FMJ.CO.UK HVAC FOCUS
In the longer term, CIBSE
expect consideration of infectious
disease transmission to be embedded
into buildinJ reJulations, speciƛ cally
those dealing with the provision of
ventilation, in the same way that
energy, comfort and air quality
REFERENCE NOTES
(i) www.gov.uk/government/publications/emg-role-ofventilation
in-controlling-sars-cov-2-transmission-30-
september-2020
(ii) www.cibse.org/knowledge/knowledge-items/
detail?id=a0q3Y00000HsaFtQAJ
(iii) www.gov.uk/government/publications/ventilationapproved
FEBRUARY 2021 31
document-f
rate is doubled”. This is in line with the
underlying principle adopted by CIBSE which
is: dilute and ultimately remove airborne
pathogens as much as possible from a
space by exhausting them to the outside to
reduce the chance they will be deposited on
surfaces or inhaled by room users.
In practice this means taking steps to
increase both the supply of outside air and
the exhaust ventilation rate in buildings
as much as is reasonably possible while
minimising, or eliminating, any recirculated
air.
MECHANICAL SYSTEMS
In buildings with mechanical ventilation
systems a key intervention recommended
by CIBSE is to extend the systems’ operation
times to start the ventilation at least an
hour before building usage time and to run
the system for at least an hour at the end
of usage to help purge the building of any
aerosols.
Similarly, for demand-controlled
ventilation systems where the rate of
fresh air supply is adjusted to maintain a
minimum CO– level, CIBSE recommends
changing the CO– set point to a lower value
to help maximise the flow of outside air.
CIBSE also recommends that air is not
recirculated between spaces, rooms or zones
occupied by di‘ erent groups of people,
unless recirculation is the only way of
maintaining adequate levels of outside air
to occupied spaces. Systems with thermal
wheels should be inspected by a competent
person to assess the risk of air leaking from
the exhaust flow into the supply air and to
adjust the system to eliminate any transfer
which could carry viral material into the
supply air stream.
If the ventilation rate in a space is
designed, constructed and maintained at
current design standards, then SAGE says
the risk of aerosol transmission is likely
to be low. In most workplaces and public
environments, the Advisory Group say this
equates to an outside air supply rate of 8-10
l/s/person; which is the minimum outside
air supply rate recommended by CIBSE and
also satisfies the building and workplace
regulations.
Providing e‘ ective ventilation is more of
a challenge in older buildings, particularly
where systems may not have been designed
in line with current ventilation standards.
According to SAGE, the risk of aerosol
transmission is likely to be significantly
higher in poorly ventilated multi-occupant
spaces, where the supply of outside air is low
(below 5 l/s/person).
CIBSE’s COVID-19 Ventilation Guidance
document(ii) provides information to help
building owners and operators understand
the types of ventilation systems commonly
encountered in buildings and how these
can best be used to reduce the risks of
airborne infection.
The Guide will help building managers
and those who operate and maintain
building systems to identify those areas
of a building and elements of ventilation
systems that may need particular attention
to reduce the risks to the building
occupants. CIBSE’s general advice is that
when it is reasonable to do so, ventilation
rates should be increased as much as
reasonably possible without compromising
thermal comfort.
AIR CLEANING
In poorly ventilated
spaces with a high
occupancy and where
it is di‘ icult to
increase ventilation
rates it may be
appropriate to
consider using
air cleaning
and ultraviolet
disinfection
devices.
Room air cleaners
are generally
standalone devices
containing a fan to pull air
through a high e‘ iciency air
filter. To be e‘ ective a High E‘ iciency
Particulate Air (HEPA) filter should be
used, which can filter out the COVID-19
virus. However, because the units are only
e‘ ective over a small area their location is
important.
Ultraviolet systems can take various
forms but most use light with a wavelength
of 254nm to render viruses ine‘ ective by
destroying genetic information in their
DNA. Any such installation should use
equipment with robust test data to show
its e‘ ectiveness and needs to be of an
appropriate size to handle the volume of
air requiring treatment. The installation
needs to be designed to meet the
particular requirements of the space and
activities being undertaken.
Getting the ventilation rate right is a
di‘ icult balance. Ventilation has wellestablished
associations with health and
has a complex relationship with other
factors including energy, noise, air quality
and thermal comfort. Those modifying an
existing system or altering its operation
need to be mindful of the possibility of
unintended consequences of their actions;
for some mechanical installations it would
be wise to seek expert advice before taking
any action.
For naturally ventilated buildings, where
a space is reliant on opening windows and
doors for ventilation, the advice is that
windows and vents should be used more
than normal. In warm weather opening
windows is a typical behavioural response,
however it is important windows are kept
open, even if only by a small amount,
to maintain ventilation when it is cooler
outside.
Of course, opening windows can result in
draughts and occupant discomfort, so where
possible desks and should be relocated away
from windows (or occupants encouraged to
wear jumpers!). An alternative solution is
to encourage all occupants to take regular
breaks simultaneously throughout the day
in order to purge the space through
opening windows and doors with
outside air to complement
background ventilation
and further dilute the
concentration of aerosols.
Unlike distancing
and hand washing,
ventilation requirements
cannot easily be distilled
into one simple approach
that everyone can follow.
Assessing ventilation
in many environments is
challenging and will require
the type of engineering expertise
CIBSE members can provide
to ensure any proposed mitigation
measures will be e‘ ective for a particular
building type, ventilation system, user group
and activity.
In the longer term, CIBSE expect
consideration of infectious disease
transmission to be embedded into building
regulations, specifically those dealing with
the provision of ventilation, in the same
way that energy, comfort and air quality
are already included. In England and Wales,
Approved Document F: Ventilation(iii) is
currently under review and CIBSE will be
contributing to the review process.
CIBSE COVID-19 ventilation Guidance
is available as a free download from the
CIBSE website: go to https://www.cibse.
org/Coronavirus-covid-19/emerging-fromlockdown
are already included.”
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