ASHRAE Journal - June 2024 - 17

COLUMN INFECTIOUS AEROSOL CONTROL
makes calculating energy costs straightforward. The
installation costs are also simple since the main driver
of cost will be the price of each device. These devices are
easy to turn on and off, making it possible to only use
them during IRMM. In addition to these benefi ts, recent
studies have shown that IRACs can achieve signifi cant
reductions in environmental contamination,4 going
beyond the performance minima set by Standard 241.
Achieving Standard 241 compliance through
adjustments to the existing HVAC system in a building
is more complicated to analyze for a variety of reasons.
First, it is possible that the clean airfl ow needed to
maintain proper indoor air quality (IAQ) as required
by ASHRAE Standard 62.1, 62.2 or 170 (prerequisites
for Standard 241) may already be suffi cient to meet the
requirements of Standard 241. This scenario can arise
when the building readiness plan of Standard 241 only
allows a low occupancy during IRMM. For rooms where
reducing occupancy to this extent is not an option,
supplemental air treatment will usually be required.
The second highly variable expense to consider
is whether the existing HVAC system is capable of
achieving the clean airfl ow requirements of Standard
241. If larger ductwork, new fi lter racks or an upsized
air-handling unit is required, the up-front installation
costs can quickly exceed any potential savings compared
to using IRACs for Standard 241 compliance. Typical
pricing for these types of installations can be:5
* $25/ft2 to $60/ft2 ($269/m2 to $646/m2) for HVAC
unit replacement.
* $12 to $25 per linear foot ($39 to $82 per linear foot)
for ductwork replacement.
* $75 to $150 per hour for labor.
Third, energy costs associated with increased airfl ow
through the HVAC system are nonlinear, and even
change day to day.6 These complexities are further
compounded when estimating how much costs will
rise when switching to a more robust fi lter, such as
upgrading from a MERV 8 to a MERV 13. For these
reasons, a qualifi ed professional should be consulted
regarding possible changes to any existing system. For
this column, energy costs associated with HVAC systems
are taken from peer-reviewed studies and governmental
reports, but each location will have unique expenses.
Energy Cost Analysis in a Sample Hospital Room
Since ASHRAE Standard 170 compliance is a
prerequisite to Standard 241 compliance in hospitals,
the actual burden of Standard 241 compliance during
IRMM will be the difference between how much clean
airfl ow is required by Standards 170 vs. 241. This
can be diffi cult to estimate, as the two standards use
different requirements for clean airfl ow. ASHRAE
Standard 170 mandates a certain amount of clean air
exchanges (airfl ow/room volume) while Standard
241 requires a certain amount of clean air per person
(airfl ow/occupancy).
The sample room that is considered here is a
500 ft2 (46 m2) emergency waiting room that meets the
minimums of ASHRAE Standard 170. This means that
the waiting room has a minimum of 2 air changes per
hour (ach) of outdoor air (133 cfm [63 L/s] with an 8 ft
[3 m] ceiling) and a minimum of 12 ach of total air, with
a minimum fi ltration of MERV 8 on recirculated air. The
recirculated air is taken as providing the other 10 ach in
this analysis, for a volume fl ow rate of 667 cfm (315 L/s).
A room this size might normally have a maximum
occupancy of 16 (30 ft2 [2.8/m2] per person), but the
building readiness plan for this facility may specify
a lower maximum occupancy during IRMM. For this
analysis, the maximum occupancy is arbitrarily chosen
as fi ve people, which may represent one staff person
who is admitting patients and up to four people who are
waiting to be seen. Table 5-1 of Standard 241 requires
that a hospital waiting room maintain 90 cfm (42 L/s) of
clean air per person, which would be 450 cfm (212 L/s)
with the reduced occupancy of fi ve people. The 133 cfm
(63 L/s) of outdoor air in the existing system would
count toward this total, but the 667 cfm (315 L/s) of
recirculated air would not because a MERV 8 fi lter does
not provide any clean airfl ow per Table 7-1 of ASHRAE
Standard 241. There is therefore a shortfall of clean
air equal to 317 cfm (150 L/s). The goal for a facilities
engineer would be to make up this difference in a low
cost and energy effi cient fashion.
While the cost of conditioning outdoor air can vary
signifi cantly by location, a survey of Texas hospitals
in 20047 found that the average annual cost for HVAC
system operation was $1.55/ft2 ($16.68/m2). If a linear
relationship exists between volume of outdoor air
and energy costs, then adding an additional 317 cfm
(150 L/s) of outdoor air at all times would be expected to
cost $3.69/ft2 ($39.72/m2), for an added annual energy
cost to this room of $1,847.18. This number must be
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17
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ASHRAE Journal - June 2024

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