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A textbook case of using zoned AHUs

Maintaining good indoor air quality in classrooms without wasting energy can be a major challenge. Lars Fabricius explains why zone-specific, decentralised air handling units with heat recovery offer a useful way forward
2005 of eight primary schools which were using open windows for ventilation, Building Research Establishment (BRE) findings revealed that, in winter, more than half the fresh air rates were below the minimum required.

As insulation and air-tightness of school buildings improve, maintaining acceptable indoor air quality (IAQ) can become problematical. Traditional options for managing IAQ have included simply opening the windows or installing a centralised air handling system. However, for reasons explained below, neither of these is entirely satisfactory.

Another option, quite new to the UK but widely used in Northern Europe, is to mount a low noise air handling unit with heat recovery in each classroom, with direct connections to the outside through the wall or roof. Such units are able to respond to key IAQ parameters in each space, providing true localised Demand Controlled Ventilation without wasting energy.

First, it's worth reviewing the traditional approaches in a little more detail. A traditional ductwork system needs to be incorporated in the early design phase of construction, often with a 'play safe' specification that assumes all spaces will be fully occupied throughout the day. In reality, occupancy varies considerably during the school day and with after-school activities. All too often this results in over-ventilation and wasted energy.

Furthermore, when existing school buildings are refurbished, improvements to building fabric often create a requirement for mechanical ventilation that didn't exist originally. Retrofitting a centralised ventilation system with associated ductwork can be expensive and technically challenging.

In contrast, decentralised air handling units are easily retrofitted to existing classrooms and are located just where they are needed. The system can be extended at any time by simply adding additional units, while retaining central control of multiple units through a PC.

Opening the windows, another traditional option for managing IAQ, is wasteful in energy terms as there is no opportunity for heat recovery. People tend to open windows in response to rising temperatures, rather than because of any fall-off in IAQ. This is a major concern as field trials by the International Centre for Indoor Environment and Energy (2009) have found that deterioration in air quality - and increased CO2 concentrations in particular - has a more profound effect on cognitive performance than does rising indoor temperature.

Quite apart from any IAQ concerns, many schools discourage the opening of windows because of security issues, air pollution, noise ingress and risk of draughts in winter.

Quick response to occupancy
An air handling unit mounted in a classroom, linked to appropriate sensors (typically CO2 or relative humidity) will adjust ventilation rates to maintain acceptable IAQ. As a result, the unit is able to respond quickly to variable occupancy. This feature may well become increasingly important, as future editions of the Building Regulations are likely to require the use of Demand Controlled Ventilation.

The units are fitted with F5 Class filters on both intake and exhaust pathways. These safeguard IAQ against air pollution from outside, while also preventing deposits from building up on the heat exchanger surfaces, which would reduce heat transfer efficiency and increase fan power consumption. F5 Class filtration complies with the requirements of EN 13779.

Crucially, the units recently introduced to the UK use high efficiency counter-flow aluminium heat exchangers to ensure heat recovery of up to 91% at 80% relative humidity. An integral data logger allows the performance of each unit to be closely monitored.

These units have very low noise levels. Typical noise emissions at 1m are only 35 dB(A) at 100% power and 30 dB(A) at 80% power. This complies with the recommended indoor ambient noise levels for classrooms and general teaching areas. The use of variable speed EC motor technology minimises the energy consumed by the ventilation fans, holding specific fan power to 0.7 W/ltr/sec for the type of unit usually specified for classrooms.

Typically, in a teaching area, local air handling units are discreetly mounted close to the ceiling with connections to the outside through the wall or roof. With suspended ceilings, the units can be recessed to 1/3, 2/3 or full depth in the ceiling void. Where pitched ceilings have been used to maximise daylight entry, units can be mounted close to the top and still provide good air distribution.

Fresh air is moved across the ceiling using the Coanda effect (whereby the flow stays close to a surface) so that it spreads across the space and falls gently into the back of the room without causing draughts. Stale, warm air is drawn into the unit and passes through the heat exchanger, enabling energy to be transferred to the incoming air. Air flows are automatically adjusted to maintain a comfortable temperature in the room. Where required, the units can also be fitted with supplementary cooling.

The arrival of such units comes at a time when schools need to increase their energy efficiency without compromising IAQ. As more schools make their facilities available to local communities, patterns of building usage will be subject to change and occupancy levels will become ever more variable. Ventilation systems that can adapt to such variations offer very significant contributions to energy efficiency, ease of use and sustained good indoor air quality.

// The author is managing director of SAV Systems //
17 April 2013


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