AUTOMATIC NATURAL VENTILATION: SIMPLE OR COMPLICATED?
Energy efficiency is now commonly at the heart of good mechanical services design. Many technologies have entered the race to become the perfect partner for energy efficiency within buildings, such as Combined Heat and Power (CHP), ground and air source heat pumps. Among these Automatic Natural Ventilation stands head and shoulders above these in the technology battle. It offers a simple yet highly effective solution for free cooling and air quality control.
Within modern construction natural ventilation is found in many guises. We see it in actuated windows, motorised louvres and roof turrets to name a few. All of these can be coupled together within different modes of ventilation strategy – such as cross ventilation, stack ventilation and mixed mode mechanical and natural ventilation.
To provide some understanding of the differences between these varied natural ventilation system designs, we have provided the following summary. It is defined from CIBSE’s Natural Ventilation Guide for non-domestic buildings, AM10: 2005.
Single sided double opening ventilation
Where multiple ventilation openings are provided at different heights within the façade the ventilation rate can be enhanced with the stack effect. Stack induced flows increase with the vertical separation of the openings and with the inside to outside temperature difference. As well as enhancing the ventilation rate the double opening increases the depth of penetration of the fresh air into the space, as opposed to single opening ventilation. As a rule of thumb the limiting depth for effective ventilation is about 2/5 times the floor to ceiling height of a building.
This occurs where there are ventilation openings on both sides of the space. Air flows in one side of the building and out the other through, for example, a window or door. Cross ventilation is usually wind driven but it can also be driven by density differences in an attached vertical chimney. As air moves across the zone there will be an increase in temperature and a reduction of air quality as the air picks up heat and pollutants from the occupied space.
A normal approach to achieving cross ventilation is in opening windows. However other approaches can also be successful. An older example is the ‘wind scoop’, built into the main infrastructure of the building. A roof-mounted ventilator represents a more modern method of this. It uses the pressure difference across a segmented ventilation device to drive air down through the segment facing the wind and into the space. The suction this creates through negative pressure draws air back out of the space. Flow rate can be controlled using a damper, with air distribution being achieved via a diffuser module.
Stack ventilation is driven by density differences. The approach draws air across the ventilated space and then exhausts the air through a vertical flow path. This means that occupied zones are cross ventilated, in that air enters one side of the space and exits at the opposite site. In order to achieve the required flow distribution without excessively large outlet ventilator sizes, the stack outlet usually needs to be at least half of one storey above the ceiling level of the top floor. This can be achieved with a dedicated chimney or through an atrium. The advantage of atrium ventilation is that air can be drawn from both sides of the building towards a central extract point, effectively doubling the plan width that can be effectively ventilated by natural means.
Different strategies may be applied to different parts of a building, or at different times. This is the so-called ‘mixed mode’ approach. For example, Changeover Mixed Mode recognises that cooling requirements of any space varies from season to season. An example of this would be to use mechanical ventilation in extreme weather conditions, both hot and cold, but rely on natural ventilation in milder weather. This reduces the problem of cold draughts in winter and allows the use of mechanical night ventilation for precooling in hot summer periods.
Each different approach to natural ventilation requires a different level of BMS control, monitoring, interface or integration to achieve a successful natural ventilation control system. A key aspect of this is that it empowers the building occupant to make adjustments to window openings in order to maintain personal comfort, without prejudicing the comfort of others. It means that automatic control strategies need to be carefully integrated with user behaviour.
Some systems, such as roof turrets, come with optional stand-alone controls, which are not required when the technology is being integrated within a site-wide BMS strategy. Window actuators can also cause complications where the typical contractual chain sees these procured as part of the façade package let directly by the main contractor and not generally in sight of the BMS specialist subcontractor of the M&E. So too often by the time the BMS contractor is engaged on the project the actuator selection has been agreed. This removes the opportunity to review and select the best technologies aligned to the overall BMS design. In an ideal situation the actuator selection will be withheld until the BMS contractor is appointed and such detail can be developed through consultation.
For an M&E buyer, clarifying which optional items are required is all too often a minefield, presenting possible cost overlap, score gap or warranty issues. A BMS specialist should be able to assist in identifying the boundary points of all packages, to ensure their alignment to provide a combined working technical solution.
If not already stipulated by the design consultants, an early review and detailed design of the operability, signals and power requirements between devices (such as the BMS, window actuators and turrets) across all suppliers (façade package, M&E procurement, BMS sub-contractors) is critical to the success of the overall outcome.
How can ABEC help?
We work closely with our customers to provide this consultation, ensuring the successful outcome of BMS automated natural ventilation projects. This has multiple benefits, ensuring the end client’s overall building control, energy efficiency and successful contractual delivery of the project.