ASHRAE Journal - April 2024 - 19

COLUMN INFECTIOUS AEROSOL CONTROL
Fans that contribute to VECAi cannot cycle on and off
with heating or cooling demand, although this is a
common thermostat control strategy ( " FAN AUTO " ).
In the classroom example shown in Online Figure 1,
HVAC equipment must be scheduled for the entire
time the classroom is occupied, including after-school
activities. If IRACs are used to provide VECAi, they
also must operate during occupied hours. Automatic
control, such as controlled receptacles, could be used
to schedule IRACs alongside other systems.
Flushing between occupied periods is not required
if the target VECAi is delivered (Section 9.1.5). Early
ASHRAE Epidemic Task Force (ETF) guidance4 for
" Pre- and Post-Occupancy Flushing " was created
when much uncertainty existed about how long
the virus responsible for COVID-19 remained
infectious in aerosols. The guidance was commonly
implemented by extending HVAC schedules by two
hours before and after a normal daily schedule.
As we learned more about particle deposition and
viral decay rates,5,6 it became clear that risk from
infectious aerosols would greatly diminish overnight
without active intervention, making unoccupied
operation unnecessary.
The 2021 ETF " Core Recommendations " refl ected
this understanding; the concept was renamed
" Flushing Between Occupied Periods " for changeover
periods in gathering spaces, like between classes
or movie showings when necessary.7 (Despite this
clarifi cation, extended-schedule fl ushing periods
remain in effect in many buildings today.) Delivering
the target VECAi during occupied periods reduces
infection risk to a very low level already, negating the
need for extra HVAC runtime in IRMM. An additional
benefi t of this approach is lower or avoided energy
consumption.
Demand-controlled ventilation (DCV) is allowed
in IRMM as long as the required VECAi is maintained
(Section 9.1.4.d). This is a shift from the ETF guidance
to disable DCV. Standard 241 allows equivalent clean
air targets to be met through any combination of
strategies as long as code-minimum ventilation rates
are maintained. If DCV is enabled, the quantity of
outdoor air contributing to VECAi must be based on
the outdoor air minimum airfl ow setpoint.
Standard 241 also requires communication with
occupants. Like fi re occupancy limits, spaces will
FIGURE 1 Prevalence of ventilation equipment failures in a study9 of commercial
buildings, emphasizing the need for regular maintenance to ensure systems can
reliably deliver VECAi.
Maintaining Positive Pressure
Broken Dampers
(Outdoor, Exhaust/Relief or Mixed Air)
Absent or Faulty CO2 Sensor for DCV
Incorrect Minimum Outdoor Airfl ow
Relief/Exhaust/Return Fans Not
Faulty Temperature or Pressure Sensors
0%
10%
20%
30%
% of Buildings
14%
24%
23%
27%
35%
40%
have infection occupancy limits that must be publicly
posted. Engineering controls that could be easily
adjusted by occupants, such as programmable
thermostats or IRAC controls, must be labeled with
the required operating settings. Labeling provides
occupants with assurance that the engineering
controls are contributing to reduced infection risk
and allows for quick intervention if equipment
settings do not match the label. Building operations
are not regulated, so this transparency allows
occupants to verify that systems are maintained and
delivering the expected clean airfl ow.
Maintenance Requirements
Maintenance might be the most crucial component
of readiness. Failed damper actuators or fi lters that
allow bypass will not provide the expected VECAi. All
buildings will experience equipment failures, and
a ventilation system that is off provides no VECAi.
Maintenance is particularly important in schools, as
capital expenditure budgets may not allow for new
equipment, and good maintenance can extend life
expectancy while ensuring proper operation. The U.S.
Government Accountability Offi ce reports that 41% of
school districts need to update or replace their HVAC
systems.8 Figure 1 illustrates how common ventilation
equipment failures can be, using data from a study
of 151 commercial buildings.9 In addition to meeting
VECAi targets, repairing such failures as part of a
" retuning " effort can save between 5% - 30% of
building energy.10
The maintenance requirements in Standard 241
aim to keep VECAi-delivering systems ready for when
they are most needed. In normal mode, semiannual
testing is required, while during IRMM, critical
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