United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-97/066 August 1997
SEPA Project Summary
Phase I Pilot Air Conveyance
System Design, Cleaning, and
Characterization
Douglas W. VanOsdell, Karin K. Foarde, and Roy Fortmann
Air conveyance system (ACS) clean-
ing, is advertised to homeowners as a
service having a number of benefits,
including the improvement of indoor
air quality (IAQ). Because ACS clean-
ing includes many procedures applied
to many different duct systems, evalu-
ation has been difficult and the effec-
tiveness of ACS cleaning has not been
adequately measured.
The objective of this project was to
develop and refine surface and airborne
contamination measurement tech-
niques that could be used to evaluate
ACS cleaning. The research was in sup-
port of a field study to be conducted
later. To this end, a pilot air convey-
ance system (PACS) using full-size resi-
dential heating and air-conditioning
(MAC) equipment was constructed and
operated to provide a controlled, artifi-
cially soiled, ACS environment. The
PACS consisted of ducts, an MAC unit,
a dust mixing room, an instrument
room, and a dust generation and injec-
tion system. Three types of duct sys-
tems were evaluated with the proposed
measurement methods under unsoiled
and soiled conditions. Each duct sys-
tems was then cleaned by professional
ACS cleaners and reevaluated.
As a result of the pilot study, the
surface contamination measurement
methods were evaluated over a range
of conditions and improvements. Sur-
face contamination (microbial and total
dust) measurement methods and visual
inspection showed that the pilot unit
was effectively cleaned by the meth-
ods applied during this study. Submi-
cron and larger particle counts were
reduced following ACS cleaning, and
respirable particle mass was reduced
for two of the three duct systems tested.
The significance of these results in an
actual residence was not determined.
This Project Summary was developed
by EPA's National Risk Management
Research Laboratory's Air Pollution
Prevention and Control Division, Re-
search Triangle Park, NC, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction
The overall objectives of the Air Pollu-
tion Prevention and Control Division's
(APPCD's) Air Duct Cleaning Program are
to determine when and how to clean ven-
tilation systems, and evaluate both the
effectiveness of such cleaning and its im-
pact on IAQ. Little published research data
are available to support the IAQ improve-
ment claims sometimes made by ACS
cleaning contractors, and the data that
are available are difficult to interpret. A
two-phase research program was under-
taken to develop evaluation methods to
achieve these objectives. This report de-
scribes research conducted under Phase
I of the program. The research was a
cooperative effort, led by RTI, with partici-
pation by personnel from APPCD, Acurex
Environmental, Inc., and the National Air
Duct Cleaners Association (NADCA). Its
objective was to develop and operate a
PACS as a test bed suitable for:
• Development and testing proposed
ACS cleaning evaluation methods,
and
• Comparison of IAQ instrumentation,
intended for use in the field, under
controlled conditions that were also
as realistic as possible.
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Phase II, a field study to be reported
separately, was undertaken to evaluate
ACS cleaning in actual residential use.
Though commonly referred to as "duct
cleaning," ACS cleaning properly includes
the cleaning of all air-side components of
a ventilation system: air handler, heat
exchanger, humidifier, blower, and duct
system. It is sometimes advertised to
homeowners as a service capable of pre-
venting and possibly mitigating IAQ prob-
lems and may improve the energy effi-
ciency of the relatively dirty systems. ACS
cleaning is a broadly defined service, with
a wide range of cleaning apparatus used
by different contractors and different parts
of the system cleaned using different
equipment. Many combinations of clean-
ing procedures can be used on any given
system, and there are many types of sys-
tems.
Procedures
Pilot Air Conveyance System
A PACS was constructed in a high-bay
laboratory to allow artificial soiling of air
duct components using a reasonable test
aerosol. As shown in Figure 1, the pilot
system included commercially available
components expected to accumulate dust
in varying degrees (e.g., bends, diffusers,
registers, grills, blowers, heat exchang-
ers, expansions, contractions, regions of
surface irregularity, and dampers) and was
designed to allow application of all as-
pects of the proposed evaluation methods
with the exception of evaluation of IAQ in
residences, including pre- and post-clean-
ing inspections and evaluations. The MAC
equipment was all scaled for a small resi-
dence. The PACS was constructed of the
following modules to simplify cleaning and
allow substitution of new test components:
1) supply and return air ducts, 2) MAC
unit including air-conditioning coil and heat
exchanger, 3) dust mixing room, 4) instru-
ment room, and 5) dust generation sys-
tem. The PACS was operated in two
modes:
• normal operation with flow into both
rooms, and
• bypass of the instrument room during
dust injection.
Standard, commercially available MAC
equipment was used when possible. So
that evaluation methods could be devel-
oped for the three major duct materials,
completely separate PACS duct systems
were constructed of the three duct materi-
als commonly utilized in residential MAC:
galvanized steel, fibrous glass duct liner
(FDL), and fibrous glass duct board (FOB).
A new air handler was installed for each
duct type.
Supply air sample locations S1-S4
53—7
Dust
feed
r^Oj
S4-
S2-
\
Supply air
Temp, RH, and flow sensors
,— S1
Dust mixing room
Humidifier
Supply air duct
I I I I I I I
Tim
Instrument
room
R1 .
Return air duct R2
R3
Air
handling
unit
R4
Return air sample locations R1-R4
Return air temp
RH, and flow sensors
Figure 1. Elevation view of PACS.
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>ACS Cleaning Measurement
Methods
The measurement parameters and sam-
pling and analysis methods evaluated
were: 1) total surface dust loading, 2)
culturable microbial surface loading, 3) re-
spirable aerosol concentrations (PM10 and
PM25), 4) size dependent particle number
concentration, 5) airborne fiber mass and
counts, and 6) fungal bioaerosol concen-
trations.
The primary sampling method being
evaluated (and hence, the parameter of
greatest interest during the study) was
measurement of the mass of dust on sur-
faces of the ACS components prior to and
following ACS cleaning. Several variations
of vacuum sample collection were evalu-
ated, the most promising being the Me-
dium Volume Dust Sampler (MVDS), which
drew 10 L/min of air through a small brush
or a custom designed slit nozzle.
A second important parameter mea-
sured was the number of culturable mi-
crobial organisms deposited on the duct
and MAC surfaces within a 10 cm2 tem-
plate area, as measured with a swab tech-
nique and a 10 L/min vacuum method.
Because of the short time between inject-
ing the dust and sampling during most of
these tests, microbial growth on the duct
was not an issue during this research.
Airborne particle mass was measured
with integrated size dependent impactor
samplers at 10 L/min. Continuous optical
particle monitors were set up in the instru-
mentation room of the PACS, measuring
number concentration in three ranges:
greater than 0.5 urn, greater than 5 urn,
and over 16 differential channels between
0.1 and about 10 |jm Bioaerosol samples
were also collected in the room using a
28.3 L/min slit to agar impactor. Although
the data, particularly those collected with
the continuous optical particle counters,
showed changes in particle concentrations
during the test, the reader is cautioned
not to draw conclusions about the impact
of ACS cleaning on airborne particle con-
centrations based on the data presented
in the report due to the limited scope of
the measurements and the artificial na-
ture of the dust deposits and physical
arrangement of the room in which the
measurements were made.
Application of the Test Methods
in the PACS
A complete test series for a single duct
system consisted of a number of operat-
ing periods: 1) installation of the new duct
system and checkout; 2) pre-soiling eval-
uation of the duct and MAC unit, 3) duct
soiling and deposited dust conditioning, 4)
post-soiling, pre-cleaning evaluation of the
duct and MAC unit, 5) post-soiling, pre-
cleaning air sampling, 6) cleaning the ven-
tilation system, 7) post-cleaning evalua-
tion before restarting the MAC unit, 8)
immediate post-cleaning air sampling,
9) post-cleaning air sampling for total par-
ticles after cleaning, and 10) post-clean-
ing (24-hr) sampling.
ACS Cleaning Methods
Several procedures used during the
cleaning phase of PACS operation were
common to all three duct systems: 1) the
supply and return grills were removed,
power washed, and replaced as one of
the final steps; 2) the interior of the MAC
unit and the blower were cleaned each
time; 3) the evaporator coil in the MAC
unit was inspected and cleaned each time
using a commercial coil cleaner; 4) nega-
tive system pressure was achieved each
time by placing large portions of the duct
system under vacuum so that the dust
and debris loosened and entrained by the
cleaning devices were transported to a
large vacuum blower/filter unit capable of
drawing 1 m3/s (2000 cfm). It remained
running as long as debris was being gen-
erated.
The three duct systems were cleaned
using similar techniques. With the duct
system isolated and under negative pres-
sure, the galvanized steel duct was
cleaned using primarily a stiff, abrasive-
coated, cylindrical rotary power brush to
loosen the dust. The test dust adhered
well, and multiple passes were required to
clean corners and the stiffening beads in
the duct. Following brushing, air washing
(configured to move debris through the
system toward the main vacuum source)
was used to entrain and transport any
remaining dislodged dust to the collector.
Cleaning of the fiberglass lined and the
duct board ducts was different primarily in
that the rotary power brush was con-
structed of cloth strips to loosen the dust
while minimizing damage to the duct ma-
terial. Frequent inspection was used to
prevent over cleaning and consequent
damage to the duct. Hand-brushing was
used where required.
Results and Discussion
An overview of the sampling conducted
during this research is provided in Table
1. The discussion below presents selected
results.
Total Duct Dust Sampling
The MVDS brush method worked well
on the galvanized steel duct surface. Be-
cause there was no concern about dis-
lodging surface materials, it could be used
aggressively to obtain maximum collec-
tion efficiency. The method also worked
well on the FDL used in this study. Al-
though background mass was collected
from surfaces of the new FDL prior to
loading of the dust into the system, the
amount of background mass from "clean"
FDL was not substantially higher than that
collected from the surface of flexible duct
and foil liner in the same system. The
amount of mass collected from "clean"
FDL surfaces was also similar to that col-
lected from galvanized steel duct surfaces,
flexible duct surfaces, and foil liner after
ACS cleaning.
The precision of the MVDS/brush sam-
pling method was generally very good for
duplicate side-by-side samples in spite of
the variability of particle deposition in the
ducts. Duplicate samples were obtained
in locations similar in apparent dust load-
ing. Over all the sampling in the three
duct types and the flexible feeder ducts,
the standard deviation of duplicates was
between 8 and 40% of the mean mass for
the pre-cleaning samples. This level of
precision is probably adequate for sam-
pling duct dust from ACS components be-
cause the dust loading at different loca-
tions in an ACS can be expected to be
highly variable.
The NADCA sampling method was used
only to collect post-cleaning samples from
galvanized steel duct surfaces. This is cur-
rently the only application for which the
method is recommended. It is not an effi-
cient sampling method.
The average mass collected on the
cleaned galvanized steel duct surfaces was
0.26 + 0.11 g/m2, while prior to cleaning
the average of all samples was 7.0 + 4.4
g/m2. On the flexible duct surface, the
bottom-of-duct average prior to cleaning
was 4.3 + 4.0 g/m2 while after cleaning
the overall average was 0.27 + 0.09 g/m2.
The mass on the cleaned foil liner of the
air handler was 0.28 g/m2 in the galva-
nized duct system. Similar results were
observed in the FDL system where the
average mass on surfaces after cleaning
was 0.39 + 0.08 g/m2 on FDL, 0.30 + 0.06
g/m2 on flexible duct, and 0.24 g/m2 for
the one foil liner sample. When efficient
sampling methods such as the MVDS
brush method are used, a more appropri-
ate criterion for cleaning effectiveness is
probably residual dust of less than 0.5 g/
m2 based on the results of these tests.
The sampling results for the duct board
system are inconclusive because all
sample contained a large fraction of fibers
that confounded efforts to measure re-
moval of deposited dust.
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Table 1. Measurements Conducted
Parameter
Dust loading
Dust loading
Dust loading
Dust loading
Microbial loading
Bioaerosol
concentration
Sampling Method
Manual
Manual
Manual
Manual
Manual
Integrated
Instrumentation
EADS - brush
EADS - nozzle
NADCA method
High volume sampler
Pipettip sampler
Mattson-Garvin slit to
agar impactor
Analysis Method
Gravimetric
Gravimetric
Gravimetric
Gravimetric
Plate counting
Plate counting
Notes
Primary method
For duct board
Un-lined galvanized only
Cooling coils only
Applied to all ducts
1-hr integrated samples
PIVL
PM10
Integrated
Integrated
MS&T impactor/filter
and 20 Ipm pump
MS&T impactor/filter
and 20 Ipm pump
Gravimetric
Gravimetric
24-hr integrated samples
24-hr integrated samples
Particles > 0.5|im
(counts)
Particles > 5.0|im
(counts)
Particle count -
16 channel
Fibers
Fibers
Continuous:
10-min averages
Continuous:
10-min averages
Continuous:
60-min averages
Integrated
Semi-continuous
ClimetCI-4100
ClimetCI-4100
LAS-X
Filter/SKC pump
MIE FAM-1
Optical (scattered light)
Optical (scattered light)
Laser aerosol
spectrometer
Phase contrast
microscopy
Optical fiber monitor
Recorded with IAQDS
and Climet
Recorded by Climet
Direct download to
laptop computer
NIOSH 7400 method -
24-hr integrated samples
PDL-10data logger
Microbial Surface Samples
The microbial surface samples showed
that duct cleaning significantly reduced the
microbial loading (by factors of 10 to 20).
However, the loadings were low and not
amplification sites, so the results cannot
be applied to microbial problem ducts. The
nozzle technique gave results comparable
to but generally lower than the swab on
the galvanized and duct liner system, and
higher on the duct board. Overall, the
nozzle system was preferable for all sys-
tems.
Aerosol Measurements
The aerosol measurements were all con-
ducted as an instrumentation and proce-
dure shakedown study, and were not in-
tended to establish the effect of ACS clean-
ing on indoor air quality. Integrated PM25
and PM10 samples were collected in the
instrument room during pre- and post-
cleaning nominal 24-hr periods for each
duct system. All concentrations were low
(1.8 to 11.8 |j,g/m3 while the National Am-
bient Air Quality PM10 Annual Primary
Standard is 50 u,g/m3). The effect of the
ACS cleaning was not clear-cut, given the
small number of samples and lack of con-
trol over the particle content of infiltrating
air. Following cleaning of the galvanized
duct system, the inhalable particle mass
was lower; it was higher for the fibrous
glass liner; and about the same for the
duct board system. The instrument shake-
down that was the study's major purpose
was successful, as pump and timer op-
eration and flow stability were verified.
The mean optical particle counter re-
sults (an integrated sample) were simi-
larly inconclusive with respect to the ef-
fect of duct cleaning but productive as an
equipment shakedown test. Post-cleaning
particle counts for particles >0.5 and
>5.0um were mostly lower following clean-
ing, but the differences are probably not
significant.
Examination of the particle counter re-
sults as a function of time during the tests
gave additional information. During most
of the 24 hour period, particle counts in
the instrument room actually were lower
following cleaning of each duct. However,
a burst of particles was emitted on start-
up that raised the after-cleaning mean.
This decrease occurred with both particle
size ranges, and was especially clear for
the >0.5 um particles. The optical particle
counter results show clearly that particle
concentrations change from day to day
even in a simple system such as the PACS
and that aerosol samples must be taken
over several days to make valid compari-
sons.
Airborne Fiber Measurements
Fibers were generally found to be be-
low detection limits using both the inte-
grated mass sampler and an optical de-
tector.
Bioaerosols
The bioaerosol concentration was low
in the instrument room for all tests. Con-
centrations high enough to allow pre- and
post-cleaning comparisons were only
reached with the galvanized duct. In this
case, the overall culturable fungal con-
centration rose following AHU start-up
(from about 20 cfu/m3 during the back-
ground, dirty duct, and during-cleaning
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samples) to 104 cfu/m3 in the hour imme-
diately following system startup. However,
over 80% of that increase was sampled in
the first 15 minutes of the 60-minute
sample. After 45 minutes, the fungal con-
centration was down to about 2 cfu/m3.
Conclusions and
Recommendations
Overall, the PACS was successful as a
test bed for sampling method develop-
ment. That is, dust could be injected and
conditioned, and the system cleaned such
that the PACS was a reasonable labora-
tory surrogate for a residential ACS. Op-
erated for only short periods, as was true
of this work, it was not suitable for
biocontaminant studies because active
growth was not present. Conclusions from
this research are summarized below:
1. Previously collected duct dust can be
dispersed into a duct system and con-
ditioned at high humidity to provide a
realistic challenge to conventional ACS
cleaning techniques. The dust deposit
was clearly artificial but, in the opinion
of experienced ACS cleaning practi-
tioners, had reasonable distribution in
the duct system and adhesion to the
duct.
2. A pilot ventilation system can be used
to investigate some aspects of ACS
cleaning under controlled conditions
and provide results that may be appli-
cable to field ACS cleaning. Additional
research is needed to understand all
the parameters involved in obtaining a
suitable ACS dust deposit, including
dust injection and conditioning.
3. The medium volume dust sampler
(MVDS), when fitted with a brush on
the nozzle, was shown to be suitable
for collection of dust from bare galva-
nized steel, FDL, and foil liner sur-
faces of ACS components. Collection
efficiency of the MVDS with the brush
was higher than the MVDS with a
slotted nozzle or the NADCA Vacuum
Test Method. The MVDS with brush is
recommended to sample dust mass
deposited on surfaces during the
Phase II field study. NADCA Standard
1992-01 should be only used as in-
tended.
4. Neither the MVDS with the slotted
nozzle nor that with the brush was
suitable for collection of dust from FOB.
The brush dislodged a substantial
amount of fibrous material from new
FOB, while the nozzle did not effec-
tively remove deposited dust on the
fibrous surface. Accurate measure-
ments of dust on FOB surfaces can
not be made with the vacuum meth-
ods used in this study. If FOB clean-
ing is to be evaluated, a suitable sur-
face sampling method must be devel-
oped.
5. The dust loading on bare galvanized
steel duct surfaces that were cleaned
was less than 0.02 g/m2 when mea-
sured with the NADCA Vacuum Test
Method, meeting the NADCA Stan-
dard 1992-01 criterion for effective
cleaning. Collocated measurements
with the MVDS-brush were 0.26, 0.37,
and 0.36 g/m2 at the three locations,
demonstrating the low collection effi-
ciency of the NADCA Vacuum Test
method.
6 For microbial sampling of dust depos-
ited on the surface of various fibrous
glass and galvanized metal surfaces,
the vacuum method provided more
consistently reliable results than the
surface swab technique and should
be used in future studies. It was par-
ticularly superior on fibrous materials.
7. Both the results of the post-cleaning
dust sampling and visual inspection
indicated that the ACS components
could be cleaned effectively by the
methods used in this study. The
amount of dust measured on ACS
components after cleaning was com-
parable to those made prior to soiling
in the PACS.
8. The impact of ACS cleaning of par-
ticle concentration indoors remains
unclear because infiltration and filtra-
tion effects confounded the results.
9. The importance of collecting multiple
integrated samples and the need to
measure particle concentrations for ex-
tended pre- and post-cleaning periods
were evident in the indoor particle data.
10. No evidence was obtained for fiber
emission from the cleaned duct sys-
tems, but the scope of this research
was too limited to allow a definitive
conclusion on fiber emissions from
FOB.
11. A brief pulse of particles was released
when the galvanized ACS was re-
turned to service following cleaning.
This phenomenon was detected by
both the bioaerosol and optical par-
ticle samplers.
12. While not a focus of the study, as the
research progressed it became ap-
parent that ACS construction quality
was an important variable in both
PACS operation and the "cleanability"
of an ACS. While poor construction
practices did not interfere with this
study, which focused on methods de-
velopment and not measurements,
they did affect the performance of the
ACS and the ease and thoroughness
with which it could be cleaned.
With regard to the duct itself, the un-
lined galvanized duct installed in the
PACS had no apparent construction
flaws. The butt joints between sec-
tions in the FDL system had been
sprayed with duct liner adhesive but
were not sealed with a mastic. A small
piece of liner near the return air inlet
was found to be loose when inspected
prior to cleaning. The cut edges in the
FOB system did not appear to be
sealed and were not coated. These
construction details, while not in ac-
cordance with applicable construction
standards, were flaws that the duct
cleaning professionals considered to
be very common.
In addition to duct quality shortcom-
ings, the air handler, though it was in
"as received" condition, was not per-
fectly sealed, and coil bypass and
leaks occurred at several points.
13. The study of biocontamination in an
ACS must be conducted over longer
time periods than were available to
the present research so that active
microbial growth can become estab-
lished in the ACS. Accomplishing this
would present some risk of exposure
for those working in the vicinity unless
the PACS was redesigned for con-
tainment to prevent exposure, and may
be impractical. Such studies are
needed, and use of smaller
biocontamination study apparatus is
thus recommended.
14. Biocides, encapsulants, and sealants
are all used in residential ACS clean-
ing in attempts to control
biocontamination without replacing
duct work. The usefulness of these
practices and their potential threats to
residents have not been determined
and should be investigated.
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Douglas I/I/ VanOsdell and Karin K. Foarde are with Research Triangle Institute,
Research Triangle Park, NC 27709; Roy Fortmann is with Acurex Environmental
Corporation, Research Triangle Park, NC 27709.
Russell N. Kulp is the EPA Project Officer (see below).
The complete report, entitled "Phase I Pilot Air Conveyance System Design,
Cleaning, and Characterization, "(Order No. PB97-189682; Cost: $25.00, subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air Pollution Prevention and Control Division
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
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POSTAGE & FEES PAID
EPA
PERMIT NO. G-35
EPA/600/SR-97/066
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