This CPD aims to provide a closer understanding of the use of carbon dioxide monitoring for ventilation control in education buildings. This module is sponsored by Medem.
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Module 6 deadline: July 29 2011.
This module covers current legislation regarding ventilation and air quality in education buildings. It will present methods of ensuring good fresh air while minimising heat loss, as well as the importance of keeping CO2 levels low in teaching areas.
Standards that improve the airtightness of buildings and reduce air leakage can lead to a build-up of carbon dioxide in classrooms, particularly as classrooms are often more densely populated than offices and have lower levels of ventilation.
Ventilation systems incorporating carbon dioxide monitors can provide optimum indoor air quality, while minimising heat loss. The monitoring of carbon dioxide levels to adjust and control heating and air conditioning ventilation rates to suit the occupancy level of a building is now a tried-and-tested method of minimising a building’s energy use.
In the absence of any major pollutants, the level of carbon dioxide is also recognised as a key indicator of general indoor air quality.
Carbon dioxide in buildings
Carbon dioxide (CO2) is a colourless, odourless gas. Outdoor air will typically contain between 350-400 parts per million (ppm) of CO2, or 0.04%. CO2 is produced when any carbon-based material (coal, oil, wood etc) is burned, and through human, animal and plant respiration.
Major contributors to CO2 levels in the air are exhaust fumes from vehicles, industrial equipment and burning fuel for power.
CO2 is not generally found at hazardous levels indoors, but it is often measured when trying to determine the indoor air quality of a building as it can provide an indication of the number of occupants. If CO2 levels are high, it is assumed that there may not be adequate ventilation, which may allow other environmental contaminants to build up.
CIBSE recommends a CO2 concentration of no more than 900ppm to control human odours and maintain comfort. This is based not on the impact of carbon dioxide itself on health or comfort but on the association of elevated CO2 concentrations with unaccept-able levels of body odour.
In some industrial settings, much higher levels are found. Exposure to CO2 levels of 10,000-15,000ppm can cause changes in breathing patterns. Occupants may experience health effects at much lower concentrations, but this is likely to be attributed to other contaminants in the air that build up as a result of insufficient ventilation.
A CO2 level of 1000ppm is equivalent to 1.8g/m3.
Carbon dioxide in schools
When an adult, or teenage student, exhales, the proportion of CO2 in their breath is 35,000-50,000ppm, which equates to about 0.01 grams per second (g/s), or 0.005 litres per second (l/s). Infants and primary school children will have lower emission rates, but as they are likely to be more active in the classroom, their overall CO2 production may be at a similar level.
In classrooms, CO2 levels well in excess of 400ppm have frequently been measured.
A study carried out in a number of UK schools indicated CO2 levels above 4000ppm with ventilation rates of less than 0.5l/s per person in some classrooms (Source: DA Coley & R Greeves, Report R102 for DfES, The Effect of Low Ventila-tion Rates on the Cognitive Function of a Primary School Class, 2004.) In a report carried out for the Department for Education, higher levels of CO2 (above 1000ppm) were seen to reduce the attention span of students, with possible negative effects on learning ability.
The requirements for ventilation in schools are laid out by the Building Regulations Part F (October 2010), supported by DfE Building Bulletin 101 – Ventilation of School Buildings. It states that ventilation should be provided to limit the concentration of carbon dioxide in all teaching and learning spaces. Measured at seated head height, during the continuous period between the start and finish of teaching on any day, the average concen-tration of CO2 should not exceed 1500ppm. This is a limiting value and it requires that at any occupied time, the occupants should have the ability to lower the concentration to 1000ppm by extra ventilation. CIBSE TM40: Health Issues in Building Services also recommends a lower limiting value of approximately 1000ppm.
Building Bulletin 101 quotes a study in a school where CO2 levels in classrooms typically rose from the start of each day, reached a peak before lunch at about 12.30pm, and then decreased over the lunch period when the classroom was empty. After lunch when the classroom was occupied, CO2 levels again increased, reaching a peak at the end of the school day at 3.30pm.
Ventilation methods and strategies
CIBSE recommends that, assuming an outdoor CO2 level of 400ppm, the fresh air supply rate should not fall below 8l/s per adult occupant to maintain an indoor level of 1000ppm. Building Bulletin 101 gives a minimum ventilation rate of 3l/s per person, a minimum daily average of 5l/s per person with a capability of 8l/s minimum per person at any occupied time.
In many cases, it may be expedient to provide increased levels of outdoor air to provide “free” cooling into the space. The amount of outdoor air supplied to a room for the purposes of CO2 dilution may be far less than is required for good air movement and for heating or cooling purposes – in this case, where mechanical ventilation is used, opportunities for recirculation air systems should be explored.
Systems based on ventilation standards such as CIBSE Guides A and B provide greater ventilation assuming continuous occupancy. This can mean unnecessary energy consumption, particularly in education buildings where occupancy in classrooms varies throughout the day.
The principal options are naturally ventilated, mechanically ventilated or mixed-mode ventilation. Whichever method is used, it is possible that the metabolic thermal gains from high occupancy in a classroom (ie from body heat) will compensate for any thermal loss, even during winter.
It is therefore important that the system has the flexibility to benefit from casual gains in winter, remove the heat load in summer and maintain an appropriate air quality. Naturally ventilated schools are reasonably common in the UK but pressures on space and location will often mean some mechanical ventilation is required.
The wide deployment of advanced building modelling tools has enabled designers to meet performance standards while exploring the use of automatic natural ventilation systems that employ CO2 sensing.
Mixed-mode systems are often specified to minimise the cost-in-use of mechanical ventilation. These systems use natural airflow through the building wherever possible, but can be fitted with additional mechanical systems to increase the ventilation rate when and where needed.
By incorporating CO2 monitoring, the ventilation rate in each room can be designed to be dependent on its occupancy level, to ensure the correct number of air changes. The CO2 monitor controls the operation of the fans to keep classroom levels below 1500ppm.
By analysing likely scenarios in the occupied space, systems can be evaluated based in terms of their total cost and total carbon emissions, while ensuring that they maintain acceptable air quality. Where demand-led ventilation systems are used, heat recovery within the mechanical extract system may cease to be cost effective. However, it is essential to remember that many indoor air pollutants are generated independent of occupancy, including volatile organic compounds from carpets and stored materials. Where pollutant sources are indepen-dent of occupancy, CO2 may not be a suitable means to evaluate the quality of the indoor air.
In schools, higher ventilation rates will be required in rooms where pollutants are likely, such as those used for science or technology lessons.
A Bunsen burner, for example, produces about double the amount of carbon dioxide of a student or teacher. It is now common practice to install gas pressure proving systems in school laboratories, and these systems are now available with integrated carbon dioxide monitoring to control the ventilation rate.
Chris Dearden is Managing Director of Medem (UK) Ltd.
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MODULE 7 DEADLINE: July 29 2011
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