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It's all a question of balance

The recession has prompted building owners to look for more ways to cut their running costs including greater use of natural ventilation, but could indoor air quality be compromised? asks David Fitzpatrick
Building ventilation has two main tasks: Maintaining a good level of indoor air quality and reducing the chance of overheating, particularly in hot summer weather. However, other factors like noise and health as well as smoke and fire safety have a big influence on the solutions available to building services engineers.

Economic pressures and the need to reduce carbon emissions are also limiting the engineering options. Commercial building users are busily looking for ways to minimise their running costs, but in offices and schools the impact on the 'productivity' of the occupants has to be carefully managed.

Natural ventilation has much to commend it - both from an initial capital cost perspective and in terms of ongoing running costs. There is less need for expensive mechanical equipment and the service and maintenance is also reduced compared with conventional mechanical systems. In a new build situation, natural ventilation allows the architect greater flexibility and by reducing the space required for plant.

Overcoming planning obstacles

From a legislative viewpoint, natural ventilation helps meet the energy efficiency reductions stipulated in Part L of the Building Regulations and overcomes planning obstacles. The ability to use 'free' night cooling can dramatically cut energy costs because the building is allowed to cool down naturally overnight, which can minimise or even eliminate the use of air conditioning during the day.

It is this ability to harness, or work in tandem with, nature's elements rather than trying to engineer them out that holds the promise of really significant progress in low energy building designs. However, for a natural ventilation strategy to work effectively there needs to be a very high level of design accuracy. Engineers should normally carry out detailed computer modelling and test their theories thoroughly before putting their chosen system together. Factors such as the orientation of the building; thermal mass; shading; and the size of the openings in the building fabric all have an impact on how a natural ventilation system will perform. It is not suitable for all buildings.

Designers also need to take on board the possible contradiction between the Government's enthusiasm for natural ventilation to meet energy and carbon reduction targets and occupants concerns and comfort requirements.

For example, it may not be possible to use openable windows in all circumstances because of possible noise disruption. Urban noise can be as high as 60-70dBa, which is loud enough to interrupt conversations and inhibit teaching. However, sound barriers and/or acoustic vents can be deployed in this type of location.

Occupants also worry about poor quality outside air in some situations as well as the security implications of open windows or easily accessible louvre systems. Any responsible engineer and equipment supplier will keep all these factors at the front of their minds when putting a system together, but all of these problems can be overcome through good design.

The extra effort needed to design a natural ventilation solution is worthwhile as there is strong evidence that naturally ventilated buildings are healthier buildings because they have lower levels of airborne contaminants.

There are three main approaches: Wind; stack and mixed mode. The first works on the principle that wind blowing against the (Windward) side creates a positive pressure on one side of the building and a negative pressure on the opposite side (Leeward). Air then flows from the windward to the leeward sides via carefully positioned ventilators, usually used in conjunction with a roof terminal.

Stack (or chimney) effect relies more on the buoyancy effect created by the warmer air inside the building, which is less dense than the outside cooler air. Warmer stale air rises up and exhausts through high level ventilators and/or a roof terminal and this movement also draws in outside air at low level. This approach is generally thought to be more reliable than wind-based systems.
However, the mixed mode approach is often a popular compromise, which involves some mechanical elements such as fans and fan coils.

Controls are critical element

The controls are the critical element in this 'hybrid' solution as they must ensure the motorised items only operate when needed to ensure low energy consumption. Mixed mode is particularly useful for winter heating and summer cooling applications when the natural system might struggle to provide the optimum internal comfort conditions.

Whichever approach we adopt, it is critical that humidity, temperature and CO2 levels are managed closely. These are the main factors that determine whether an internal environment is healthy and productive or not. For example, the CIBSE Natural Ventilation and Schools Groups have established a clear link between ventilation strategies and academic performance.

Students will not perform at their best unless relative humidity is between 40 and 60 per cent; and temperature is between 20 and 22degC, a recent joint conference heard. The groups also recommend that the ventilation system provides eight litres of fresh air per second for each classroom occupant to dilute CO2 levels.

In many instances, it will be hard for a natural ventilation system to achieve that level of 'fresh' air without some mechanical intervention at some times. However, in a large number of cases it is perfectly possible for a primarily natural ventilation system to deliver a high level of indoor air quality that meets health, safety and productivity criteria, but at far lower capital and running costs than 'conventional' fully mechanical ventilation systems.

Each project has to be assessed on its merits - this is not a one-size-fits-all issue. Ventilation engineers, with close assistance from specialist equipment suppliers, have to find the right balance in each case.

//The author is sales and marketing director of Ruskin Air Management //
1 October 2012

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