.

The panel is the part that gets forgotten in the budget. It monitors the actuator circuit for open and short circuit faults, provides the standby supply, and takes the inputs from detectors and manual controls. Standby batteries are specified to hold the system for a defined period after mains failure, commonly 72 hours followed by the ability to operate, and the battery capacity is a design calculation rather than a guess. Panels need a permanent unswitched supply on its own protective device, labelled and not sharing a circuit with anything a cleaner can turn off.

How the vent hears about the fire

Three routes, and a building often uses more than one. A dedicated smoke detector at the head of the stair or in the protected space, wired directly to the AOV panel. A volt-free interface from the building’s fire alarm system, so a general alarm triggers the vent. And manual controls for the fire service.

The interface is where coordination fails on site more often than anywhere else, because the AOV panel and the fire alarm are usually supplied by different parties. The connection needs to be agreed at design stage: which system provides the interface, who provides the cable, what the cause and effect matrix says, and who witnesses it working. Left to sort itself out, it is discovered at commissioning with everybody already off site.

Manual control points and firefighter override

Fire service controls are normally located at the point of entry into the protected space, clearly labelled, with the ability to open and to reset. Where a residential block is involved, the control is typically at the stair entrance on the ground floor, in a position agreed with the fire and rescue service.

The override matters. A firefighter needs to be able to open the vent regardless of what the detection is doing, and to close it again once ventilation is no longer wanted. Controls hidden in a cupboard, unlabelled, or behind a locked door defeat this. Position is worth agreeing early rather than being decided by whoever is holding the drill.

Dual purpose units: smoke and daily ventilation

Many stair and atrium vents are also used for comfort ventilation, which is attractive because the aperture is already there. It is a legitimate arrangement, with conditions.

The unit must be classified for the cycle count that daily use implies, which means a high reliability class rather than the minimum. The control logic must give the fire signal absolute priority, so a vent held closed for comfort reasons still opens on alarm. A rain and wind sensor is needed for the comfort function and must never be able to inhibit the fire function. And the comfort controls have to be arranged so that nobody can isolate the unit thinking they are only turning off a draught.

Kerbs, upstands and the detailing an AOV still needs

For all its electrical complication, an AOV is still a hole in a roof and the same rules apply. A minimum 150mm upstand from the finished covering surface, falls that take water away from the kerb rather than into it, the covering dressed up and terminated correctly, and the flashing lapped in the right order.

The mechanism adds two things. The kerb has to be rigid enough not to flex when the actuator drives the sash against a wind load, which on a large vent means a properly built and braced kerb rather than a thin insulated one. And the cable entry has to pass through the kerb without becoming a water route, which means a sealed gland below the flashing line rather than a hole drilled through the top after everything else is finished.

Cabling, containment and who does which bit

AOV wiring is normally in fire resisting cable, in dedicated containment, routed to avoid areas where a fire would disable the circuit before the vent operated. It is not general power wiring and it is not run in the same containment as everything else.

On a job of any size the division of work needs writing down: who supplies and installs the panel, who runs the cable, who terminates at the actuator, who provides the fire alarm interface, and who commissions. We install the vent and the roof detailing around it, and we work to whatever the electrical contractor and the fire alarm contractor need at the roof. Where a client wants all of it under one order, the electrical package sits with a specialist and we coordinate.

Commissioning, witnessing and the certificate

A smoke ventilation system is commissioned as a system, not as a set of parts. That means demonstrating that each trigger causes the correct vent to open within the required time, that the free area achieved matches the design, that the standby supply holds the system for the specified period, and that the reset works.

The result is a commissioning certificate and a cause and effect record, and both belong in the building’s fire safety file. Building control and, in a residential block, whoever holds the ongoing safety duty will ask for them. On a job we install into, we provide the installation detail and the product documentation; the system certificate comes from whoever commissions the system.

Servicing intervals and the duty holder’s log

Once installed, the vent has an ongoing regime. Common practice is a monthly functional test operated from the control point, with a full service by a competent party at least annually covering the actuator, the battery, the seals and the free area achieved.

Each test goes in a log kept with the fire safety documentation. This is a duty holder responsibility under fire safety legislation rather than something a supplier discharges for them, and it is the part most often neglected on smaller buildings. A vent that has never been tested since handover is not a working vent, whatever the paperwork from installation says.

What we install and where a specialist takes over

We install and replace AOV rooflights and the kerbs, upstands and weathering details around them, including replacing an existing failed vent with a compliant unit on a properly formed upstand. We will work to the free area, the classes and the location a fire strategy specifies.

We do not write fire strategies, size free areas, design or install mechanical smoke shafts, or commission fire systems. Those need a fire engineer and a smoke control specialist, and on any job where one is not already appointed we will say so before quoting rather than after.

What to send with an AOV enquiry

The extract of the fire strategy or specification giving the required free area and whether it is geometric or aerodynamic. The location: stair head, corridor, atrium, basement lightwell. The roof construction and the existing kerb height if there is one. Whether the unit is also wanted for daily ventilation. Whether a control panel and detection already exist or are part of the work. Photographs from above and below.

With that we give a range by email and a fixed price after survey. Call 01865 704245 or email info@heritageskylightsoxford.co.uk, and if the strategy has not been written yet, say so, because that is the thing to sort out first.