Designing for a higher IAQ

When designing services for a learning environment it is essential to ensure good air quality. Lars Fabricius of SAV Systems explains how school-specific decentralised air handling units with heat recovery provide an ideal solution.

As classrooms become better insulated with improved air-tightness, it becomes increasingly important that the ventilation design maintains acceptable indoor air quality (IAQ) without energy wastage. Neither of the main traditional options – central air handling units with distribution ductwork or the opening of windows – provides a wholly satisfactory solution.

A traditional centralised ventilation system needs to be planned from the early design phase, with a specification based on the assumption that all of the spaces will be fully occupied for all of the time. The reality, though, is that occupancy of teaching spaces varies considerably through the course of the day. In many schools where the facilities are also used by the local community, this variation in occupancy may continue through to the evening. This can result in the continuous ventilation of unoccupied or partially occupied spaces, thereby wasting energy.

Nor is this challenge confined to new schools as refurbishment programmes often include measures to improve air-tightness through improvements to the building fabric. With less reliance on the ‘leakiness’ of the buildings for ventilation, other measures will be required to maintain good IAQ. Retrofitting additional ventilation to an existing centralised system can be expensive and disruptive.

There are also issues with opening windows for ventilation, not least because it offers no opportunity to recover heat from the outgoing air and is thus potentially wasteful. Additionally, school staff tend to open windows in response to rising temperatures rather than because of any feeling of stuffiness (the classic indicator of rising CO2 levels). Field trials by the International Centre for Indoor Environment and Energy have found that deterioration in air quality has a more profound effect on cognitive performance than is the case for rising indoor temperatures.

Furthermore, many educational organisations discourage the opening of windows because of concerns about security, air pollution and noise ingress.

Look at alternatives

An alternative to these traditional options for controlling IAQ is to use low noise mechanical ventilation units with heat recovery (MVHR), individually mounted in each space with direct connections to the outside through the wall or roof. Such units are easily retrofitted and respond rapidly to changes in IAQ so that the rate of ventilation is aligned to the needs of the space (demand controlled ventilation). A combi design can also be employed to ventilate two rooms with a single unit, though this reduces the precision of the demand control.

The units can be configured to modify their ventilation rate in relation to the key IAQ parameters of CO2 concentration or relative humidity. Control can also be arranged via PIR movement sensors, simple timers or through an interface with a building management system (BMS).

Decentralised units use high efficiency counter-flow aluminium heat exchangers that comply with the EN308 standard. They recover at least 84% on dry bulb (or 91% at 80% relative humidity) of the heat energy contained in the outgoing air. An integral data logger constantly measures the performance of the unit.

Further energy savings are achieved by the use of variable speed EC motor technology to minimise the energy consumed by the ventilation drives (specific fan power between 0.7 and 1.2W/ltr/sec at maximum airflow).

Key advantages

A key advantage of these MVHR units in educational applications is their very low noise levels. Typical sound pressure levels 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, and is especially valuable for designers of facilities for children with Special Educational Needs.

As standard, decentralised units use F5 Class filters (in compliance with EN 13779) on both intake and exhaust pathways, protecting against external air pollution. They also prevent any build-up of deposits on the heat exchanger surfaces, which means that heat transfer efficiency and low fan power consumption can be safeguarded for the long term. Where required, the units can be supplied with filters up to Class F7.

A decentralised system can be extended at any time simply by adding additional units. Control can be maintained by one control panel using a master/slave arrangement with up to 20 units in a cluster.

How it works

Local air handling units are mounted discreetly at ceiling level with connections to the outside through either the wall or roof. Where false ceilings are used, each unit can be semi-recessed so as to minimise visual impact. In the event of pitched roofs being used to maximise daylight entry, units can be mounted at the higher elevation and still remain effective.

Fresh air is distributed across the ceiling using the Coanda effect (whereby the air ‘hugs’ the ceiling) so that it spreads along the room and falls gently into the space without causing draughts. Stale, warm air is drawn into the unit and passes through the heat exchanger before being exhausted to the outside. The proportion of heat recovered is automatically adjusted to stay as close as possible to the desired value of room temperature. In more spacious areas such as atria larger floor mounted models with ducted extraction at high level can be used.

The MVHR units can also offset the effect of high temperatures in summer by using cooler night-time air to reduce the temperature of the building fabric. For areas with above average heat gains, the decentralised units can be fitted with additional cooling modules.

While this concept of zone-specific air handling is relatively new to the UK, it has been in use for many years in Scandinavian countries, where building tightness regulations were introduced over a decade ago.

The availability of such MVHR units in the UK comes at a time when the education sector needs to increase its energy efficiency and become more responsive to change. Thus the use of responsive building services systems is clearly a sensible way forward.

Effects of CO2 levels on student performance

Studies carried out by the University of Exeter have shown that CO2 levels exceeding 1,500 ppm can lead to a 5% reduction in Power of Attention measures. CO2 levels in naturally ventilated classrooms frequently exceed 2,000 ppm.

Reducing CO2 levels with adequate ventilation improves the performance of students and reduces the risk of accidents in practical subjects. Studies in Denmark have shown there is an improvement in school performance when classroom ventilation is increased from around 3 l/s/person to 8.5 l/s/person (Wargocki & Wyon 2007).