SCHOOL BUILDINGS VENTILATION STRATEGIES
design template for schools Building Bulletin 101. The same study showed that mechanically ventilated international schools achieved a median of 8.5 l/s. ‘A temperature-based control strategy is
not adequate for natural ventilation,’ said Jones. ‘Lots of companies are claiming that louvre-based, roof-mounted systems will deliver the desired result, but they can’t claim that unless they can give the design engineer specific figures about pressure loss coefficients.’ To get this required level of accuracy into
The City Academy, east London (see previous page)
their designs, engineers need to carry out detailed computer modelling before putting the system together, according to Malcolm Cook of Loughborough University. Factors such as the orientation of the building, thermal mass, shading and the size of the openings in the building fabric need to be fed accurately into the model to get a clear picture of how natural ventilation will perform. He also called for the ‘three Cs’ to be employed on every project: client, commissioning and control. The conference discussed the potential contradiction between the government’s enthusiasm for natural ventilation to meet energy and carbon reduction targets, and teachers’ worries about the impact of background noise, poor- quality outside air, and security implications of open windows or louvre systems.
Overheating ‘The fact is, schools have always been naturally ventilated and passively cooled – by using their windows,’ said Martin Liddament, managing director of Veetech. ‘Overheating was minimal in the past. If you seal up your building and use filtration, you will have difficulty getting chemicals out.’ He agreed that noise was a problem –
urban levels are around 60-70dBa, which will disrupt teaching – but Liddament said the use of sound barriers and acoustic vents would overcome this problem. John Palmer of AECOM added that recent tests carried out by the Department for Communities and Local Government (DCLG) showed that the perception of noise did not always match reality. ‘It is an operational fact that schools have windows, but don’t necessarily use them.
26 CIBSE Journal January 2012
Most of those tested said it was because it was too noisy, but the tests showed outside noise was not as bad as they assumed.’ Buro Happold’s Mike Entwisle said the
BSF programme had been helpful in giving the industry experience of these issues and allowing engineers to establish ‘what works and what doesn’t’. ‘The less money you have, the more you need good design,’ he told the conference. ‘Classrooms are now filled with students and computers, but can still be ventilated naturally – but it is vital that we get our calculations right.’ The use of weather data and design
School facilities managers are hard to find, so we need to intervene by doing basic systems well but not increasing complexity
models based on previous experience should allow engineers to design a classroom that would rarely rise above 25C, according to Entwisle. Night-time cooling is an excellent strategy for schools, he believes, but site managers are often reluctant to leave windows open at night because of security fears. There are a number of ways round this, he said, including the use of louvres and only using windows at high level to provide the night cooling. ‘Simple solutions
are best,’ added Entwisle. ‘If schools weren’t so densely occupied there would be a case for
mixed-mode ventilation with heat recovery, but it doesn’t always work and schools are risk averse.’ However, simplicity is not what has been
delivered in many cases. Schools have suffered from design-and-build teams piling on complexity, according to Rod Bunn of BSRIA. He outlined the results of several post-occupancy surveys of school buildings, and condemned the control strategies employed in most. ‘Buildings are getting too complex,’ he
said. ‘The ventilation strategy is usually left to a specialist ventilation provider at the tail end of the contract chain, so the design team has lost control of the detail. Design and build is the norm, and means that the designer can’t talk to the client and can’t involve the occupants in the design.’
Complexity As a result, according to Bunn, teachers are occupying buildings with little or no idea how they work or how to operate the ventilation systems in their classrooms. He said, in many cases, there were too many layers of bespoke control packages,
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