ASHRAE Journal - May 2022 - 24

TECHNICAL FEATURE
resulting EOA delivery is not sufficient, it could be combined
with other sources.
For the two hot climates (in rows two and three), we
note there is no heating season, and thus the coolingseason
trends apply throughout the year. Under these
conditions, extra ventilation is particularly costly and
should likely be avoided in favor of an alternative strategy.
Once again, raising the supply temperature setpoint
is a particularly attractive option based on this comparison,
but some care is needed to ensure that the HVAC
system still provides adequate dehumidification. Finally,
the marine climate in the last row is generally similar to
the cold climate, although with significantly lower average
energy cost.
The key observation from this analysis is that ventilation
may be a particularly costly source of EOA, and so,
if a building is already receiving the ASHRAE minimum
outdoor air (or if CO2 levels indicate acceptable air
quality and transmission risk25), then other strategies
should be examined to provide additional EOA and further
reduce infection risk. In particular, upgrading to
a MERV 13 filter provides extremely cheap EOA and is
almost equivalent to operating at 100% outdoor air due
to its high filtration efficiency.
After upgrading the in-duct filter, a higher supply temperature
during the cooling season is worthy of exploration,
but other system limitations may prevent the use
of this strategy. In such cases, the most robust option
for EOA delivery is the installation of in-zone filtration
units, although capital and operating costs will depend
on the particular devices that are selected.
Conclusions
In this article, we have analyzed the dynamics of
infectious aerosol particles in buildings, formulating
their removal in terms of EOA. Based on our model,
we have suggested possible actions that could be taken
to increase EOA delivery (thus reducing infection risk)
along with the corresponding effect on energy consumption.
We have then illustrated these considerations via
simulations of HVAC systems typical to commercial
buildings across a range of climates, making the following
key observations:
* Outdoor air ventilation is a direct source of EOA, but
it may already be provided by an economizer or made
redundant by a high-efficiency filter; thus, it is likely
to be the most energy-inefficient source of EOA un24
ASHRAE
JOURNAL ashrae.o rg M AY 2022
der many weather conditions in the absence of energy
recovery.
* Improved in-duct filtration efficiency provides additional
EOA with only a small increase in energy cost;
thus, it is the first infection mitigation action that should
be taken in the cases considered.
* In variable volume systems, raising the supply
temperature setpoint is often the only direct way to
increase total supply airflow and EOA while often reducing
energy cost, but it may lead to unacceptably high
indoor humidity or temperature due to reduced cooling
capacity.
* Stand-alone filtration units or other disinfection
devices provide EOA independent of the HVAC system,
and they are cost-competitive with some HVAC sources.
We hope these insights may help guide operational
strategies during the current COVID-19 pandemic and
beyond. Furthermore, we hope the underlying analysis
can be applied to a wider variety of buildings and
HVAC systems to determine the appropriate course of
action in cases where these observations may not apply.
More broadly, similar analysis can be applied to indoor
air quality to help justify HVAC design and operational
changes, which may be further informed by carbon
dioxide monitoring.25
References
1. Morawska, L., J. Allen, W. Bahnfleth, P.M. Bluyssen, A.
Boerstra, et al. 2021. " A paradigm shift to combat indoor respiratory
infection. " Science 372(6543):689- 691.
2. Risbeck, M., M. Bazant, Z. Jiang, Y. Lee, et al. 2021. " Modeling
and multi-objective optimization of indoor airborne disease
transmission risk and associated energy consumption for building
HVAC systems. " Energy and Buildings 253:111497.
3. Risbeck, M., M. Bazant, Z. Jiang, Y. Lee, et al. 2021.
" Quantifying the trade-off between energy consumption and the
risk of airborne disease transmission for building HVAC systems. "
Science and Technology for the Built Environment 28(2):240-254.
4. Bazant, M.Z., J.W Bush. 2021. " A guideline to limit indoor
airborne transmission of COVID-19. " Proceedings of the National
Academy of Sciences 118(17).
5. Morawska, L., D.K. Milton. 2020. " It is time to address
airborne transmission of COVID-19. " Clinical Infectious Diseases
71:2311 - 2313.
6. Burkett, J. 2021. " Virus transmission modes and mitigation
strategies, part 2: airborne transmission and distribution. " ASHRAE
Journal 63(4):10 - 16.
7. Burkett, J. 2021. " Virus transmission modes and mitigation
strategies, part 1: defining viruses and droplet release. " ASHRAE
Journal 63(3):24 - 29.
8.Wells, W.F. 1955. Airborne Contagion and Air Hygiene. An Ecological
Study of Droplet Infections. Cambridge, Mass: Harvard University
Press.
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ASHRAE Journal - May 2022

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