Meeting the standards for ventilation in schools
New ventilation regulations for schools are designed to ensure comfortable temperatures with acceptable indoor air quality. Jonathon Hunter Hill considers the issues and the challenges they pose. The latest version of Building Bulletin 101 (BB101) ‘Guidelines on ventilation, thermal comfort and indoor air quality in schools’ demands better control of draughts, temperature and carbon dioxide (CO2) levels – as well as reiterating the noise criteria of the previous version. And, of course, energy efficiency is also an important consideration.
In doing so, it lays out new standards for the design and performance criteria of ventilation systems in schools, including acceptable levels of carbon dioxide (CO2). This is particularly important as many studies have shown that excessive CO2 levels can have a detrimental effect on children’s learning performance.
However, there is also an anomaly in the BB101 criteria, insofar as it allows higher levels of CO2 when natural ventilation systems are in use, compared to mechanical ventilation systems. Given the importance of maintaining low CO2 levels, I would argue that the standards should be the same, irrespective of the type of equipment – BB101 should be focused on the result, not how it is achieved!
This article aims to discuss this issue further, as well as expanding on the importance of using ventilation systems capable of controlling CO2 levels and temperature independently of each other – something that is proving difficult with traditional system designs.
CO2 and types of ventilation system
To set the scene, mechanical ventilation systems use a fan to move air into and out of buildings. Natural ventilation systems depend on wind or buoyancy to achieve the required air movement. There are also so-called hybrid ventilation systems that combine or switch between natural and mechanical modes, with their main operational mode dictating into which category they fall.
For mechanical ventilation systems, BB101 requires that the daily average CO₂ level be kept below 1,000 parts per million (ppm), with a temperature differential (ΔT) between the air in and room and the air entering the room from outside of between 1.5°C and 4°C (depending on the room type).
These levels stipulated for mechanical ventilation systems are broadly in line with those that other countries require, irrespective of the type of system used. For example, Norway and France both have 1,000 ppm limits on classroom CO₂ levels, whilst Denmark and France limit classroom CO₂ levels to 900 ppm.
For natural ventilation systems, however, BB101 requires a daily average CO₂ level of below 1,500 ppm, with a ΔT of 5°C for all room types. Thus, for some reason, the bar has been lowered for natural ventilation systems, compared to mechanical systems. I would suggest this reason is that most natural ventilation systems are unable to comply with the 1,000 ppm requirement throughout the course of the year.
It’s also worth noting that the average CO₂ levels can each be exceeded by 500 ppm for no more than 20 consecutive minutes a day for both system types.
Pollution and filtration
As well as the need to control indoor pollutants such as CO2, there is growing concern about the impact of poor outdoor air quality on the indoor air quality (IAQ) within school buildings. A recent study of London schools by University College London and the University of Cambridge found that a significant proportion of indoor air pollution is due to outdoor air pollution – sometimes resulting in illegally high levels of pollutants indoors.
In city and town centres, at least, this means incoming air must be filtered to higher levels than has traditionally been the case; ideally to ensure compliance with the new (July 2018) ISO 16890 standard.
Filtration of both incoming and outgoing air is also necessary to protect the heat exchangers from pollutants, thereby maintaining optimum heat exchange and reducing maintenance requirements.
Natural ventilation systems typically lack the fan power needed to drive the air through the filters, thus limiting the filtration opportunities. For this reason, I suggest that natural ventilation systems should not even be considered for schools in city and town centres. However, BB101 does not make this compulsory.
Controlling comfort as well as IAQ
The control of indoor air quality (IAQ) must be considered in the context of the other parameters that need to be controlled at the same time to comply with BB101. For example, there are also issues with natural ventilation systems in relation to their ability to maintain acceptable temperatures while avoiding uncomfortable draughts. In fact, failing to control temperature adequately may result in draughts that are unacceptable in the new BB101.
A draught can be simply described as unwanted local cooling and a critical element here is the difference between the air temperature in the classroom and the temperature of the outside air that is being drawn into the room for ventilation (supply air). The acceptable supply air temperature for any room type is typically 16°C (assuming a room temperature of 21°C).
The velocity of the air is also a contributory factor in draught control and EN 7730:2005 Ergonomics of the Thermal Environment shows that air velocity should be kept within a range of approximately 0.12 m/s to 0.25 m/s within a temperature range of 19°C to 27°C to avoid draught.
BB101 contradicts EN7730:2005 for the purposes of natural ventilation, where supply air temperature is allowed to fall below 19°C and velocity is not a considered factor.
Taking full control
This interplay of temperature and air velocity is critical to the ability of ventilation systems to deliver full compliance with BB101’s criteria for IAQ, temperature and draughts.
For instance, when outdoor air temperatures are reasonably high, natural ventilation systems can operate at high flow rates of supply air to dilute CO2 levels, with a limited requirement to raise the temperature of the outdoor air before it enters the room.
However, in winter this situation changes as outdoor air temperatures are typically much lower than 16°C, while the indoor temperature is still at 21°C. In such cases, many natural or hybrid ventilation systems reduce the flow rates of the supply air, so that less cold outside air is being introduced to the classroom. As a result, there is less dilution of the stale air in the space so there is a danger of CO2 levels rising to unacceptable levels.
This approach, therefore, prioritises thermal comfort over indoor air quality, compromising the latter by allowing CO2 levels to rise.
This issue was clearly illustrated at a new school in Scotland, which was initially designed to use a ventilation system that mixes stale warm air with incoming air so that the temperature of the incoming air is increased slightly.
However, computational fluid dynamics modelling showed that with an outdoor temperature of -2°C (not uncommon in Scotland) such a system would not provide BB101-compliant control of both draughts and CO2 levels at the same time. Meeting the requirements of one would inevitably lead to compromising the other under such conditions.
The key point here is that if there is an increase in CO2 levels, this can lead to a demand for higher levels of incoming air. Such an increase in air velocity risks causing draughts when outdoor temperatures are low unless appropriate control is included. The only workable solution is to control IAQ and temperature independently of each other, something that most natural ventilation systems are not designed to do.
In contrast, a suitable mechanical ventilation system will use a heat exchanger, two fans that can be controlled independently of each other, grilles and CO2 sensors in the ventilated spaces and, in centralised ventilation systems, appropriately positioned dampers in the ductwork – as well as temperature sensors and sophisticated control algorithms.
This enables a predictable output and performance as such systems are not limited by variable environmental factors in the way that natural ventilation systems are. It also underpins highly efficient heat exchange, so that the heat energy from the stale warm air in the space can be utilised to heat the incoming supply air when required.
Noise levels
In meeting all the criteria described above, it’s also essential to meet the strict noise criteria cited in BB101: noise levels in standard classrooms should not exceed 35 dB(A), including noise from outdoors. Spaces for pupils with special educational needs (SEN) should not exceed 30 dB(A).
Typically, this requires a combination of quiet operation of the ventilation system itself and attenuation of external noise using either extra attenuation of ductwork in a centralised system or standalone units that incorporate high levels of acoustic insulation in the air pathways.
As SEN classrooms may have occupants more sensitive to changes in temperature, I would suggest that ventilation systems with more effective temperature control should be used.
This enables a predictable output and performance as such systems are not limited by variable environmental factors in the way that natural ventilation systems are. It also underpins highly efficient heat exchange, so that the heat energy from the stale warm air in the space can be utilised to heat the incoming supply air when required.
Summary
The options available for ventilating school spaces range from manually opening windows through to balanced mechanical systems, with numerous variations in between. The requirements of BB101, particularly the need to control temperature and IAQ independently of each other, now favour the use of mechanical ventilation systems with heat recovery.
With increasing pressure on reducing pollution within classrooms and the requirement to maintain low internal noise volumes, mechanical ventilation systems will soon be a requirement within city centres and other urban areas.
In designing such systems, account needs to be taken of all of the control parameters discussed here, which often necessitates additional cost for extra components such as sensors, dampers, attenuators etc. An alternative is to opt for a decentralised system using individual units in each space, which incorporate all of the required components and controls.
Author: Jonathon Hunter Hill, Sector Manager
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