The pandemic has focused attention on the need for better building
ventilation, but this need not come at the expense of saving energy and cutting
carbon, according to a leading building services engineer.
Speaking at a webinar hosted by the Building Engineering Services Association (BESA), AECOM’s Ella Clark said this was “not an either/or
issue”
“We now know that doubling the rate of ventilation
reduces the spread of Covid-19 by around half and that the World Health
Organisation (WHO) recommends ventilation as a first-line strategy for getting
people back to the office,” she said.
“We can make our HVAC systems pandemic resilient
and design buildings with excellent indoor air quality (IAQ) that are also low
carbon.”
Clark, who is a mechanical engineer and committee
member of the British Council for Offices (BCO), said the use of technologies
like radiant heating and cooling panels, mechanical ventilation with heat
recovery (MVHR), and direct evaporative cooling can all improve the indoor conditions
without driving up energy consumption.
“Displacement ventilation is a good option because
it is more effective at drawing away pathogens compared with top-down air
mixing,” she explained. “It can reduce energy cost by 20% because it allows air
to be introduced into the occupied space at higher temperatures so reducing the
need for cooling power. However, it does require larger volumes of air so needs
bigger air handling units etc.”
Natural ventilation is the most energy efficient
option with energy savings as high as 79% depending on what it is replacing –
equivalent to as much as £30,000 per annum in running costs for a commercial
building, according to Clark. However, it increases the risk of introducing
polluted air into the indoor space.
“Many studies show that the IAQ can be considerably
worse than the outdoor air in these circumstances…and 75% of urban areas in the
UK are above the WHO recommendations for air quality.”
Health experts have been quick to highlight the
crucial role played by ventilation in maintaining health and comfort through
temperature and humidity control and by reducing moisture, odours, and gases. A
NASA study also showed that human productivity fell by 3.6% for every 1degC the
indoor temperature rises above 22degC.
The BCO recommends temperature in offices should be
maintained at between 20 and 24degC and there should be 12 litres per second
per person (l/s pp) of clean air introduced with an additional 10% in high
density occupied spaces. It also says that controlling humidity is crucial.
Clark told the BESA webinar that BCO studies showed
the average relative humidity (RH) in offices was 38% whereas for good health
it should be between 40 and 60%. At 35% people will experience eye irritation,
nasal dryness, and sore throats. This also has important implications for
future health emergencies.
“At 23% RH more than 70% of flu particles remain
infectious after an hour in the air, but at 43% that falls to just 14% according
to a range of studies,” she explained “However, providing those conditions can
lead to an energy penalty if you use humidifiers. Adding an extra 0.28 l/s pp
can increase energy demand by 0.6kWh/m2 per annum which is
5,000 kw annually for a large office.”
Mechanical ventilation offers a more controllable
approach that can also make use of filtration to reduce the ingress of
pollutants into occupied spaces. “We want to ensure that we are filtering down
to PM2.5. There are HEPA filters and ULPA filters which are typically used in
clean rooms in hospitals,” said Clark.
“You will get a higher pressure drop across these
filters and they do also have to be maintained, but we can still find the
balance between healthy indoor conditions and sustainability if we use the
right design approach.”
Clark’s presentation followed the launch of BESA’s
third piece of IAQ guidance in just over a year.
‘Buildings as Safe
Havens – a practical guide’ outlines strategies
facilities managers and building owners can follow and provides targeted
questions they can put to ventilation experts to establish the right air
quality approach for their building.
It can be downloaded for free along with the
Association’s other IAQ guides here.
