A breath of fresh air
How many buildings are using large amounts of energy to maintain indoor air quality when spaces are empty or only lightly occupied? Chris Dearden explains how carbon-dioxide monitoring systems provide an energy-saving and cost-effective approach to controlling ventilation and indoor air quality in buildings.
Businesses and educational establishments are increasingly concerned over the level of carbon dioxide in indoor air. Although generally not found at hazardous levels indoors, carbon dioxide is often measured when trying to assess indoor air quality. If the levels of CO2 are too high, the ventilation may not be adequate and this may allow the build up of other environmental pollutants such as volatile organic compounds.
With new standards and legislation emerging from Government departments such as the Health & Safety Executive (HSE) and the Department of Education & Skills (DES), there is a need for design solutions for schools that are not only reliable, easy to use and install, but also cost-effective and energy-saving.
Generally the main indoor source of carbon dioxide is people. Carbon dioxide is a product of metabolism, so the higher the room occupancy, the higher the levels of carbon dioxide can become. Unusually high levels of carbon dioxide indoors can affect a person’s performance and wellbeing. Occupants are likely to grow drowsy, get headaches, suffer a loss of concentration and function at lower ability and activity levels — though in the case of young people, research has shown that levels do not have to be much above ambient levels to affect concentration.
The education sector has been one of the first to respond to the need to find an effective means of improving air quality. The DES Building Bulletin 101 is a second-tier document to the Building Regulations and so is a mandatory document. It stipulates that the levels of carbon dioxide in all teaching and learning spaces, when measured at seated head height and averaged over the whole school day should not exceed 1500 parts per million (ppm).
Since the introduction of air-tightness standards for new buildings to reduce air leakage around doors and windows and reduce energy consumption, there has been a rise in carbon dioxide in classrooms. The primary method used by most buildings to reduce carbon dioxide is ventilation. To comply with the Building Regulations, many buildings use ventilation systems that often operate at a fixed rate based on an assumed number of occupants. However, if occupancy level is likely to vary throughout the day, there is often much more ventilation than is actually needed, resulting in unnecessary energy consumption.
Medem consulted with the DES, design engineers and local authorities to identify an efficient ventilation strategy that would not only improve indoor air quality but simultaneously bring energy- and cost-effective results. The most effective solution was found to be carbon-dioxide monitoring systems that adjust and control heating and ventilation rates, depending on the occupancy level of the building, to ensure the correct amount of air changes. Carbon-dioxide monitors based on ventilation standards are now a tried-and-tested method of minimising a building’s energy usage.
The principal options are naturally ventilated, mechanically ventilated or mixed-mode ventilation.
The most energy efficient is the mixed-mode system, which uses natural airflow throughout the building where possible. However, some classrooms need more ventilation than others — food technology rooms, for example, as well as other technology rooms where pollutants are likely, such as from brazing hearths and woodworking machines, and science laboratories, where equipment such as Bunsen burners can significantly increase carbon-dioxide levels. In these classrooms, additional mechanical ventilation systems can be added to produce a higher ventilation rate to keep carbon-dioxide levels within the appropriate limits.
A ‘stand-alone’ monitoring system can advise teaching staff when carbon-dioxide levels are rising. The teacher can then adjust the ventilation by opening windows or switching on fans to increase ventilation depending on the carbon-dioxide levels and temperature.
A more sophisticated system not only monitors carbon-dioxide levels but then automatically controls the ventilation by means of, for example, stepped-motor fans — a method known as on-demand ventilation. The higher the occupancy of the room, the higher the ventilation rate needs to be to keep carbon-dioxide level within the required concentration bracket. Net heat loss is minimised because of the metabolic gains due to the higher room occupancy. This gain is surprisingly high and so helps to offset the heat lost through the increased ventilation in a fully occupied room. As the room occupancy drops, so does the level of ventilation required, resulting in energy and costs savings.
In the case of an educational establishment or office, extracting air can remove artificially heated air. On-demand variable ventilation minimises the removal of air that may have been heated by a gas boiler or electric heaters, again producing energy and costs savings.
We have focused here on educational establishments, but the importance of keeping carbon-dioxide levels within the appropriate parameters is no less important in the workplace. The HSE (Catering Sheet 23 Revision One) sets a maximum level of carbon dioxide at 2800 ppm for commercial kitchens. Again on-demand variable ventilation can be used to minimise energy consumption whilst at the same time ensuring adequate air change. This being particularly important because of the presence in the air of polycyclic aromatic hydrocarbons (carcinogens) where cooking with oil is carried out with inadequate ventilation.
For people to function safely and at their maximum potential, it is essential that the carbon dioxide in the air is monitored. Knowing that these systems also bring long-term energy-saving and cost-effective benefits, making them an attractive proposition as well as a necessary course of action.
Chris Dearden is a director of Medem