ASHRAE Journal - November 2019 - 77

COLUMN IEQ APPLICATIONS

middle, and end of each filter's life and long-term measurements of indoor particle concentrations and HVAC
operation status. A central theme of the project was on
redundant measurements: using different approaches
and/or different instruments to calculate important performance parameters. The idea was to ensure data quality
and also be able to tell as complete a story about filtrations as possible.
One of the most important measurements is the in-situ
removal efficiency, where we sampled 10 nm - 10 µm
particle concentrations up and downstream of the filter
with isokinetic nozzles and wide range particle counters. One of the challenges with measuring efficiency in
residential systems is that they are messy: airflows are
turbulent and poorly mixed, systems aren't standardized in terms of layout or geometry, and the installer
makes a lot of decisions about everything from airflow
rates to the layout for the ducts. We conducted the test
with and without the filter in place to account for any
difference in particle concentrations because of things
other than the filter. To make sure that we had enough
particles for the measurement, we elevated particles
inside the home by boiling water on the stove to generate small particles and shaking pillows to generate large
particles. We thus had 160 measurements of new and
used filter size-resolved efficiency at the end of the project (20 homes × 4 filters/home × 2 measurements/filter),
excluding repetitions. The results present a pretty clear
picture: when new, the MERV 14E filters were usually
better than the other filters in the same home, but there
was a lot of variation between homes. Overall, there was
less of a difference in efficiency between MERV 8 and
MERV 14E than the ANSI/ASHRAE Standard 52.2-2017,
Method of Testing General Ventilation Air-Cleaning Devices for
Removal Efficiency by Particle Size, test results would suggest. Things got even messier after three months. On
average, the non-electret (MERV 8) filters performed
about the same, but the electret filters declined in efficiency, on average. As a general phenomenon, this is
well-known within the filtration community, but the
range of change in performance for electret was quite
large: some actually improved a little and some declined
considerably. This whole idea that filtration efficiency
is dynamic is an important concept-specifying a filter
with a certain efficiency from a laboratory test is not a
guarantee that you will actually get that performance
when the filter is installed in a home.

The primary way that I like to think about these results
is through one of feedback mechanisms. When you
add a filter to a system, it changes the system. One big
change has to do with the fan. For many fans in residential systems, a filter with a bigger flow resistance (either
because it has more filter media in it or because it is
dirty) will cause airflow to go down. When airflow goes
down, filtration efficiency changes (because it is dependent on the air velocity) and pressure drop decreases
from what it would be at the original flow. These are not
simple impacts: every fan is different, and every system
is different, meaning that in some homes this is a big
impact and in others it is a small impact. Furthermore,
there is an important particle size dependence: in general, lower airflow will lead to higher efficiency for very
small particles (because they have more time to diffuse
to filter fibers) and lower efficiency for larger particles
(because they are less likely to impact on filter fiber).
Depending on what size particles you care about (and
what your chosen filtration standard measures) this can
be a good or a bad thing. One final complication is that
there are different types of fan motors used in residential systems: permanent split capacitor (PSC) motor fans
are the conventional choice and they are usually sensitive to filter pressure drop, and electrically commutated
motor (ECM) fans often have speed control and increase
the fan speed if there is increased pressure drop (i.e.,
from a filter). This means that to really understand how
a filter is going to perform you need to know a lot about
the system and the fan, not just about the filter.
Another good example of a feedback mechanism is
filter bypass. Filters don't fit perfectly in their slot and
there is often an air gap around the filter. If a filter has
a higher flow resistance, more air gets pushed through
the gap than would happen for a lower-flow resistance
filter. This gap is usually not very good at filtering particles. Laboratory test standards don't include bypass
(for the obvious reason that the geometry of the bypass
gap is different in every situation) and this is one of the
big differences in in-situ performance. Another kind
of bypass, which happened in 12 of the homes, was that
they had a filter slot that was open to the basement
on one side. This makes it easy for the homeowner to
change the filter, but it also means that one edge of the
filter has a big bypass gap. How much air flows through
this gap depends on the characteristics of the filter and
fan, as well as the geometry of the gap. One difference
N O V E M B E R 2 0 19

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ASHRAE Journal - November 2019

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

Contents
ASHRAE Journal - November 2019 - Intro
ASHRAE Journal - November 2019 - Cover1
ASHRAE Journal - November 2019 - Cover2
ASHRAE Journal - November 2019 - 1
ASHRAE Journal - November 2019 - Contents
ASHRAE Journal - November 2019 - 3
ASHRAE Journal - November 2019 - 4
ASHRAE Journal - November 2019 - 5
ASHRAE Journal - November 2019 - 6
ASHRAE Journal - November 2019 - 7
ASHRAE Journal - November 2019 - 8
ASHRAE Journal - November 2019 - 9
ASHRAE Journal - November 2019 - 10
ASHRAE Journal - November 2019 - 11
ASHRAE Journal - November 2019 - 12
ASHRAE Journal - November 2019 - 13
ASHRAE Journal - November 2019 - 14
ASHRAE Journal - November 2019 - 15
ASHRAE Journal - November 2019 - 16
ASHRAE Journal - November 2019 - 17
ASHRAE Journal - November 2019 - 18
ASHRAE Journal - November 2019 - 19
ASHRAE Journal - November 2019 - 20
ASHRAE Journal - November 2019 - 21
ASHRAE Journal - November 2019 - 22
ASHRAE Journal - November 2019 - 23
ASHRAE Journal - November 2019 - 24
ASHRAE Journal - November 2019 - 25
ASHRAE Journal - November 2019 - 26
ASHRAE Journal - November 2019 - 27
ASHRAE Journal - November 2019 - 28
ASHRAE Journal - November 2019 - 29
ASHRAE Journal - November 2019 - 30
ASHRAE Journal - November 2019 - 31
ASHRAE Journal - November 2019 - 32
ASHRAE Journal - November 2019 - 33
ASHRAE Journal - November 2019 - 34
ASHRAE Journal - November 2019 - 35
ASHRAE Journal - November 2019 - 36
ASHRAE Journal - November 2019 - 37
ASHRAE Journal - November 2019 - 38
ASHRAE Journal - November 2019 - 39
ASHRAE Journal - November 2019 - 40
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ASHRAE Journal - November 2019 - 88
ASHRAE Journal - November 2019 - Cover3
ASHRAE Journal - November 2019 - Cover4
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