ASHRAE Journal - May 2022 - 20

TECHNICAL FEATURE
FIGURE 4 Histograms of zone humidity and dew point for the hot humid climate during occupied hours throughout the year. Bins for humidity are spaced every 1%, while
bins for dew point are every 1°F. Note that the " MERV 13 Filter " and " +1 ACH In-Zone Filtration " cases are identical to " ASHRAE Baseline " and thus omitted.
800
600
400
200
20
40
0.5× Minimum Ventilation
60
Zone Humidity (%)
ASHRAE Baseline
Guideline 3624 suggests an upper limit of 65°F [18°C] in
mild or dry climates and 60°F [16°C] or lower in humid
climates.)
We note also that the EOA provided by deactivation
and deposition (which are directly affected by humidity)
increases by no more than 0.2 ACH compared to the
baseline case, which underscores the fact that, as long
as humidity is maintained within the moderate range,
further humidity control is generally not an effective
mitigation strategy (although this observation may need
to be revisited as viral deactivation models are improved
to account for possible discontinuous effects near the
efflorescence point).17
Thus, while raising the supply temperature setpoint
is an intriguing possibility for many spaces, the most
widely applicable solution for EOA delivery (of the
options considered here) is to improve in-duct filtration
to MERV 13 or better.
Energy Costs
Consistent with the EOA observations in the previous
section, we turn now to the associated energy
consumption. For this purpose, in Figure 5 we show
monthly average EOA and energy cost for each climate
and case. In addition, to determine which of the EOA
sources are most energy-efficient, we plot a monthly
" price " of the additional EOA as the ratio between the
change in energy cost and change in EOA delivery
from the baseline case. This value essentially normalizes
all strategies to providing an additional 1 ACH of
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ASHRAE JOURNAL ashrae.o rg M AY 2022
2× Minimum Ventilation
80
100
2,000
1,500
1,000
500
50
55
60
Zone Dew Point (°F)
Higher Supply Temperature (55°F
62°F)
EOA and then estimates the energy impact. In general,
we prefer strategies with a low (and ideally negative)
EOA price.
Starting in the first column of Figure 5, we note that the
plotted values for EOA delivery are the same values as in
Figure 3 but re-averaged on a monthly basis and with a
new curve for the in-zone filtration case. With the values
overlaid, it is much easier to see which strategies are
providing the most EOA. We see that in-zone filtration is
the only consistent strategy throughout the year, as those
devices can be deployed at will independent of the HVAC
system. Both the increased ventilation rate and the
MERV 13 filter also provide additional EOA each month,
but the magnitude varies throughout the year. Finally,
we see that the higher supply temperature is the most
inconsistent source of EOA, generally providing no additional
EOA during heating conditions but significantly
more during cooling conditions.
Moving to the second column, the most obvious trend
is that energy consumption is driven primarily by the
annual climate cycle, with the disinfection strategies
shifting costs only marginally from the baseline case.
The obvious outlier to this trend, however, is ventilation
during heating conditions. For the 0.5× Ventilation
case, internal building loads are often enough to meet
the heating requirements of the outdoor air, and thus
energy consumption is quite low. However, as the outdoor
air rate is increased to the " ASHRAE Baseline, " some
supplemental heating is required, and at 2× Ventilation,
a very large amount of heating is needed.
65
70
Frequency (h)
Frequency (h)
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ASHRAE Journal - May 2022

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