United States National Risk Management

Environmental Protection Research Laboratory

Agency Research Triangle Park, NC 27711

Research and Development EPA/600/SR-97/066 August 1997

Project Summary

Air conveyance system (ACS) cleaning, is advertised to homeowners as a

service having a number of benefits,

including the improvement of indoor

air quality (IAQ). Because ACS cleaning includes many procedures applied

to many different duct systems, evaluation has been difficult and the effectiveness of ACS cleaning has not been

adequately measured.

The objective of this project was to

develop and refine surface and airborne

contamination measurement techniques that could be used to evaluate

ACS cleaning. The research was in support of a field study to be conducted

later. To this end, a pilot air conveyance system (PACS) using full-size residential heating and air-conditioning

(HAC) equipment was constructed and

operated to provide a controlled, artificially soiled, ACS environment. The

PACS consisted of ducts, an HAC unit,

a dust mixing room, an instrument

room, and a dust generation and injection system. Three types of duct systems were evaluated with the proposed

measurement methods under unsoiled

and soiled conditions. Each duct systems 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. Surface contamination (microbial and total

dust) measurement methods and visual

inspection showed that the pilot unit

was effectively cleaned by the methods applied during this study. Submicron and larger particle counts were

reduced following ACS cleaning, and

respirable particle mass was reduced

Phase I Pilot Air Conveyance

System Design, Cleaning, and

Characterization

Douglas W. VanOsdell, Karin K. Foarde, and Roy Fortmann

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, Research 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 Pollution Prevention and Control Division’s

(APPCD's) Air Duct Cleaning Program are

to determine when and how to clean ventilation systems, and evaluate both the

effectiveness of such cleaning and its impact on IAQ. Little published research data

are available to support the IAQ improvement claims sometimes made by ACS

cleaning contractors, and the data that

are available are difficult to interpret. A

two-phase research program was undertaken to develop evaluation methods to

achieve these objectives. This report describes research conducted under Phase

I of the program. The research was a

cooperative effort, led by RTI, with participation 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.
2

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 preventing and possibly mitigating IAQ problems and may improve the energy efficiency 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 cleaning procedures can be used on any given

system, and there are many types of systems.

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 exchangers, expansions, contractions, regions of

surface irregularity, and dampers) and was

designed to allow application of all aspects of the proposed evaluation methods

with the exception of evaluation of IAQ in

residences, including pre- and post-cleaning inspections and evaluations. The HAC

equipment was all scaled for a small residence. 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) HAC

unit including air-conditioning coil and heat

exchanger, 3) dust mixing room, 4) instrument room, and 5) dust generation system. 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 HAC

equipment was used when possible. So

that evaluation methods could be developed for the three major duct materials,

completely separate PACS duct systems

were constructed of the three duct materials commonly utilized in residential HAC:

galvanized steel, fibrous glass duct liner

(FDL), and fibrous glass duct board (FDB).

A new air handler was installed for each

duct type.

Figure 1. Elevation view of PACS.

Dust mixing room

Humidifier

Supply air sample locations S1-S4

S3 S2

Supply air

Temp, RH, and flow sensors

S1

Air

handling

unit

Instrument

room

R4

Dust

feed

Blower

S4 R1 R2

Return air temp

RH, and flow sensors Return air sample locations R1-R4

Supply air duct

Return air duct R3
3

ACS Cleaning Measurement

Methods

The measurement parameters and sampling and analysis methods evaluated

were: 1) total surface dust loading, 2)

culturable microbial surface loading, 3) respirable aerosol concentrations (PM10 and

PM2.5), 4) size dependent particle number

concentration, 5) airborne fiber mass and

counts, and 6) fungal bioaerosol concentrations.

The primary sampling method being

evaluated (and hence, the parameter of

greatest interest during the study) was

measurement of the mass of dust on surfaces of the ACS components prior to and

following ACS cleaning. Several variations

of vacuum sample collection were evaluated, the most promising being the Medium 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 measured was the number of culturable microbial organisms deposited on the duct

and HAC surfaces within a 10 cm2 template area, as measured with a swab technique and a 10 L/min vacuum method.

Because of the short time between injecting 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 instrumentation room of the PACS, measuring

number concentration in three ranges:

greater than 0.5 μm, greater than 5 μm,

and over 16 differential channels between

0.1 and about 10 μm. 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 concentrations based on the data presented

in the report due to the limited scope of

the measurements and the artificial nature 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 operating periods: 1) installation of the new duct

system and checkout; 2) pre-soiling evaluation of the duct and HAC unit, 3) duct

soiling and deposited dust conditioning, 4)

post-soiling, pre-cleaning evaluation of the

duct and HAC unit, 5) post-soiling, precleaning air sampling, 6) cleaning the ventilation system, 7) post-cleaning evaluation before restarting the HAC unit, 8)

immediate post-cleaning air sampling,

9) post-cleaning air sampling for total particles after cleaning, and 10) post-cleaning (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 HAC

unit and the blower were cleaned each

time; 3) the evaporator coil in the HAC

unit was inspected and cleaned each time

using a commercial coil cleaner; 4) negative 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 generated.

The three duct systems were cleaned

using similar techniques. With the duct

system isolated and under negative pressure, the galvanized steel duct was

cleaned using primarily a stiff, abrasivecoated, 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 constructed of cloth strips to loosen the dust

while minimizing damage to the duct material. 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. Because there was no concern about dislodging surface materials, it could be used

aggressively to obtain maximum collection efficiency. The method also worked

well on the FDL used in this study. Although 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 collected from galvanized steel duct surfaces,

flexible duct surfaces, and foil liner after

ACS cleaning.

The precision of the MVDS/brush sampling 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 loading. 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 sampling duct dust from ACS components because the dust loading at different locations 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 currently the only application for which the

method is recommended. It is not an efficient 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 galvanized 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 appropriate 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 removal of deposited dust.
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Table 1. Measurements Conducted

Parameter Sampling Method Instrumentation Analysis Method Notes

Dust loading Manual EADS - brush Gravimetric Primary method

Dust loading Manual EADS - nozzle Gravimetric For duct board

Dust loading Manual NADCA method Gravimetric Un-lined galvanized only

Dust loading Manual High volume sampler Gravimetric Cooling coils only

Microbial loading Manual Pipet tip sampler Plate counting Applied to all ducts

Bioaerosol Integrated Mattson-Garvin slit to Plate counting 1-hr integrated samples

concentration agar impactor

PM2.5 Integrated MS&T impactor/filter Gravimetric 24-hr integrated samples

and 20 lpm pump

PM10 Integrated MS&T impactor/filter Gravimetric 24-hr integrated samples

and 20 lpm pump

Particles > 0.5μm Continuous: Climet CI-4100 Optical (scattered light) Recorded with IAQDS

(counts) 10-min averages and Climet

Particles > 5.0μm Continuous: Climet CI-4100 Optical (scattered light) Recorded by Climet

(counts) 10-min averages

Particle count - Continuous: LAS-X Laser aerosol Direct download to

16 channel 60-min averages spectrometer laptop computer

Fibers Integrated Filter/SKC pump Phase contrast NIOSH 7400 method -

microscopy 24-hr integrated samples

Fibers Semi-continuous MIE FAM-1 Optical fiber monitor PDL-10 data 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 systems.

Aerosol Measurements

The aerosol measurements were all conducted as an instrumentation and procedure shakedown study, and were not intended to establish the effect of ACS cleaning on indoor air quality. Integrated PM2.5

and PM10 samples were collected in the

instrument room during pre- and postcleaning nominal 24-hr periods for each

duct system. All concentrations were low

(1.8 to 11.8 μg/m3 while the National Ambient Air Quality PM10 Annual Primary

Standard is 50 μg/m3). The effect of the

ACS cleaning was not clear-cut, given the

small number of samples and lack of control 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 shakedown that was the study’s major purpose

was successful, as pump and timer operation and flow stability were verified.

The mean optical particle counter results (an integrated sample) were similarly inconclusive with respect to the effect of duct cleaning but productive as an

equipment shakedown test. Post-cleaning

particle counts for particles >0.5 and

>5.0μm were mostly lower following cleaning, but the differences are probably not

significant.

Examination of the particle counter results 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 startup that raised the after-cleaning mean.

This decrease occurred with both particle

size ranges, and was especially clear for

the >0.5 μm 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 comparisons.

Airborne Fiber Measurements

Fibers were generally found to be below detection limits using both the integrated mass sampler and an optical detector.

Bioaerosols

 The bioaerosol concentration was low

in the instrument room for all tests. Concentrations high enough to allow pre- and

post-cleaning comparisons were only

reached with the galvanized duct. In this

case, the overall culturable fungal concentration rose following AHU start-up

(from about 20 cfu/m3

 during the background, dirty duct, and during-cleaning
5

samples) to 104 cfu/m3 in the hour immediately 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 concentration was down to about 2 cfu/m3.

Conclusions and

Recommendations

Overall, the PACS was successful as a

test bed for sampling method development. That is, dust could be injected and

conditioned, and the system cleaned such

that the PACS was a reasonable laboratory surrogate for a residential ACS. Operated 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 conditioned 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 practitioners, 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 applicable 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 galvanized steel, FDL, and foil liner surfaces 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 intended.

4. Neither the MVDS with the slotted

nozzle nor that with the brush was

suitable for collection of dust from FDB.

The brush dislodged a substantial

amount of fibrous material from new

FDB, while the nozzle did not effectively remove deposited dust on the

fibrous surface. Accurate measurements of dust on FDB surfaces can

not be made with the vacuum methods used in this study. If FDB cleaning is to be evaluated, a suitable surface sampling method must be developed.

5. The dust loading on bare galvanized

steel duct surfaces that were cleaned

was less than 0.02 g/m2 when measured with the NADCA Vacuum Test

Method, meeting the NADCA Standard 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 efficiency of the NADCA Vacuum Test

method.

6 For microbial sampling of dust deposited 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 particularly 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 comparable to those made prior to soiling

in the PACS.

8. The impact of ACS cleaning of particle concentration indoors remains

unclear because infiltration and filtration effects confounded the results.

9. The importance of collecting multiple

integrated samples and the need to

measure particle concentrations for extended pre- and post-cleaning periods

were evident in the indoor particle data.

10. No evidence was obtained for fiber

emission from the cleaned duct systems, but the scope of this research

was too limited to allow a definitive

conclusion on fiber emissions from

FDB.

11. A brief pulse of particles was released

when the galvanized ACS was returned to service following cleaning.

This phenomenon was detected by

both the bioaerosol and optical particle samplers.

12. While not a focus of the study, as the

research progressed it became apparent 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 development 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 unlined galvanized duct installed in the

PACS had no apparent construction

flaws. The butt joints between sections 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

FDB system did not appear to be

sealed and were not coated. These

construction details, while not in accordance with applicable construction

standards, were flaws that the duct

cleaning professionals considered to

be very common.

In addition to duct quality shortcomings, the air handler, though it was in

“as received” condition, was not perfectly 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 established in the ACS. Accomplishing this

would present some risk of exposure

for those working in the vicinity unless

the PACS was redesigned for containment 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 cleaning 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.
6

Douglas W. 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

EPA/600/SR-97/066

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT NO. G-35