ASHRAE Journal - December 2020 - 40

TECHNICAL FEATURE

highly variable depending on open area, wind velocity
and temperature differentials, open windows are not
evaluated here as an independent option.

HVAC System Characteristics
For central HVAC systems, the fan can be run continuously by switching to fan-on mode on the thermostat. If
not switched to " on, " the fan runs when heating or cooling is needed based on the thermostat setting (unless it
is programmed to run on a schedule). Because the HVAC
is a protection system, for this evaluation assume the
fan runs 100% of the time to obtain maximum benefit of
the filter. The fan with the filter is like the mask for your
house; it doesn't work if it's not on.

Filters
The level of filtration can be improved by purchasing higher-rated filters. In the U.S., filters are rated by
MERV10 ranging from MERV 4 to MERV 16, with higher
numerical ratings indicating better filtration. Typical
residential filters are MERV 4. Better filters commonly
available to consumers include MERV 7, MERV 8 and
MERV 11. Filters higher than MERV 11 are specialized and
may not be suitable for installing in many residential
HVAC systems because of higher pressure drop (resistance to airflow that cannot be overcome by the fan).
In this case, we assume that the filter face velocity is
250 fpm (1.3 m/s) for which MERV 8 and MERV 11 filters
are available with an initial pressure drop of 0.1 in. w.c.
(25 Pa) or lower.
Many residential central HVAC systems have a total fan
pressure capacity of 0.5 in. w.c. (125 Pa),11 which must
include the filters, coils and ductwork. Some low pressure drop filters rated higher than MERV 11 may be available; however, caution is advised since pressure drop is
velocity dependent, and filter face velocity in residential
HVAC can range from 160 fpm to 500 fpm (0.8 m/s to
2.5 m/s).12 A hypothetical example for a MERV 14 filter is
illustrated in case one can find such a filter with acceptable pressure drop. The filter efficiency is taken from
Stephens13 based on the size distribution of viral particles and shown in Table 2.

Ventilation
Ventilation for residences is specified by ASHRAE
Standard 62.2-2019.14 Ventilation may be provided by
mechanical ventilation, exhaust systems, infiltration or
40

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TABLE 2 Assumed filter efficiency by MERV. This filter efficiency assumes the

following particle size distribution: 15% are 0.3 µm - 1 µm, 25% are 1 µm - 3 µm,
and 60% are 3 µm - 10 µm.
MERV

0.3 µm - 1 µm

1 µm - 3 µm

3 µm - 10 µm

DROPLET
NUCLEI-WEIGHTED
h FILTER

15%

11%

17%

46%

50%

44%

30%

65%

85%

72%

70%

90%

87%

80%

90%

89%

90%

95%

HEPA

99.90%

a combination. The relevant effect on exposure in this
case is air changes with the outdoors with an implicit
assumption that no infectious particles are brought in
from the outdoors. The term " air change " is the variable used in the model and will be used to describe
ventilation.

Portable Air Cleaning
Stand-alone portable air cleaning devices are rated in
terms of clean air delivery rate (CADR) by the Association
of Home Appliance Manufacturers (AHAM).15 Certified
ratings are similar to those used in commercial applications such as AHRI or AMCA. Manufacturer's claims about
effective square foot coverage are not the same as a certified CADR. CADR rates the measured system effectiveness, which is often not the same as an assumed cfm multiplied by an assumed filter efficiency. Filters or devices
without MERV or CADR ratings are not recommended
because no basis exists for comparison or assurance of
performance. For more information on residential filtration, including information on ozone and other " purifiers, " please refer to the " ASHRAE Position Document
on Filtration and Air Cleaning, " 16 the " EPA brochure
Residential Air Cleaners,17 and EPA web pages.18,19,20

Infection Risk
One often-used approach to estimate the risks associated with airborne transmission of respiratory diseases
is the Wells-Riley model. The Wells-Riley model is based
on a concept of " quantum of infection, " whereby the rate
of generation of infectious airborne particles (or quanta)
can be used to model the likelihood of an individual in
a steady-state, well-mixed indoor environment being

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ASHRAE Journal - December 2020

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