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On-demand ventilation, coupled with high efficiency energy recovery successfully manages air quality as well as reducing a school's carbon footprint and energy costs, says David Cook
With schools accounting for 2 per cent of UK greenhouse gas emissions and 15 per cent of the country's public sector emissions, the Government is keen to cut their carbon footprints. The challenge is to create a healthy learning environment, facilitated by excellent indoor air quality where problems with moisture, CO2 and external fume are eliminated.

Building Bulletin 101 specifies limiting CO2 levels within teaching and learning spaces to 1,500 parts per million. However, fresh air supply rates per person in schools are often so low that CO2 levels are well above this recommended level, leading to adverse health effects and impacting on the learning performance of pupils.

In 2004, research from the University of Exeter on the effect of low ventilation rates on the cognitive function, concluded that in a classroom with high CO2 levels students are likely 'to be less attentive and concentrate less well on what the teacher is saying which may possibly over time lead to detrimental effects on learning and educational attainment.'

This was followed by research a few years ago from Reading University and University College London revealing that new energy efficient schools were being designed to be more airtight to reduce heat loss, but as a result the schools had appalling ventilation rates with CO2 levels exceeding targets. This research followed the Commission for Architecture and the Built Environment's (CABE) worrying findings that drew specific attention to HVAC issues with classrooms as part of its detailed review of 40 proposed designs for schools under the former Building Schools for the Future initiative.

So how can contractors help schools achieve a balance between reducing their carbon footprints and maintaining good ventilation? Research undertaken in a sample of school projects points to schools that have on-demand mechanical ventilation installed as showing the lowest energy consumption while providing good indoor air quality.

This demonstrates how on-demand ventilation both improves indoor air quality in schools and is a solution to the energy/ventilation conundrum. With traditional fixed volume ventilation the system is either on or off irrespective of the number of people in the room. Here, you run the risk of creating a mediocre and unproductive classroom environment through over or under ventilation with a resulting waste of energy.

Instead, demand ventilation works by responding to the exact ventilation demands of a room, supplying or extracting air only when and to the level it is required. The system is activated according to sophisticated control and sensing options. A range of sensors, such as CO2, PIR occupancy detection, humidity or temperature are employed to determine the room air quality state and adjust the ventilation requirements automatically in the classroom, managing the system's ventilation rates accordingly. The sensors communicate with the main unit which, in turn, drives the fan to the required speed to deliver the airflow and respond exactly to classroom conditions.

Hierarchy of control
Sensors can be combined to generate a hierarchy of control for the ventilation system, which can also be easily linked in to a building management system for full control and monitoring, if required. Therefore, only the energy that is needed to ventilate is actually used. Demand ventilation control also limits summer gains and winter losses as only the required ventilation is delivered based on occupancy and indoor air quality need, reducing summer overheat and heating losses in winter.

However, on-demand ventilation is just the first step to reducing school carbon emissions. As we move towards zero carbon buildings the energy efficiency improvements needed to meet these carbon goals will result in buildings becoming increasingly air tight. As a result newer ventilation technologies, such as mechanical ventilation with energy recovery, will grow in popularity as a method to increase efficiency by recovering heat/cool while substantially lowering the carbon footprint.

The latest on-demand ventilation systems provide especially impressive energy savings because they integrate 90 per cent energy recovery into the ventilation system. This type of system extracts the energy from the warm, stale air taken from classroom areas before it is exhausted to outside whilst fresh, incoming air is preheated via the high efficiency plate heat exchanger and supplied into the classroom. The energy recovery process is proven to use up to 94 per cent of the heat energy which would otherwise be wasted to outside. The result is a massive further boost to the energy performance credentials of the ventilation system especially in larger school buildings.

This type of system also offers schools further energy saving potential. To overcome the increasing problem of thermally efficient school buildings overheating D-ERV systems are available with automatic 100 per cent summer bypass. When temperatures rise the automatic summer bypass closes off the airflow from the heat exchanger while simultaneously opening a bypass which the air flows through, thus avoiding overheating. This also allows the building to take advantage of any free cooling available when the ambient temperature is below the room design condition, something found particularly during the spring and autumn seasons.

Since an on-demand system is automatically controlled, it also eliminates the need for manual intervention by teachers. At the same time, it improves comfort, keeping CO2 levels within prescribed regulatory limits for school classrooms, as defined by Building Bulletin 101.

At a time when many schools are feeling pressure to cut their carbon emissions but also improve indoor air quality, contractors can offer them the solution - on-demand ventilation can bring significant advantages creating a productive and energy efficient academic environment.

// The author is product marketing manager - non-residential at Vent-Axia //
17 April 2013

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