CPD Programme
> constant volume flowrate, with a combination
of cooling and (re)heating to meet design
conditions. With the convergence of the
1970s energy crisis and improved control
technology variable air volume (VAV)
systems that were conceived to alter the
volume flowrate to match thermal demand
became popular. By regulating the flowrate
of air through a properly designed system,
(as well as eliminating reheating) significant
energy savings could be achieved
[3]
. The
concept has been developed and refined
to produce the modern DCV systems that
frequently not only control temperature (and
humidity) but also optimise the supply of air
volumes, and particularly outdoor air, so that
appropriate levels of IAQ are maintained
whilst minimising energy costs.
The contaminated internal
environment
The continued drive to save energy, together
with novel construction techniques and
materials in the litigious 1980s and ‘90s,
focused building users and operators on
IAQ as ‘Sick Building Syndrome’ became
a household term. The ‘contaminants’ that
A naturally-ventilated Edwardian school
actually affect building IAQ are various and
numerous. been shown to significantly add to the by humans than had previously been thought
Both particulate and gaseous matter perception of odours (as measured by the (hence the long lingering perception of
are likely to be present in the cocktail that unit of ‘decipol’)
[6]
and hence IAQ. odours from materials used in constructing
makes up the indoor air. Pollutants can be The occupants themselves will contribute and furnishing buildings). This olfactory
generated through indoor activities such to the cocktail by adding their bio-effluents affect, exacerbated at low humidity levels,
as cooking (particles, carbon monoxide, that will include perspiration, chemicals clearly indicates that the overall perception
nitrogen oxides, NOx) and cleaning and gases in their breath. Unless infectious, of comfort may be influenced by factors that
(volatile organic compounds, VOCs) human bio-effluents are not a health hazard, are occupancy independent and hence have
while others are emitted from building, though CO2 can build up in unventilated little or no correlation with the number of
furnishing and consumer products such spaces and cause headaches; moisture can people present. The combined effect of all
as carpets, adhesives, paints, houseplants raise humidity levels and cause condensation the contaminating influences on human
and toiletries
[4]
. Carbon dioxide (CO2), and corrosion; and odour levels may become comfort (let alone human health) is difficult
as breathed out by people, is frequently unacceptable. Since the early 20th century, to clearly identify, although there has been
included in the list of contaminants. the general assumption has been that if much research in the area
[9]
.
However, unless it goes above the
recommended 1,000 ppm (compared Sensors
with outdoor CO2 levels of about 400ppm)
The effect of non-occupant
In buildings where the use is principally
it will have a negligible affect on the
occupants
[5]
. And, of course, pollutants
related, ‘building-borne’
characterised by moderate to high occupant
densities (eg classrooms, theatres, assembly
generated outdoors from vehicle traffic contaminants challenges halls) CO2 is most frequently sensed to
and combustion will pass into the space
previous assumptions.
predict the magnitude of the occupancy
through cracks and openings, mechanical (and by inference, the IAQ). CO2 sensors
systems, doors and windows. These will there is sufficient ventilation to control are commonly based on a technology known
include PM10 (particles measuring 10µm human odours, there will be enough as non-dispersive infrared detection (NDIR)
or less); fine particles, PM2.5 (the size that ventilation to control the health effects where the absorption of the infrared red light
are manifested as haze in the atmosphere); from other contaminants
[7]
. However, the by the sampled air in the sensor provides a
and the more recently highlighted ultrafine effect of non-occupant related ‘building- measure of the CO2 content.
(particles measuring 0.01µm to 0.1µm borne’ contaminants (for example VOCs The technology is reasonably mature,
characteristically associated with diesel released from materials such as particle having been used for more than 20 years in
engine emissions). board) in modern buildings challenges that HVAC applications. However, the sensors are
The very mechanism of passing air assumption. Recent reviews
[8]
have shown sensitive to temperature and humidity (as well
through poorly cleaned ductwork has itself that far lower levels of VOCs can be sensed as tobacco smoke, which is now not such a
60 CIBSE Journal September 2009 www.cibsejournal.com
CIBSEsep09 pp59-62 cpd.indd 60 8/20/09 7:09:51 PM
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