HVAC for schools
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Making sense of standards
ense
rds ens
T
helatestversiono f BB101 introduces rigorous ventilation systems in
requirements for v
schools. However, there is an anomaly, insofar as BB101 allows higher levels of carbon dioxide (CO2)whennaturalventil ation systems are in use, compared to mechanical ventilation systems. Given that high CO2 the same, irrespective of equipment type – BB101 should be focused on the result, not how it is achieved. For mechanical ventilat requires that the daily ave below 1,000ppm, with a te (∆T) between the air in the entering the room of betw depending on the room typ mechanical ventilation sy other countries, irrespect
should be focused on th
eroomandtheair emperature differential rage CO2 level be kept tion systems, BB101
tive of the type of ystems are close to pe. These levels for een 1.5°C and 4°C,
system. For example, No rway and France have 1,000ppm limits on class
sroom CO2 levels, whilst
Denmark and France limit CO2 levels to 900ppm. For natural ventilation systems, however, BB101 requires a daily average CO2 level of below 1,500 oom types. Thus, it seem
systems, however, BB1 with a ∆Tof5°Cforallro
the bar has been lowered for natural ventilation, compared to mechanical because most natural ven to comply with the 1,000p the course of the year. Average CO2 levels can 500ppm for no more than day for both system types.
ppm requirement throughout ntilation systems are unable systems. This is possibly
n20consecutiveminutesa neachbeexceededby
CO2 Levels and thermal comfort quality (IAQ) – and the
The control of indoor air q choice of ventilation syste the context of other BB10
em – needs to be seen in 01 requirements, especially
avoidance of uncomfortable draughts.
For instance, when outdoor air temperatures are reasonably high, natural ventilation systems can use high flow rates of supply air to dilute CO2 levels, with a limited requirement to temper supply air. In winter this situation changes as outdoor air
!" April 2020
This approach, therefore, prioritises thermal mfort over indoor air quality, comprom latter by allowing CO2 levels to rise. The key point is that an increase in CO
com
O2 levels mising the
ess appropriate control is included. T rkable solution is to control IAQ and te ependently of each other, somethingm ventilation systems are not designed to do. In contrast, a suitable mechanical ventilation tem will use a heat exchanger, two fa controlled independently of each othe sors in the ventilated spaces and, in c tilation y, pp
vent sens be c syst
dam
unc allo N
systems, appropriately positioy p p oned
mpers in the ductwork – as well as tem sensors and sophisticated control algorithms. Natural ventilation can be wasteful of energy, wing warm air to escape the space in ontrolled fashion. The use of heat rec
mperature
can lead to demand for higher levels of incoming air, increasing the risk of draughts in cold weather unle wor inde
most natural emperature he only
temperatures are typically much lower t han 16°C, le the indoor temperature is still at 2 1°C. In such nsystems
whil case
risin of st air i
es, many natural or hybrid ventilation reduce the supply air flow rates so that cold outside
ngtounacceptablelevels. tale air in the space and a danger of C
sintroduced.Consequently,thereis
CO2 levels less dilution
External air pollution
Another consideration is the potential impact of external air pollution on indoo
orIAQ,whichisanissueformost
(July 2018) ISO 16890 standard. ensure compliance with the new levels than has traditionally been reas. This means incoming air must
e ;
ideally to e dtohigher nbuilt-upa
18) ISO 16890 standard.
er needed to drive the air through entilation systems typically lack and reducing maintenance requirements. ntaining optimum heat exchange eat exchangers from pollutants, oth incoming and outgoing air also
o h
ereby main tects the he tration of b
nd reducing
hefanpowe Natural ve
in city and town centres. ot even be considered for schools efore, that natural ventilation us limiting filtration opportunities.
e g
m
sshouldno gest, there filters, thu
Conclusion The options availab
nd town cen lusion
range from manual
ble for ventilating school spaces lly opening windows through
to balanced mechanical systems, with numerous variations in betwe en. The requirements of BB101, particularly the nee independently of ea
en BB101
ed to control temperature and IAQ ach other, now favour the use of
mechanical ventilation systems with heat recovery. With increasing p within classrooms a v
centralised er and CO2 ans that can
low internal noise volumes, mechanical ventilation systems will soon be
b a requirement within city centres and other urban areas.
taken of all the cont In designing such
which often necess components such a
covery in nan
as sensors, dampers, attenuators itate additional cost for extra p,
trol parameters discussed here, hsystems,accountneedstobe
etc. An alternative i sadecentralisedsystemusing individual units in e
of the required components and controls.
www.heatingandventilating.net each space, which incorporate all
pressure on reducing pollution and the requirement to maintain
e
The release of BB101: Ventilation, thermal comfort and indoor air quality 2018 lays out new standards for designers of ventilation systems in schools, Jonathon Hunter Hill of SAV considers the apparent disparity between requirements for mechanical and natural systems
mechanical ventilation units can deliver a minimum of 40% energy savings compared to natural systems.
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