Ensuring acceptable indoor air quality in classrooms whilst maintaining comfort levels requires ventilation systems that can control several parameters independently of each other. *Jonathan Hunter Hill explains
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 parallel, there is growing concern around the impact of external air pollution on the indoor environment of schools in built-up areas, introducing a range of pollutants to classrooms and other spaces.
Addressing these concerns requires ventilation systems that meet BB101 criteria and provide protection against outdoor air pollution. In most cases simply opening windows does not provide adequate control, so that mechanical ventilation is the obvious alternative.
A major challenge for mechanical ventilation systems is being able to control draughts, temperature and carbon dioxide (CO2) levels simultaneously. In particular, the necessity of controlling both CO2 levels and temperature independently of each other is proving difficult with traditional system designs. Failing to control temperature adequately may result in draughts that are unacceptable in the new BB101.
For example, a new school in Scotland 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 -2⁰C (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.
It is this consideration that is now leading many ventilation designs to use mechanical ventilation with heat recovery (MVHR), as was the case with the school mentioned above. With the right design these units enable independent control of indoor air quality and supply air temperature to ensure compliance with BB101.
However, not all MVHR systems are equal.
Draught control and CO2
Preventing draughts requires effective control of both temperature and velocity of incoming air – whilst maintaining CO2 at acceptable levels (daily average of no more than 1,000 ppm between 9am and 4pm).
With traditional ventilation systems, if there is an increase in CO2 levels this can lead to demand for higher levels of incoming air at higher velocity, risking draughts in cold outdoor conditions unless appropriate control is included.
Such appropriate control requires a heat exchanger, two fans that can be controlled independently of each other and, in centralised ventilation systems, appropriately positioned dampers in the ductwork, with diffusers and CO2 sensors in the ventilated spaces. To avoid overheating, temperature sensors and sophisticated control are also a necessity.
Thus, use of a centralised system to meet requirements necessitates many extra components, with added cost. Alternatively, decentralised units that include these functions as standard can be installed each ventilated space.
Pollution and filtration
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 efficiency and reducing maintenance requirements.
It’s also essential to meet the strict noise criteria of BB101: noise levels in standard classrooms should not exceed 35 dB(A), including noise from outdoors. Spaces for pupils with special educational needs 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.
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.
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.
*Jonathan Hunter Hill is sector manager for education with SAV.