ASHRAE Journal - May 2022 - 16

TECHNICAL FEATURE
However, in the cooling season, the cooling energy
consumption is likely to decrease, as the latent load
required to meet the supply temperature setpoint is now
smaller. Because the fan and cooling effects are opposite,
the net overall effect may be positive or negative
depending on which dominates.
When changing the in-duct filter type, the predominant
energy impact is on fan power, as different filter
grades impose different pressure drops that must be
overcome by the fan to meet airflow requirements. For
standard MERV filters, the change in pressure drop is
often small, which makes high-efficiency in-duct filtration
an extremely cost-effective source of EOA.19
Finally, activation of in-zone devices requires additional
electricity consumption. In many cases, these
devices must be " on " or " off, " so the only required data is
power consumption when active. However, the energy
intensity of different devices can vary significantly, with
common residential devices requiring 0.2 W/cfm to
1 W/cfm (0.34 W/(m³/h) to 1.7 W/(m³/h)) of total flow (see
Table 3 in Reference 20) and similar variation for larger
devices that would be more applicable in commercial
settings. Variable-speed devices may have more complicated
power characteristics, which present a further
opportunity for optimization.
We note that when comparing in-zone devices it is
important to focus on total EOA delivery and power
consumption, rather than other metrics like filtration
efficiency. Indeed, HEPA and similar filters impose
significantly larger pressure drops than MERV filters.19
Thus, a device using a high-efficiency MERV filter can
often deliver the same EOA as a HEPA device but with
significantly lower power consumption. Similarly, UVGI
devices in ducts14 or free-standing units15 can almost
completely deactivate infectious particles. As with all
removal devices, if total flow through the device is small
or carries a low concentration of infectious aerosols,
then the overall impact on infection risk will be correspondingly
small.
Overall, the main observation is that the energy impact
of ventilation and supply temperature changes is very
tightly coupled to outdoor air conditions, whereas the
cost of higher-efficiency filtration or in-zone devices
is relatively constant. Thus, when choosing a particular
strategy, it is important to consider how long the
measures are likely to be used and what the weather
conditions will be over that time period. Given these
16
ASHRAE JOURNAL ashrae.o rg M AY 2022
TABLE 1 Information for example climate zones. Minimum ventilation is chosen by
EnergyPlus in accordance with ASHRAE Standard 62.1-2019.3 Economizer cutoff
temperature is taken from ASHRAE Standard 90.1-2019.23
NAME
Cold
Hot Dry
Marine
CITY
IECC
ZONE
Chicago 5A
Hot Humid Houston 2A
Phoenix 2B
Seattle 4C
ASHRAE MINIMUM
VENTILATION (ACH)
0.9
0.9
0.9
0.9
DESIGN
FLOW (ACH)
5.9
5.7
6.3
6.0
ECONOMIZER
CUTOFF (°F)
70
65
75
75
complexities, an opportunity exists to optimize the
chosen strategy on a regular basis using relevant predictive
models.2 Otherwise, a suitable course of action is to
choose a robust strategy that can be applied consistently
and explore other options only when deemed necessary.
Annual Trends
To illustrate the key considerations for EOA delivery
and associated energy consumption, we present example
cases of how these quantities vary throughout a typical
year for a typical office building in various climates.
For this purpose, we use EnergyPlus21 simulations of
the same " Large Office " reference building21 and apply
each of the discussed infection mitigation strategies.
Information about the chosen climate zones, along with
key HVAC parameters, is shown in Table 1. Note that the
HVAC system's design flow varies slightly across the cases,
as HVAC equipment is automatically sized by EnergyPlus
based on the different design climates for each.
For our purposes, we consider the baseline case to be
the 2019 version of ASHRAE Standard 62.1's specified
minimum required ventilation rate with a MERV 8 induct
filter and a supply temperature setpoint of 55°F
(13°C), which we refer to as the " ASHRAE Baseline. "
From here, we then consider five different alternative
cases:
* 0.5× Minimum Ventilation. Minimum ventilation
rate set to half the ASHRAE Baseline value (to represent
buildings that are not adequately ventilated).
* 2× Minimum Ventilation. Minimum ventilation
rate set to double the ASHRAE value.
* MERV 13 Filter. In-duct filter switched from
MERV 8 to MERV 13.
* Higher Supply Temperature. Supply temperature
setpoint increased from 55°F (13°C) to 62°F (17°C).
* +1 ACH In-Zone Filtration. Add portable HEPA
filtration units to provide an extra 1 ACH of EOA (device
power 0.65 W/cfm (1.1 W/(m³/h)).
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