Delivering good indoor air quality and reducing carbon footprint
Published: 08 March, 2010
With schools under Government pressure to reduce their carbon footprints, but not at the expense of good indoor air quality, Nygel Humphrey explains how on-demand ventilation, coupled with high efficiency, energy recovery, successfully manages air quality — as well as reducing carbon footprints and energy costs.
With schools accounting for 2% of UK greenhouse-gas emissions and 15% of public-sector emissions, the Government is keen to cut their carbon footprints. However, caution needs to be taken to ensure emissions are not reduced at the expense of good ventilation. The schools sector is undergoing the ambitious construction programme, Building Schools for the Future (BSF), which aims to see almost every school in England rebuilt or refurbished by 2020. The BSF programme offers an ideal opportunity to construct low- to zero-carbon schools, but the challenge is to create a healthy learning environment, facilitated by excellent indoor air quality where problems with moisture, CO2 and external fumes are eliminated.
Building Bulletin 101 provides the regulatory framework in support of the Building Regulations for the adequate provision of ventilation in schools and specifies limiting CO2 levels within teaching and learning spaces to 1500 ppm. However, so little fresh air is often supplied that CO2 levels are well above this recommendation, leading to adverse health effects and also 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’.
Just over a year ago research from Reading University and University College London revealed 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 & 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 BSF initiative.
So how can consultants help schools achieve a balance between reducing their carbon footprints and maintaining good ventilation? Recent research undertaken in a sample of recent school projects points to schools with on-demand mechanical ventilation showing the lowest energy consumption while providing good indoor air quality. On-demand ventilation therefore 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 too much or too little ventilation, with a resulting waste of energy.
Demand ventilation responds 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 quality of the air in a classroom and adjust ventilation automatically.
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. Sensors can be combined to generate a hierarchy of control for the ventilation system; they 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.
However, on-demand ventilation is just the first step to reducing school carbon emissions. As we move towards zero-carbon buildings the improve-ments needed to meet these carbon goals will result in buildings becoming increasingly airtight. As a result, newer ventilation technologies, such as mechanical ventilation with energy recovery, will increase in popularity to increase efficiency by recovering heat/coolth and substantially reducing carbon footprint.
The latest on-demand ventilation systems (such as Sentinel Totus, which also has energy recovery) provide especially impressive energy savings because they integrate 90% energy recovery into the ventilation system. The energy-recovery process is proven to utilise up to 94% 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. An automatic summer bypass will take advantage of any free cooling available when the ambient temperature is below the room design condition, particularly in Spring and Autumn. The Totus system also incorporates interlocks for associated heating and cooling equipment installed to optimise energy recovery potential and eliminate any possibilities of systems conflict. There is an installer-settable night-time purge facility to reduce Summer start-up temperature and loads and help reduce over heat in summer from spaces that are not air conditioned.
Since an on-demand system is automatically controlled, there is no need for manual intervention by the 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 under pressure to cut carbon emissions and improve indoor air quality, consultants can offer them the solution — on-demand ventilation can bring significant advantages, creating a productive and energy efficient academic environment. For more information, visit Vent-Axia's web site.
Nygel Humphrey is product marketing manager — non-residential with Vent-Axia.
The benefits of combining on-demand ventilation with energy recovery*
• Up to 78% reduction in power consumption versus AC fixed ventilation.
• Up to10 440 kWh per year in a 20-classroom school.
• Up to 5600 kg per year carbon saving in a 20-classroom school.
• Reduction in ventilation heat losses in Winter by up to 7.93 kW in a classroom during maximum winter design;
• Reducing ventilation gains in Summer by up to 2.1 kW in a classroom during maximum summer design conditions.
Further energy and carbon savings are then made through the improvement of building performance through integration with associated heating/cooling systems.
* This analysis is based on Vent-Axia’s Sentinel Totus.