ASHRAE Journal - May 2022 - 15

TECHNICAL FEATURE
required to cool zones, providing additional fi ltration for
recirculated air and possibly also additional ventilation
during economizer conditions.
* In-Duct Filter Type. Using a higher-effi ciency fi lter
results in more particles being removed from the recirculated
air; however, if there is no recirculated air, the
fi lter does not provide any EOA.
* Activation of In-Zone Disinfection Devices. Portable
air fi lters or upper-room UVGI lamps can directly
remove or deactivate infectious particles from zones
independently of the operation of the HVAC system.
In different systems, different variables may exist that
can be specifi ed (e.g., demand-controlled ventilation
setpoint or economizer suitability temperature), but we
select these variables as a representative cross section. A
key observation is that the fi rst three variables listed just
above all operate on the same supply airstream; thus,
they cannot always provide an independent source of
EOA. For example, if the minimum outdoor fl ow setpoint
is high enough that the system is operating near
100% outdoor air, the in-duct fi lter essentially provides
zero EOA, as there are no infectious particles for the fi lter
to remove.
The maximum possible EOA that can be delivered by
these variables is the total supply fl ow provided by the
HVAC system, which is tightly coupled to the thermal
loads experienced by the space and thus often cannot be
controlled directly. In addition, we note that the fi rst two
variables, if poorly chosen, can compromise occupant
comfort. When outdoor air conditions are particularly
hot or cold, the system may not be able to provide the
heating or cooling required to adequately condition
additional outdoor air. Similarly, if the supply temperature
setpoint is adjusted too far from its design value,
the system may hit fl ow constraints without being able
to meet the thermal loads in the space. Alternatively, setting
the supply temperature setpoint too high may not
provide suffi cient dehumidifi cation, leading to excessive
zone humidity.
These concerns are less relevant during milder shoulder
seasons, but it is important to recognize that the
HVAC setpoints may not provide as much fl exibility as
expected, and they may need to be adjusted gradually to
ensure comfort is maintained.
Finally, we note that the selected variables are primarily
applicable to fairly modern commercial buildings
with central HVAC systems. Therefore, the conclusions
that follow may not apply to older buildings that are
naturally ventilated or otherwise lack central HVAC. For
example, in classrooms without mechanical ventilation,
none of the indicated HVAC setpoints are available to be
manipulated. While the use of stand-alone in-zone EOA
sources is possible, such devices may be costly to deploy
or lead to unacceptable noise levels. In such situations,
the only readily available way to increase EOA delivery
is to manually open windows.18 Unfortunately, this
strategy may compromise energy effi ciency (and possibly
also occupant comfort), but it may nevertheless be
necessary to achieve acceptable risk levels. Similarly, in
more modern buildings with energy-recovery ventilation
or other advanced capabilities, the relative costs of
the mitigation strategies could vary signifi cantly.
The cases presented in this article were chosen to represent
a large class of buildings and weather typical of
the United States with a concise presentation of results.
However, the appropriate course of action will need to
be modifi ed for spaces with different HVAC confi gurations
or other conditions, and will thus require separate
application of the underlying analysis techniques.
Energy Analysis
Although the mitigation strategies can provide infection
risk reduction via increased equivalent outdoor
air (EOA) delivery, using these strategies almost always
leads to an increase in overall energy consumption. We
summarize this energy impact as follows:
Energy Impact = Heating + Cooling +
Fan + Devices
∆∆
∆∆
(2)
When increasing the minimum ventilation rate, the
primary change to energy consumption is to heating and
cooling for the outdoor air, and the magnitude of these
changes varies signifi cantly with outdoor air conditions.
It is possible that energy use may decrease in some conditions,
but a well-functioning economizer would likely
already capture these savings in the baseline operation.
Fan power may also increase if the operational changes
increase total fl ow to the zone. For an increase to the
supply temperature setpoint, energy analysis is more
complicated. Whenever this change leads to additional
supply fl ow, fan power is increased.
In the heating season, heating energy consumption
may increase, although the effect will depend on
equipment confi guration and sequence of operation.
M AY 2 0 2 2 ashrae.o rg ASHRAE JOURNAL
15
http://ashrae.org
ASHRAE Journal - May 2022

Table of Contents for the Digital Edition of ASHRAE Journal - May 2022
