ASHRAE Journal - June 2024 - 18

COLUMN INFECTIOUS AEROSOL CONTROL
TABLE 1 Cost comparison of various interventions.
INTERVENTION
Outside Airflow
IRAC Installation
Increased Filtration
COST/YEAR WITH 10% IRMM
$535.68
$54.87
$24.82
COST/YEAR WITH 50% IRMM
$2,678.41
$274.37
$248.18*
*Increased filtration would remain installed in normal mode.
increased to account for the difference in energy prices
since 2004, which were an average of $0.06/kWh. Using
the 2023 average cost of electricity for cities in Texas
($0.174/kWh8), this annual energy cost would go up to
$5,356.82. Moreover, increasing the demands on the
heating and cooling components of this HVAC system
by more than 230% will likely require installing a new
system, further increasing the expense of this option.
The clean air shortfall can also be addressed by
installing one or more IRACs that contribute toward
317 cfm (150 L/s) of clean airflow. Four devices that
provide 95 cfm (45 L/s) of clean air each could be
used to exceed the Standard 241 requirements with
a total of 380 cfm (179 L/s) of equivalent outdoor air.
A commercially available device that provides this
amount of clean airflow reports an energy use of 90 W of
electricity per device, for an annual energy consumption
of 788.4 kWh per device. At the same electricity cost of
$0.174/kWh, the annual energy cost for all four devices
used to clean the room would be a total of $548.73. As
a cautionary note, IRACs must be tested according to
Appendix A of Standard 241 to verify the amount of
clean air that they provide. Make sure to ask the device
manufacturer for their test report.
The third option is to meet the clean airflow
requirement by increasing the filtration of recirculated
air above a MERV 8. To achieve the target 317 cfm
(150 L/s) of additional airflow, a minimum of 47.5% of
the existing recirculated airflow would have to be clean
air. This could be achieved using a MERV 11 filter, but
this analysis will consider the upgrade to a MERV 13
filter since better data on energy use is available. A
simulation of energy costs associated with different
HVAC configurations9 found that increasing filtration
to MERV 13 would increase annual fan energy by 14.3%.
When a system used 100% outdoor air, fan energy
made up 28.9% of the total energy expenses. A crude
approximation of the cost of upgrading the sample room
to a MERV 13 filter can therefore be made by taking
28.9% of the cost for using all outdoor air above (the fan
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ASHRAE JOURNAL ashrae.org J U N E 2024
energy of that solution) and multiplying by 14.3%. This
results in an estimated cost of $221.38.
A survey of filter costs for three common filter sizes
(30 × 36 × 1, 24 × 24 × 1, 24 × 36 × 1) showed the average
difference in filter cost when upgrading from a MERV
8 to a MERV 13 is approximately $6.70 per filter.10 If
filters are replaced every 90 days, the annual added
filter cost will be $26.80. (All prices are from Dec. 2023.)
One assumption is the existing HVAC system is capable
of supporting the filters' increased back pressures.
Otherwise, HVAC upgrade costs also need to be included.
These energy costs must be multiplied by an
assumption of how often IRMM will be used. The
outdoor airflow adjustments and IRAC operation are
easily adjustable via central controls and are therefore
assumed to represent variable costs that will only be
incurred during IRMM. It is uncertain whether a facility
would choose to maintain two filter stockpiles and swap
all of its filters from MERV 8 to MERV 13 and back again
whenever IRMM was initiated, or whether they would
leave the MERV 13 filters installed all the time. For this
analysis, it is assumed MERV 13 filters would be swapped
out if IRMM was infrequent, but would be constantly
used to avoid higher maintenance costs if IRMM is
frequent. This results in the final costs shown in Table 1.
In this sample room, increased filtration ends up
having the lowest energy costs by $26-$30 per year.
One last point to consider is how the magnitude of these
savings compares to a potential HVAC system upgrade.
This would be necessary either if the existing system
cannot overcome the back pressure of the finer filter
or if the existing system hadn't already been providing
such a high amount of recirculated airflow (a different
setup than the sample room). Taking the lowest end of
the numbers given above and neglecting labor, replacing
the HVAC unit would have a cost of approximately
$25/ft2 ($269/m2). For this sample room, that would be
a cost of $12,500. Even after including the cost of purchasing
and installing IRACs, the savings of the filtration option compared
to IRAC installation would not add up to the HVAC upgrade cost
over the course of the HVAC unit's entire lifetime. Therefore,
increasing filtration is the lowest cost option only when
no upgrades to the existing system are required.
Conclusions
There are many factors to consider in creating a
cost estimate for different methods of Standard 241
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