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Supplemental Information for
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Ultrafine particle removal by residential HVAC filters
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Brent Stephens1,* and Jeffrey A. Siegel2,3
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Department of Civil, Architectural and Environmental Engineering, Illinois Institute of
Technology, Chicago, IL USA
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Department of Civil, Architectural and Environmental Engineering, The University of Texas at
Austin, Austin, TX USA
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Department of Civil Engineering, The University of Toronto, Toronto, ON Canada
Author contact information:
Brent Stephens, Ph.D.*
Department of Civil, Architectural and Environmental Engineering
Illinois Institute of Technology
Alumni Memorial Hall Room 212
3201 South Dearborn Street
Chicago, IL 60616
Phone: (312) 567-3540
Fax: (312) 567-3519
Email: brent@iit.edu
*Corresponding author
Jeffrey A. Siegel, Ph.D.
Department of Civil Engineering
University of Toronto
35 St. George St.
Toronto, ON M5S 1A4
Phone: (416) 978-7975
Fax: (416) 978-6813
Email: jeffrey.siegel@utoronto.ca
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UTest House
CO2
CO2
LAUNDRY ROOM
E
LAUNDRY ROOM
LAUNDRY ROOM
CO2
CO2
MASTER
BATHROOM
DINING
ROOM
CO2
MASTER
BATHROOM
CO2
MASTER
DINING E
BATHROOM
ROOM
CO2
ECO
CO2
CO2
CO2
BATHROOMROOM
CONTROL
CONTROL ROOM
CO2
CO2
UP
FLOW
HVAC
E CONTROL ROOM
HVAC
CO2
CO2
CO2
2
DINING
KITCHEN
UP
ROOM
FLOW
OPC
BATHROOM
KITCHEN
BATHROOM
KITCHEN
CO2
E
CO2
CO2
E
UP
FLOW
HVACCO2
OPC
DOWN
FLOW
HVAC
DOWN
FLOW
HVAC
OPC
DOWN
FLOW
HVAC
CO2
COLIVING
ROOM
2
LIVING ROOM
MASTER BEDROOM
SMALL BEDROOM
SMALL E
BEDROOM
E
CO2
SMPS
LIVING ROOM
MASTER BEDROOM
SMALL BEDROOM
E
HEPA
HEPA
MASTER BEDROOM
CO2
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Figure S1. Photograph and floor plan of the test house. The floor plan includes locations of the AHUs,
mixing fans, CO2 instrumentation, incense emission sources (labeled “E”), and the particle
instrumentation (labeled “SMPS”).
HEPA
Airflow Rate Measurement
The airflow measurement procedure was performed once using an Energy Conservatory
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TrueFlow plate and DG-700 digital manometer installed; the airflow rate for each filter condition
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was then estimated by correcting for the measured operating supply plenum pressure using a
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procedure in the TrueFlow plate manual (where the airflow rate with a filter installed is equal to
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the airflow rate with the TrueFlow plate installed multiplied by the square root of the ratio of the
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supply plenum pressure with the filter installed to the supply plenum pressure with the TrueFlow
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plate installed). The flow plate has a manufacturer-reported uncertainty of ±7% of its reading,
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although conversations with the manufacturer suggest lower actual uncertainties for situations
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where repeated flows in the same system are compared; therefore, ±5% was used in previous
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work (Stephens et al., 2010a; Stephens et al., 2010b) and was also used here. Indoor temperature
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and relative humidity were measured upstream of the HVAC system using an Energy
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Conservatory Automatic Performance Testing System with thermistors and capacitance sensors
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connected, both logging at 1-minute intervals (manufacturer-reported accuracies were ±0.2°C for
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temperature and ±5% for RH). The same system also measured and logged the pressure drop
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across the filter (if installed) and the pressure in the supply plenum at the same intervals.
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Air Exchange Rate Measurement
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To measure the air exchange rate (AER) during each test condition, CO2 was injected
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into each room of the house from a cylinder connected to a mass flow controller. Injection
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occurred during the particle concentration elevation phase and proceeded until the CO2
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concentration in each room was at least 500 ppm or greater above background. CO2
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measurements were made at one-minute intervals in 7 locations (6 within the test house and one
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outdoors, as shown in Figure S1) with dual beam infrared absorption CO2 monitors (GE Telaire
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Model 7001) connected to a data acquisition system in the control room. Only data from the CO2
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monitor in the central living room were used to estimate whole-house air exchange rates (AERs)
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using the subsequent tracer gas decay rate, in accordance with ASTM E 741 (2006).
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Estimation of Parameters
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The filter test method relies on a well-mixed size-resolved number balance of particles of
diameter i in the space, assuming no indoor sources of particles, as shown in Equation S1.
dCi ,in

Q
 Pi Ci ,out  Ci ,in  i Ci ,in  i ,HVAC HVAC Ci ,in
dt
V
(S1)
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where Ci,in is the size-resolved indoor particle concentration (# m-3), t is time (hr), Ci,out is the
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size-resolved outdoor particle concentration (# m-3), λ is the air exchange rate (AER, hr-1), Pi is
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the envelope penetration factor (dimensionless), βi is the size-resolved deposition rate of particles
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to indoor surfaces (hr-1), ηi,HVAC is the size-resolved particle removal efficiency of the HVAC
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system (dimensionless), QHVAC is the airflow rate through the HVAC system (m3 hr-1), and V is
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the volume of the building (m3).
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The size-resolved indoor particle loss rate was then estimated using the first-order decay
model in Equation S2.
Ci ,in t   Ci ,in,t  0e
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 i , HVAC Q HVAC

     i 
V


t


(S2)
All of the loss mechanisms present in Equation S2 (λ, βi, and ηi,HVACQHVAC/V) were
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combined into one lumped loss term for each particle size in the number balance and the test
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procedure was repeated for the three basic test conditions: (1) with no filter installed and the
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HVAC system off (i.e., “background” case), (2) with the HVAC system operating and no filter
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installed (i.e., “no filter” case), and (3) with the HVAC system operating with a test filter
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installed (i.e., “filter” case). Because AER was measured simultaneously, λ was subtracted from
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the total loss rate to determine the “effective” loss rate, Li (hr-1), due to surface, HVAC system
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component, and duct interactions alone (in the no filter case) or the combined effects of surface,
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HVAC component, duct, and filter interactions (in the filter installed case).
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Air exchange rates (AERs) were measured during each test in accordance with ASTM E
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741. AERs were estimated using a nonlinear least squares regression on the solution to a well-
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mixed balance on the indoor concentration of tracer gas (CO2), as shown in Equation S3.

Ct ,in t   Ct ,in,t 0 e  t  Ct ,o u t 1  e  t

(S3)
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where Ct,in is the time-varying indoor tracer gas concentration (ppm of CO2), Ct,in,t=0 is the initial
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indoor tracer gas concentration (ppm of CO2 at time t = 0), and Ct,out is the mean outdoor tracer
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gas concentration during each experiment (ppm of CO2). Equation S3 assumes no indoor sources
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of tracer gas during the decay period, which is reasonable for the unoccupied house.
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A nonlinear least-squares regression was performed on the time-series of concentration
data from each aggregate particle size bin to estimate total loss rates using Equation S2. The
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indoor particle concentration [Ci,in(t)] was the dependent variable and time (t) was the
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independent variable, and two size-resolved parameters were estimated: an initial concentration
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(Ci,in at time t = 0) and the overall particle loss rate (λ + Li). Similarly, a nonlinear least-squares
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regression was performed on the CO2 time-series to estimate AER for each test condition using
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Equation S3. The indoor CO2 concentration [Ct,in(t)] was the dependent variable and time (t) was
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the independent variable; again, two parameters were estimated: an initial concentration (Ct,in at
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time t = 0) and the AER (λ).
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Particle Binning
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Because there were more bins associated with very small particles in the raw data (e.g.,
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seven bins for 3.11-3.85 nm particles, five bins for 5.14-5.94 particles, and three bins for 9.14-
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9.82 nm particles, etc.), a greater number of bins were combined for small sizes than for large
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sizes. Particle binning was also done in this manner because very small particles (i.e., < 8 nm)
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were not detected in great enough quantity after incense burning to treat those bins individually.
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This resulted in the following aggregation: 20 bins from 3.11 to 6.15 nm were combined and
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represented as 4.4 nm particles; 13 bins from 6.38 to 9.82 nm were combined and represented as
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7.9 nm particles; 11 bins from 10.2 to 14.6 nm were combined and represented as 12.2 nm
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particles; 8 bins from 15.1 to 19.5 nm and from 20.2 to 25.9 nm were represented as 17.2 nm and
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22.9 nm particles, respectively; the remaining subsequent aggregate size bins were summed
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across five sizes until the last size range, represented by 100 nm, was summed over only four
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bins.
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UFP Generation and Loss Rates
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Figure S2 shows mean (± s.d.) values of the initial indoor particle concentration
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measured across all 40 experiments (i.e., Ci,in at time t = 0 for each experiment), which provides
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an idea of the typical ultrafine particle size distribution resulting from incense burning. Initial
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particle concentrations generated by incense burning were very low for the smallest geometric
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mean particle size (< 5 nm), which led to high uncertainties later in the analysis. Therefore, the
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smallest particle size is excluded from the rest of the analysis; this was not an issue for larger
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particle sizes because initial concentrations were much greater.
-3
)
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1000
500
100
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10
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5
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10
50
Particle Diameter (nm)
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Figure S2. Mean (± s.d.) of indoor particle concentrations measured at the beginning of each of the 40
test runs (when t = 0 in the exponential decay model). Concentrations were intentionally elevated by
burning incense, which produced very few particles less than 10 nm in diameter. The rest of this analysis
excludes the smallest particle size (with a geometric mean dp of 4.4 nm) because of high uncertainties
associated with low concentrations.
Table S1 provides estimates of size-resolved effective loss rates (Li, total particle loss
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rates minus AER) for each of the 8 test conditions. Values represent mean and standard
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deviations of estimates resulting from five replicates for each particle size and test condition.
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Table S1. Mean size-resolved effective loss rates (Li) measured during all test conditions
Geometric
Effective loss rate (hr-1)
mean
No filter
MERV 4 MERV 6 MERV 11 MERV 10 MERV 13 MERV 16
diameter (nm) HVAC off
7.9
2.93 ± 0.95 3.73 ± 0.76 3.26 ± 1.09 4.17 ± 1.21 4.50 ± 0.93 7.06 ± 1.20 9.15 ± 1.50 8.33 ± 3.48
12.2
1.66 ± 0.66 2.50 ± 0.52 2.51 ± 0.65 2.86 ± 0.48 3.54 ± 0.43 3.97 ± 0.78 5.57 ± 1.07 6.28 ± 0.69
17.2
1.14 ± 0.43 1.60 ± 0.49 1.39 ± 0.54 1.77 ± 0.37 2.51 ± 0.41 2.79 ± 0.57 3.86 ± 0.54 5.04 ± 1.51
22.9
0.67 ± 0.30 1.19 ± 0.39 1.13 ± 0.22 1.40 ± 0.28 2.05 ± 0.17 2.14 ± 0.37 3.30 ± 0.33 5.05 ± 0.45
28.9
0.51 ± 0.19 1.07 ± 0.20 1.00 ± 0.16 1.19 ± 0.19 1.82 ± 0.14 1.92 ± 0.56 2.78 ± 0.53 4.66 ± 0.39
34.6
0.43 ± 0.13 0.94 ± 0.13 0.91 ± 0.10 1.06 ± 0.13 1.78 ± 0.11 1.80 ± 0.44 2.70 ± 0.31 4.75 ± 0.35
41.4
0.36 ± 0.11 0.83 ± 0.09 0.81 ± 0.08 0.95 ± 0.09 1.76 ± 0.08 1.76 ± 0.48 2.72 ± 0.28 4.73 ± 0.31
49.6
0.29 ± 0.10 0.74 ± 0.07 0.69 ± 0.08 0.79 ± 0.09 1.67 ± 0.10 1.83 ± 0.27 2.79 ± 0.21 4.82 ± 0.46
59.3
0.22 ± 0.10 0.65 ± 0.06 0.58 ± 0.09 0.69 ± 0.08 1.62 ± 0.15 1.86 ± 0.12 3.04 ± 0.15 5.06 ± 0.69
71.1
0.18 ± 0.09 0.59 ± 0.05 0.50 ± 0.08 0.60 ± 0.07 1.50 ± 0.18 1.85 ± 0.10 3.24 ± 0.13 5.22 ± 0.60
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0.13 ± 0.08 0.53 ± 0.05 0.44 ± 0.06 0.54 ± 0.07 1.43 ± 0.21 1.77 ± 0.15 3.41 ± 0.17 5.57 ± 0.54
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0.11 ± 0.08 0.48 ± 0.05 0.41 ± 0.06 0.47 ± 0.07 1.33 ± 0.21 1.66 ± 0.17 3.61 ± 0.18 5.71 ± 0.56
Values represent the mean ± standard deviation of five replicate tests for each condition.
MERV 4, 6, and 11 are 2.5 cm filters; MERV 10, 13, and 16 are 12.7 cm filters.
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UFP Removal Efficiency
Table S2 summarizes mean and absolute uncertainty values of particle removal efficiency
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measured during seven test cases: one with the HVAC system on without a filter, followed by
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the six filter test conditions.
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Table S2. Measured removal efficiency of the duct system and six test filters across 12 particle sizes
Geometric
mean
diameter
(nm)
Duct
removal
efficiency
No filter
Filter removal efficiency
MERV 4
MERV 6
MERV 11
MERV 10
MERV 13
MERV 16
7.9
12 ± 5%
-5 ± 2% 12 ± 5%
17 ± 5%
54 ± 15%
12.2
12 ± 6%
2 ± 1%
9 ± 3%
20 ± 5%
25 ± 7%
17.2
7 ± 3%
-2 ± -1%
5 ± 2%
17 ± 6%
20 ± 7%
22.9
8 ± 4%
0 ± 0%
5 ± 2%
15 ± 5%
16 ± 6%
28.9
8 ± 3%
0 ± 0%
3 ± 1%
13 ± 3%
14 ± 5%
34.6
7 ± 2%
0 ± 0%
3 ± 1%
15 ± 2%
14 ± 4%
41.4
7 ± 2%
0 ± 0%
3 ± 0%
16 ± 2%
15 ± 4%
49.6
7 ± 2%
0 ± 0%
2 ± 0%
16 ± 2%
17 ± 3%
59.3
6 ± 3%
-1 ± 0%
2 ± 0%
16 ± 3%
19 ± 3%
71.1
6 ± 3%
-1 ± 0%
1 ± 0%
15 ± 2%
20 ± 2%
85.0
6 ± 4%
-1 ± 0%
1 ± 0%
15 ± 3%
20 ± 3%
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5 ± 4%
-1 ± 0%
0 ± 0%
14 ± 3%
18 ± 3%
Values represent the mean ± absolute uncertainty across five replicate tests.
91 ± 25%
52 ± 15%
38 ± 13%
35 ± 12%
28 ± 8%
29 ± 6%
31 ± 5%
33 ± 5%
39 ± 5%
43 ± 5%
46 ± 6%
50 ± 6%
75 ± 35%
61 ± 15%
55 ± 24%
61 ± 21%
57 ± 12%
60 ± 10%
62 ± 9%
64 ± 10%
70 ± 13%
73 ± 12%
79 ± 12%
82 ± 13%
MERV 4, 6, and 11 are 2.5 cm filters; MERV 10, 13, and 16 are 12.7 cm filters
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References in the SI
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ASTM E 741, (2006). Standard Test Method for Determining Air Change in a Single Zone by
Means of a Tracer Gas Dilution.
Stephens, B., Novoselac, A., Siegel, J.A., (2010). The effects of filtration on pressure drop and
energy consumption in residential HVAC systems. HVAC&R Research 16, 273–294.
Stephens, B., Siegel, J.A., Novoselac, A., (2010). Energy implications of filtration in residential
and light-commercial buildings (RP-1299). ASHRAE Transactions 116, 346–357.
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