Cracking the secret of draught-free air movement
Effective ventilation and draught management achieve even distribution of air without compromising end user comfort or efficiency. Tim Mitchell, sales and marketing director of Klima-Therm, explains how.
At the turn of the last century, an unusual experiment took place at the Millennium Dome in Greenwich, London. The main roof vents of the building were opened, and all the perimeter doors other than the main entrance closed. It was then discovered that the resulting internal air velocities were so high that it was possible to fly a kite inside the building.
Draughts in buildings are rarely, if ever, this extreme. However, they remain the scourge of any ventilated space because they can have a dramatic negative impact on the comfort and wellbeing of building occupants as well as wasting valuable energy.
Draughts are usually caused by air passing from the outside to the inside of a structure, or vice versa, because of pressure differences caused by wind or warmer indoor temperatures. They cause heat loss and uncomfortable air movement, and they can occur for several reasons, including:
• Defective or poorly set up building services.
• Penetrations through walls and ceilings caused by pipes and wires.
• Trickle vents in windows and grilles in walls.
• Badly fitting, poor quality or faulty windows or doors.
• Open chimneys, letterboxes, or keyholes in homes.
Moreover, architectural features can restrict circulation or create pockets of uneven air flow.
Managing draughts involves a subtle balancing act – allowing air to move to achieve the right number of air changes while, at the same time, ensuring the velocity of air currents doesn’t reach a level that causes distress or discomfort.
The answer to the draught problem is effective ventilation. Indeed, ventilation offers a useful double whammy – not only can it prevent draughts, but, with COVID-19 still lurking in the wings, it has a crucial role in preventing the aerosol transmission of the virus in enclosed spaces.
As well as mitigating the impact of draughts, studies have shown that good ventilation is associated with improved health, better concentration, higher levels of satisfaction with an environment, lower rates of absence from schools and work, better quality of sleep and reduced exposure to a wide range of air pollutants.
The perception of draughts in buildings is intrinsically linked to indoor air velocities. The greater this is, the greater the heat exchange between people in a space and the air around them. Of course, the velocity of air inside buildings is usually low compared to the outside. However, in tall spaces, or those that are mechanically ventilated, uncontrolled air velocities can reach several m/s.
According to Designing Buildings Wiki, an industry-wide, cross-discipline forum for finding and sharing information, 0.1 to 0.15 m/s and above may be felt as a draught in a cold climate in the winter, 0.3 m/s and above can be felt as a draught in a cold climate in summer, and 0.8 to 1 m/s and above might be felt as a draught in a hot climate.
There are of course common standards by which ventilation velocities are rated, notably British Council for Offices (BCO) and DIN standards.
Although the ventilation method itself is important to ensuring occupant comfort, the terminal devices (grilles, louvres and diffusers) used to distribute the air are arguably most crucial.
By specifying inappropriate terminal devices for an air distribution system, you run the risk of condemning the entire system to poor performance, resulting in energy inefficiency, high costs, compromised comfort, and unhappy occupants.
An analogy might be helpful here – in a hi-fi system, no matter how good the amplifier and CD player are, the quality of the sound will be compromised if the speakers are of poor quality.
Specified well, high performance terminal devices will ensure the conditioned supply air will mix homogenously with the room air to minimise temperature gradients and achieve better comfort levels.
The primary requirement of an air terminal device is simply to introduce air into a space without causing draughts. But, although they do a straightforward job, they are far more than mere ancillaries.
Indeed, diffusers have a particular significance because they are the air distribution system’s customer interface; they are the product that affects people’s perception of the installation as a whole, because they are often the only part of the system that people feel, hear and potentially see. However, ambience, acoustics and aesthetics are not the only considerations when it comes to diffusers.
Draught prevention in action
Potential draught problems were mitigated at Öschberghof, an impressive 126-room luxury hotel complex on the edge of the Black Forest in Germany by fitting INDUL linear diffusers from Kiefer, and supplied in the UK by Klima-Therm.
These were installed throughout the resort, including the Esszimmer and Ösch Noir restaurants, and in the hotel’s lobby area.
The Esszimmer restaurant features a particularly high gabled roof and there were concerns that this would result in uneven ventilation in the restaurant. To ensure that this didn’t happen, the diffusers are arranged in an unusual configuration as horizontal line elements within the pitched roof.
Fitting the linear diffusers at different heights enables a consistent flow pattern and uniform ventilation throughout the restaurant. Track lighting between the diffusers continues the linear look to create a harmonious and aesthetically pleasing visual impression.
