Controlling roof vents….
11 December 2009
Whilst the benefits are introducing a natural daylight element into the fabric of a roof, there is nothing that seemingly strikes more fear in the hearts of those concerned with the roof than the incorporation of electric opening devices, known as “actuators” and their controls required to make them behave as the end user wishes.
With a careful considered approach and the assistance of a specialist ventilation partner. it is possible to utilise opening modular rooflights and the vents within structural glazing to aid in the building ventilation and lighting strategy to create a comfortable and sustainable environment for the occupants
Electric window and roof vent actuators come in two voltage options, 24 V DC and 230 V AC. The actuators generally work on the same principle, namely “power open and power close”. Therefore at the very least any actuator will require an open/close switch, usually described as “three position return to centre”. If the actuator is 24 V DC an appropriate power supply will be required, if the requirement of the vent is to allow for the ventilation of smoke then this power supply may well have to be battery backed. Finally, for the incorporation of thermostats, room by room operation, CO2 sensors, time clocks, rain and wind detectors, a control panel would be required.
Actuators vary in force and capability, their quality and performance broadly echoes the sort of variance one would find in any market place and it is vitally important that the correct actuator is selected to ensure that it is fit for purpose. There are no “bad” actuators, but there are actuators that are only suitable for certain applications.
There are two types, “linear” and “chain” – the former are cylindrical in appearance and an electric motor powers a push rod to open the vent. Chain actuators are rectangular extrusions housing an electric motor that winds out a chain which attaches to the vent to push it open and pull it closed again.
Actuator Selection is a product of the purpose of the vent
It is vital to select an actuator that is strong enough for the job over the life of the installation. If the primary purpose of the vent is to exhaust smoke then it may be the case that the weight of the vent with glass or polycarbonate is augmented by the weight of snow as laid out in the European smoke ventilator product standard EN12101-2. Remember, if the purpose of a vent is to exhaust smoke under various aspects of the building regulation, BRE or BS5588, then it may be case that practically that vent might be opening in the depths of Winter with 60kg per m² of snow on it – if it doesn’t open, or the actuator fails it could be a life or death situation which is the responsibility of the building owner under the Regulatory Reform Order of 2005. Even if the vent is not for smoke ventilation, there may still be local environmental factors that make the requirement for extra force necessary, such as prevailing wind. It is important that aesthetics don’t win over the practicalities of proper actuator selection. On a lighter note, the correct selection of actuator might allow for powder coating, bespoke installation bracket for more discreet fixing positions. These are all elements that a responsible partner will highlight and offer.
The general rule of thumb is that electric actuators are “dumb” – they will only do what they are told to do by a control panel. Functionality, price and quality of control panels vary as widely as the variety of actuator and it is important to consider the standard of manufacture. Is the panel tested to Low Voltage Directives BS60335-1 and 2:200/3, does it conform to EMC directives on Electromagnetic Compatibilty and display a CE mark?. If the product implementation is for smoke extraction, has the panel been tested to prEN 12101-9 to ensure that the product is tested against its intended functionality, environmental implications and to ensure that the manufactured control process is maintained throughout the panels integrity. If the equipment doesn’t and something goes wrong the ramifications can be huge.
Who’s package is it?
Compatible control systems will also require co-ordination between trades interfacing with BMS systems and fire alarms. Often the supply and installation of the electric actuators falls into the roofing contractor or vent manufacturer’s package, where it sits like a festering boil, no-one daring to expand upon the maxim “controls by others”. Commonly the response is to supply the actuator “any old actuator” and leave the cable “dangling” for others. When this happens unsuitable actuators might be installed, the electrical team mightn’t be aware or have budgeted the correct control incorporation to make the vents work as the design team expects and the functionality of the installation and hence the building is compromised. Frequently the practicalities of who connects the cable to the local junction box?, who makes the cable tidy?, who provides warranty on the performance of the roof-light unanswered. The answer can be found in the assistance of a specialist ventilation partner can alleviate a good deal of the heartache and uncertainty and leave a smoother handover for the installation as a whole. Involving a partner will ensure that all these critical elements of co-ordination are attended to and there will be far fewer post handover cases of actuator failure or mal-performance.
Next time your package incorporates electric actuators and a performance specification for their operation for smoke ventilation or comfort ventilation, think about whether the actuator you plan to use will perform effectively and efficiently in its intended application, make clear your inclusions and exclusions, make it clear at an early stage who is providing control and commissioning, and strongly consider the close involvement of an industry specialist to ensure that the installation goes as expected.