ASHRAE Journal - May 2022 - 18

TECHNICAL FEATURE
These cases represent the possible mitigation strategies
discussed in the Mitigation Strategies section. For
cost calculation purposes, we assume cooling is provided
with a COP of 3 and heating with an efficiency of 90% for
electricity at $0.12/kWh and gas at $8/MMBtu.
EOA Breakdown
To gain insight into how each of the mitigation strategies
provide additional equivalent outdoor air (EOA),
Figure 3 provides a weekly breakdown of EOA delivery
by source for each climate and case. All values are
averaged over occupied hours for each week. Both the
" Economizer " and " Minimum Ventilation " categories
are due to ventilation, with the former indicating discretionary
extra ventilation above the minimum provided
by the economizer (which varies with outdoor air
temperature).
For reference, these figures also show the change in
total energy cost throughout the year relative to the
baseline case. For brevity we omit the plots for the inzone
filtration cases, as it simply shifts the baseline
case up by 1 ACH of EOA. In each set of axes, the black
" Max EOA " line represents the maximum possible EOA
delivery for that system assuming 100% clean supply
air. These conditions would be achieved under either
100% outdoor air operation or using an in-duct filter
with 100% capture efficiency. Although system capacities
are generally near 6 ACH, such high flows are only provided
on the hottest hours of the hottest days, and thus
the weekly averages shown in the figure are always well
below that value.
Starting in the upper-left corner of Figure 3 (cold climate
EOA) and moving right to the ASHRAE Baseline,
we note that increasing from half of to the full ASHRAErequired
ventilation rate often does not have a significant
impact on EOA delivery but does noticeably
increase energy costs. The explanation is that under
economizer conditions, the minimum ventilation rate
does not matter (because the system is operating at 100%
outdoor air), and otherwise the in-duct filter is able to
partially clean the extra recirculating air (roughly 40% of
it, in the case of the MERV 8 filter) that would otherwise
come from outside. Thus, as the orange bars increase,
the red and yellow bars also shrink.
Moving further to the right to the 2× Minimum
Ventilation category, we see that doubling the minimum
ventilation rate from ASHRAE requirements does
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ASHRAE JOURNAL ashrae.o rg M AY 2022
provide increased EOA, but not as much as might be
expected. In this case, the reason is exactly the same
as before: the increase in the " Minimum Ventilation "
category comes with a corresponding decrease in the
" Economizer " and " Filtration " categories. Note that at
this ventilation rate, the system is at 100% outdoor air for
nearly the entire year, and thus EOA delivery is near its
maximum value.
However, moving to the fourth column, we see that
similar EOA delivery can be achieved by leaving the ventilation
rate at its ASHRAE value and instead switching
to a MERV 13 filter. Finally, moving to the last column
we see the most interesting results associated with the
higher supply temperature. In this case, whenever the
HVAC system is operating above minimum flow constraints,
the total supply airflow is increased by the control
layer as required to maintain temperature setpoint.
Thus, both the maximum possible and actual EOA delivery
rates increase relative to the ASHRAE baseline case.
Moving down to the next two rows associated with hot
climates, the observed trends are similar but with different
annual distribution due to the relevant seasons.
In these climates, the economizer season is shifted primarily
from spring and fall months to the winter. Thus,
the increased energy costs associated with ventilation
are almost exclusively due to cooling, in contrast to the
mixture of heating and cooling that affected the cold
climate.
We note also that the total cost penalty for the hot
dry climate is less than that of the hot humid climate
because the dry climate does not see the significant
latent cooling load observed for the humid climate. As
before, we see that the higher supply temperature case
in column four is the only case that increases total supply
flow, but unfortunately not all of this potential EOA
can be realized due to operating at minimum ventilation
conditions.
By contrast, moving to the marine climate in row four,
we see that economizer operation means the system
is operating near 100% outdoor air throughout much
of the year, and thus near-maximum EOA is achieved
regardless of the operating strategy. (We do note that the
large increase in energy cost for the 2× ventilation case is
primarily due to heating costs, as will be discussed in the
next section.)
Overall, these results illustrate that due to the coupling
of ventilation and in-duct filtration, diminishing
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ASHRAE Journal - May 2022

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