ASHRAE Journal - September 2019 - 20

TECHNICAL FEATURE

FIGURE 4 Modeled fine particle removal.

FIGURE 5 CADR rating label for portable air cleaners.

100
80
MERV 13

70
60
50
40

MERV 8

30
20
0
0

10

20

30
40 50
60
70
80 90 100
Air Handler Fan Runtime (%)
Monte Carlo model simulating removal of 1 to 3m particles from a home, based on a range of
air leakage rates from outdoors. Upper boundaries estimate effectiveness of a perfectly installed
system and filter in a tight house. Lower boundaries estimate effectiveness in a leaky house.

been changed for a few years, or if the system simply
does not run for more than a small percentage of the
hours in a year, then its effectiveness (removal of fine
particles from the space) will be far below the lab-tested
removal of particles from the airstream.
Ultimately, the advice to consumers to choose a MERV
13 filter is based on the fact that with the low run-times
typical of houses and apartments in North America, the
MERV 13 filter has a better chance of removing a significant percentage of the fine particles of greatest health
concern. At present, an informal survey of residential air
filter distribution conducted by one of the authors suggests that filters rated at MERV 13 and above currently
account for about 15% of annual residential air filter
sales.14 It could be interesting to follow how this percentage changes over the next few years, in light of the EPA
guidance.
Figure 4 shows the results of a mathematical model that
compares the best-case effectiveness of an ideal system
with respect to removing fine particles from a home.
(Note that perfect installation and maintenance of filters
is the modeling assumption, namely: no air bypasses the
filter, the filter is clean and remains so over a year, and
that air velocity is ideal rather than the lower-than-ideal
flow that is more commonly-observed in domestic HVAC
systems.) This Monte Carlo simulation of 1,000,000
cases estimates removal of fine particles (from the
space) using MERV 8 and MERV 13 filters. Recall that
domestic systems typically operate less than 20% of the
20

ASHRAE JOURNAL

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S E P T E M B E R 2 0 19

AHAM

10

MARIANNE TOUCHIE & JEFFREY SIEGEL

Modeled Removal Effectiveness (%)

90

hours in the year.13 The model suggests that if all other
factors were equal, a perfectly installed MERV 13 filter
has the potential to remove 3x more fine particles than a
MERV 8 filter (45% v. 15% of particles in the 1-3 micrometer range).
HVAC professionals will also understand that
improved indoor air quality comes with some increase
in operational cost. The technical summary addresses
the issues of energy and HVAC system capabilities.
Certainly for all portables, better filtration effectiveness
(through increasing operating hours) comes at the cost
of its fan energy consumption. Also, in central systems
more hours of operation mean better filtration-but
also more fan energy cost. Also higher levels of filtration
can sometimes add pressure drop that reduces airflow
low enough to affect heating and cooling effectiveness,
although certainly not in all systems.15,16 So the advice
to select a MERV 13 filter was not arrived at casually. The
technical summary addresses these issues in considerable detail, based on field measurements of both energy
consumption and airflow rates through different MERVrated filters in typical residential HVAC systems.

Portable Air Cleaner Recommendations
The EPA recommends choosing a portable air cleaner
that provides a high clean air delivery rate (CADR)
when removing smoke-sized particles. There are challenges when providing technically-robust guidance that


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ASHRAE Journal - September 2019

Table of Contents for the Digital Edition of ASHRAE Journal - September 2019

Contents
ASHRAE Journal - September 2019 - Intro
ASHRAE Journal - September 2019 - Cover1
ASHRAE Journal - September 2019 - Cover2
ASHRAE Journal - September 2019 - 1
ASHRAE Journal - September 2019 - Contents
ASHRAE Journal - September 2019 - 3
ASHRAE Journal - September 2019 - 4
ASHRAE Journal - September 2019 - 5
ASHRAE Journal - September 2019 - 6
ASHRAE Journal - September 2019 - 7
ASHRAE Journal - September 2019 - 8
ASHRAE Journal - September 2019 - 9
ASHRAE Journal - September 2019 - 10
ASHRAE Journal - September 2019 - 11
ASHRAE Journal - September 2019 - 12
ASHRAE Journal - September 2019 - 13
ASHRAE Journal - September 2019 - 14
ASHRAE Journal - September 2019 - 15
ASHRAE Journal - September 2019 - 16
ASHRAE Journal - September 2019 - 17
ASHRAE Journal - September 2019 - 18
ASHRAE Journal - September 2019 - 19
ASHRAE Journal - September 2019 - 20
ASHRAE Journal - September 2019 - 21
ASHRAE Journal - September 2019 - 22
ASHRAE Journal - September 2019 - 23
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ASHRAE Journal - September 2019 - 25
ASHRAE Journal - September 2019 - 26
ASHRAE Journal - September 2019 - 27
ASHRAE Journal - September 2019 - 28
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ASHRAE Journal - September 2019 - 30
ASHRAE Journal - September 2019 - 31
ASHRAE Journal - September 2019 - 32
ASHRAE Journal - September 2019 - 33
ASHRAE Journal - September 2019 - 34
ASHRAE Journal - September 2019 - 35
ASHRAE Journal - September 2019 - 36
ASHRAE Journal - September 2019 - 37
ASHRAE Journal - September 2019 - 38
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ASHRAE Journal - September 2019 - 49
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ASHRAE Journal - September 2019 - 51
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ASHRAE Journal - September 2019 - 55
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ASHRAE Journal - September 2019 - Cover3
ASHRAE Journal - September 2019 - Cover4
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