Gaining a ventilation advantage
The Covid pandemic highlighted the importance of good indoor air quality (IAQ) and ventilation and has given rise to CO2 monitors being used to assess the air within schools. However, guidance already exists for ventilation in schools with BB101. Louise McHugh, Product Manager at Breathing Buildings, explains how the document sets out requirements for educational premises and how by employing the latest hybrid ventilation, consultants can help schools maximise health, comfort and sustainability without significant building works.
Prior to the pandemic there was already a rising awareness of how indoor environments impact on our health and wellbeing. Guidance for schools and ventilation also already existed in the form of Building Bulletin 101 (BB101). The document’s last revision, the 2018 edition of BB101: ‘Guidelines on ventilation, thermal comfort and IAQ in schools’, looks at the key design elements of modern ventilation systems in schools. The document sets out requirements for schools to improve ventilation to help create a healthy and comfortable environment.
Aimed at technical professionals involved in the design, specification and construction of new school buildings and the refurbishment of existing buildings, the BB101 2018 guidance changed significantly since the previous BB101, published in 2006. The guidance aims to help ensure schools have efficient, healthy and comfortable environments so children will benefit from effective teaching and learning spaces. It sets out more stringent air quality targets, is tougher on summertime overheating, and recommends a pre-mixing of air to eliminate cold draughts.
BB101 also sets much clearer and more onerous limits on peak and daily average CO2 limits which have a hugely beneficial impact on IAQ in classrooms.
Where natural ventilation is used, or when hybrid ventilation systems are operating in natural mode, in general teaching and learning spaces the daily average concentration of CO2 must be less than 1,500ppm when occupied. The maximum concentration should not exceed 2,000ppm for more than 20 consecutive minutes each day. For mechanical ventilation the daily average CO2 level must be less than 1,000ppm and must not exceed 1,500ppm for more than 20 consecutive minutes each day.
Although IAQ is vital, consultants also need to consider thermal comfort when ventilating classrooms. According to BB101, overheating needs also to be assessed in terms of operative temperature using the DSY1 2020 weather file during the occupied period 9am – 4pm Monday to Friday between 1st May and 30th September, including the summer holiday period. It also states that elimination of cold draughts is important since draughts can either raise heating bills as there will be an increased use of radiators, or affect the air quality since teachers will close windows to reduce draughts. So how can consultants solve this school ventilation conundrum to balance thermal comfort and air quality?
Hybrid ventilation systems meet this need and were introduced in BB101 2018 alongside traditional natural and mechanical ventilation, offering specifiers an alternative ventilation design strategy for schools. By employing hybrid ventilation, schools can maximise health, comfort and sustainability without having to invest in significant building works, making it ideal for retrofitting.
What is hybrid technology?
For commercial buildings with high heat gains, such as schools, the ideal ventilation solution to invest in is hybrid ventilation. Hybrid ventilation is highly efficient and makes use of the free heat (and so free energy) that has already been built up within a space. Hybrid ventilation focuses on the balance of IAQ, thermal comfort and efficiency by choosing the most appropriate mode of ventilation based on the internal and external conditions. Supplied with an external temperature sensor and down, an internal temperature and CO2 sensor as well as an intelligent controller, the latest hybrid systems monitor conditions to create an ideal indoor environment, automatically switching between natural, hybrid and mechanical ventilation, allowing the unit to be in the most energy efficient mode possible at all stages, thus making it an ideal choice for schools.
For example, schools, have an abundance of heat generated by children, lighting and other school equipment, such as computers, making them ideal places to benefit from hybrid systems. What hybrid systems do perfectly is to make use of the heat to provide excellent thermal comfort and IAQ while keeping overhead costs down, offering energy savings and so reducing operating costs. Hybrid ventilation is the ideal ventilation for schools that would like to improve their energy efficiency and benefit from free heat gains.
Heat recovery
However, the latest innovative hybrid ventilation has now gone one stage further saving even more free heat for schools and so offering more cost savings. The key difference between original hybrid ventilation and the new systems is the addition of a low resistance heat exchanger cell within the unit. This allows the unit to benefit from both heat recycling and heat recovery, reclaiming even more heat than previous models, saving more energy, providing greater occupant comfort, and allowing users to include it within a building’s energy assessments (SBEM).
By including a low resistance aluminium cross heat exchanger in the unit, it further lowers energy costs by reducing the reliance on space heating to maintain thermal comfort in a room. It operates during colder external temperatures, typically below 7ºC when mixing recycled air alone is not enough to maintain the desired temperature for occupants. The low resistance heat exchanger cell also ensures it does not overheat
a space. Buildings with high heat gains already contain a significant amount of heat therefore the heat exchanger only adds back what is necessary to maintain the ambient temperature, balancing this with optimising IAQ. This type of heat recovery is ideal for buildings with high heat gains within UK temperature ranges.
The latest hybrid heat recovery ventilation units combine 46% heat recovery efficiency with low Specific Fan Power (SFP) of 0.075 W/l/s to maximise energy savings. In addition, they feature several different operating modes to minimise energy use, enhance IAQ and improve occupant comfort. An intelligent hybrid system, the units automatically determine when and if mechanical operation is required, ensuring it only operates when absolutely necessary. It optimises IAQ, comfort and efficiency by automatically switching between natural, hybrid and mechanical ventilation, maximising benefits throughout the year.
Cross-flow
However, when it comes to designing ventilation systems in schools, consultants do not only need to consider BB101. In November 2022 the Department of Education produced the ‘School Output Specification Technical Annex 2F: Mechanical Services and Public Health Engineering’. Within this document another consideration for consultants is school buildings should be designed to employ cross-ventilation or stack effect ventilation for thermal comfort in summer temperatures. With hybrid ventilation this can be achieved by siting a unit at opposite ends of a classroom if there are two external walls. Alternatively, with classrooms located on a corridor a hybrid unit can be sited one end of a classroom on an external wall with a passive stack at the other or a passive attenuator exhausting into the corridor.
With the requirement for schools to improve indoor environmental performance, NVHRs and e-Stacks tick all the right boxes for both BB101 and School Output Specification Technical Annex 2F. Offering energy savings and sustainable development by optimising the balance between IAQ, thermal comfort, energy use and environmental impact, this form of hybrid ventilation reduces cost whilst improving wellbeing and learning outcomes.
