NATIONAL BUREAU OF STANDARDS REPORT

2540

CRITERIA FOR SUBSTITUTE DUCT MATERIALS

(Project No. l-T-102)

by

O. N. McDorman

A. C. Hutton

P, R. Achenbach

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards is suggested in the following listing of

the divisions and sections engagt i in technical work. In general, each section is engaged in specialized research, development, and engineering in the field indicated by its title. A brief description

of the activities, and of the resultant reports and publications, appears on the inside of the back

cover of this report.

Electricity. Resistance Measurements. Inductance and Capacitance. Electrical Instruments. Magnetic Measurements. Applied Electricity. Electrochemistry.

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Mineral Products. Porcelain and Pottery. Glass. Refractories. Enameled Metals. Concreting Materials. Constitution and Microstructure, Chemistry of Mineral Products.

Building Technology. Structural Engineering. Fire Protection. Heating and Air Conditioning. Floor, Roof, and Wall Coverings. Codes and Specifications.

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Development.

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Department of Defense. • Office of Basic Instrumentation • Office of Weights and Measures.
NATIONAL BUREAU OF STANDARDS REPORT

June 5, 1953

MBS REPORT

2 540

CRITERIA FOR SUBSTITUTE DUCT MATERIALS

(Project No. l-T-102)

by

0. N. McDorman

A. C. Hutton

P- R. Achenbach

to

Office of the Housing Administrator

Housing and Home Finance Agency

Washington, D. C.

The publication, reprlnt|

unless permission is obta

25, D, C, Such permlssi

cal ly prepared If that a

Approved for public release by the ' ,i ls prohlbl,ed

Director of the National Institute of rds ’ Washln s

ton

Standards and Technology (NIST)

1,33

1

een sp9C'"

for Its own use. on October 9, 2015 .

CRITERIA FOR SUBSTITUTE DUCT MATERIALS

Abstract

A study was made by the National Bureau of Standards to

establish a set of performance criteria by which substitute materials for metal ducts could be judged for use

in warm air heating systems. This investigation was performed under an agreement with the Office, of the Administrator, Housing and Home Finance Agency, authorized under

Title III of the Housing Act of 194 , as amended, and

w^s identified as Project No. l-T-102 "Criteria for Substitute Duct Materials".

The study led to the conclusions that a substitute duct

material should be tested after the material was formed

into ducts and that these ducts should be evaluated for

their acceptability by means of tests for Flame Spread,

Deformation at Elevated Temperature Penetration of Flame,

Rigidity, Deterioration, Odor, and Practicability o

_ Joints and Fittings. The application of the fire resistance tests are to be governed by the type of furnace and

ai r distribution system involved and the type ox bunaing in which the ducts are to be installed. The proposea

criteria were developed primarily for application o residential duct systems.

I. INTRODUCTION

At the request of the Housing and Home Finance Agency a study

was made to determine what criteria should be recommended as a

basis for evaluating materials offered to the Federal housing

agencies as substitutes for metal ducts in warm airheating systems.

This study was to be principally concerned with residential applications although it was recognized that the requirements for ot

applications would be similar, but not necessarily identical to

those in dwellings.

After several conferences between representatives of the Housing and Home Finance Agency and members of the . staff of the Nat

al Bureau of Standards the following characteristics were listed as

worthy of consideration and investigation m establishing a set

criteria for substitute duct materials.

1. Rate of flame spread inside the duct.

2. Deformation of ducts with interior and exterior exposure to

elevated temperature.

2

3. Flame penetration of duct wall by an outside source of heat»

4. Weight gain when soaked in water (new material).

5. Weight gain when soaked in water after duct has been heated

to 200°F for a period of time.

6c Weight gain after extended exposure to high humidity at

room temperature.

7o Dimensional changes at elevated temperatures.

Sc Dimensional changes caused by high humidity.

9.

Mold growth at high humidity and at room temperatures.

10. Odor emission at operating temperatures.

11. Rigidity when heated to operating temperatures.

12. Rigidity after exposure to high humidity.

13. Rigidity when surrounded by damp sand or wet concrete.

14. Susceptibility to damage by dropping.

15. Deterioration resulting from alternately heating and

cooling the duct.

16. Ease of painting.

17. Practicability of joints and fittings for fabrication.

Laboratory tests were made of a few commercial materials proposed for substitute ducts to study flame spread, ease of ignition,

moisture absorption, dimensional changes when heated, mold growth,

odor emission, rigidity, damage from dropping, and deterioration

due to alternately heating and cooling. Other tests were made to

determine what temperatures were produced in a warm air heating

system employing a hand-fired coal furnace and forced circulation

of the air through the system during normal operation and during

a simulated electrical failure cf the blower motor when all dampers

on the furnace remained open. The test procedures used and the

results of these tests are presented in separate NB3 reports numbered 1947 and 2541.
I
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PROPOSED CRITERIA FOR SUBSTITUTE DUCT MATERIALS

The , criteria considered essential for selecting satisfactorysubstitute materials for metals in warm air heating systems are:

I. Fire Resistance

II. Rigidity.

III. Deterioration.

IV. Odor

V. Practicability of Joints and Fittings.

A description of the test procedures to be used, the limits

to be applied in determining acceptance or rejection, and the

recommended application of the tests to different types of warm

air systems for the above criteria follows.

I. Fire Resistance

It is proposed that one or more of the following tests be

made of substitute duct materials depending on the type of fur

nace and air distribution system used and the type of building

involved as shown in Part D below.

Part A.

1 .

2 .

3.

Part B.

Type of Test

Flame Spread Test on inside of the duct.

Deformation Test with interior and exterior exposure

to elevated temperatures.

Penetration Test with exterior exposure to hot gases.

Test Procedures

1. Flame Spread Test (as described in report 1947)

A 3-foot section of duct of the test material shall be mounted in an inclined position with a 20 degree angle between its

longitudinal axis and the horizontal. A Tirrell burner (3/$-inch

tip) , for which the height of the flame with the burner vertical

has been adjusted to 5 inches with an inner cone 2 inches high,

shall be mounted on a stand at a 45-degree angle with respect to

the horizontal. The stand shall be placed so that the flame impinges

at a 45-degree angle to the side of the interior surface of the

duct at a point 4 inches from the lower end. This flame shall be

applied continuously to the inside surface of the duct until the

flame emerges from the upper end or until it penetrates the wall

surface, or until it becomes evident that neither will happen.

4 -

The appearance of the test specimen, the extent of burning of the

material, and the flame travel shall be recorded at regular intervals during the test.

2. Deformation Test

A piece of duct of such length that it includes any supports

furnished as attached supports, but not less than 12 inches long,

shall be simply supported at each end in an oven and exposed to a controlled temperature of 400°F for high-temperature systems or

350°F for low-temperature systems for a period of 24 hours and the

deformation recorded at least twice during the test and at the end

of the test. Ducts intended for vertical placement shall be mounted vertically in the furnace and restrained at top and bottom while

being exposed to the same temperature as for the horizontal specimens. Lateral deflection of the specimen shall be observed. Other

evidences of structural failure or deterioration shall be noted

during these tests.

3. Penetration Test

A six-foot length of the specimen duct, which has been conditioned to constant weight at 90 percent relative humidity, at 75°F,

shall be placed horizontally in a test furnace having a combustion

space about two feet square with the two open ends of the duct protruding equal distances from the furnace wall into the ambient air.

Four thermocouples shall be placed 12 in. below the test specimen

and about £ in. from the walls of the furnace to average the furnace temperature. The thermocouples shall be constructed in accordance with the requirements outlined in Standard Methods of Fire Test

of Building Construction and Materials (ASTM Designation E119-50)

Hot gases shall be passed upward around the test specimen with the

gas temperature controlled in accordance with the Standard TimeTemperature Curve, Fig. 1, shown in this same ASTM Standard. The

test shall be continued until flame first penetrates the duct wall

or until the duct wall is ruptured by physical failure and the time

required for either type failure to occur shall be noted. The exterior of the furnace shall be insulated to reduce heat transmission

to the ambient air. The ambient temperature as measured by a shield

ed instrument along the axis of the duct and at a distance of 2 ft

from the open ends of the duct shall not exceed 100°F during the

test

.

(Note: It is considered doubtful that aluminum duct would withstand the Penetration Test described above and it is uncertain

whether or not cement-asbestos or fiber glass ducts would withstand these test conditions either. However, the Penetration Test

is recommended only for those buildings for which a fire rating is

required - see Part D - and therefore, would not be applicable to

one- and two-family dwellings in many cities.

)

- 5 -

Part C. Types of Furnaces and Air Distribution Systems Considered

1. Up-draft through furnace casing (either basement or 1st floor

mounting)

.

a. Overhead supply and underfloor return ducts.

b. Overhead supply and no return ducts.

c. Overhead warm air panel and no return ducts.

d. Overhead warm air panel and connected return ducts.

e. Floor furnace with supply plenum and connected return ducts

2. Down-draft through furnace casing (1st floor mounting).

a. Underfloor supply ducts and overhead returns.

b. Underfloor supply ducts and underfloor returns.

c. Supplies in concrete and overhead returns.

d. Supplies in concrete and no returns,

e. Crawl space plenum and floor or baseboard supply

registers with no return ducts.

f. Crawl space plenum and floor or baseboard supply

registers with overhead returns.

3. Horizontal furnace position (1st floor ceiling or attic

mounting)

.

a. Attic supplies and attic returns.

b. Overhead supplies and overhead returns near 1st floor

ceiling.

c. Ceiling panel, closed circuit.

Part D. Recommended Application of Fire Resistance Tests.

1. All substitute duct materials must pass the Flame Spread

test for all applications.

- 6 -

2. Substitute duct materials must pass the Deformation test for

all applications in both the supply and return duct systems

of furnaces mounted horizontally, or those mounted vertically

with down-draft in the furnace casing, and for all applications in the supply duct system of vertically-mounted furnaces with up-draft in the casing.

3. Substitute duct materials for use in structures requiring a

fire rating must pass the Penetration test for all applications in both the supply and return duct system except when

a duct is embedded in concrete or when a duct is used in a

crawl space plenum but does not connect to a wall stack,

floor register, or grille.

Part E. Recommended Limits for Acceptance of Substitute Duct

Materials for above Applications.

1. In the Flame Spread test the flame shall not issue from the

end of the 3-foot test section in less than 40 minutes. A

hole burned through the wall of the test specimen during

the test shall cause rejection of the material unless it is

being proposed only for use embedded in concrete.

2. In the Deformation test the specimen tested horizontally

shall not deflect more than 3/l6-inch in any direction in

24 hours and the specimen tested in a vertical position

with end restraint shall not deflect laterally more than

1/2 inch in the same length of time.

3. In the Penetration test the time required for flame or

hot gases to pass through the duct wall or for physical

failure to occur shall be equal to or greater than the

required fire resistance of the floor, wall, or ceiling

through which the duct system passes in any given installation.

Part F. Required Installation Procedures.

1. Coverings or exterior insulations that are combustible shall

not be applied to any type of warm air duct (metal or nonmetal) during installation. Clearance shall be provided

between duct walls and any combustible material, in accordance with Pamphlet No.. 90 of the National Board of Fire

Underwriters, entitled ""Standards for Air Conditioning,

Warm Air Heat, Air Cooling and Ventilating Systems”.

2. Combustible materials shall not be used for ducts within

6 feet of a furnace in either the supply or return duct system.

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Note: It has not been established that such a severe restriction

on material adjacent to a furnace is necessary for the supply

system of vertical furnaces with downward flow of air in the

casing or for the return system of vertical furnaces with

upward flow in the casing when the Flame Spread and Deformation tests described above have been met. However, ducts in

these two locations would be heated by conduction from the

furnace casing and, in some cases, by radiation from heating surfaces. Additional laboratory tests would be required

to determine what temperatures would be attained on these

ducts at the junction with the furnace casing.

II. Rigidity

Before and during 2 weeks (14 days) exposure in a room maintained at a temperature of 75°F and a relative humidity of 90

percent the specimen ducts shall not sag visibly in a longitudinal direction and the cross sectional dimensions shall not change

by more than 5 percent as a result of carrying their own weight

on supports placed 4 ft apart.

III. Deterioration

The specimen ducts shall not delaminate, crack, collapse,

spall, peel, blister, or wrinkle during 2 weeks (14 days) exposure

in a room maintained at a temperature of 75°F and a relative

humidity of 90 percent or when alternately heated and cooled

for 15 cycles between the temperatures of 250°F and 75°F.

IV. Odor

The specimen ducts shall not emit a noticeable odor at a

temperature of 250°F after the first 4 hours exposure.

V. Practicability of Joints and Fittings.

Materials used as joints and fittings for connecting separate

pieces of substitute duct materials shall be subject to the same

criteria as the duct material.

When the joints and fittings for making up a duct system are

supplied by a manufacturer of a substitute duct, it shall be

possible to readily assemble separate lengths of the duct using

these joints and fittings after a weeks exposure of all parts

to either high humidity (90 percent) or low humidity (20 percent)

by using simple hand tools without trimming, dressing, stretching, or lubricating the separate pieces. Joints shall be substantially air tight and, if slip joints are used, they shall

have a lap of least one inch.
\
DISCUSSION OF OTHER REQUIREMENTS

A. Moisture Absorption

The weight increase of a substitute duct when immersed in

water might be indicative of the amount of damage it would suffer

if such material were exposed to rain at a building site before

installation or if a below-grade system were flooded with water

after installation. However, duct materials do not consistently

receive such exposure to water so it was our opinion that a

material, otherwise suitable for warm air ducts, should not be

rejected because it would absorb water.

The weight of moisture absorbed under high humidity conditions is only important insofar as it affects rigidity, deterioration, or mold formation. The two of these effects are evaluated under the proposed criteria.

B. Dimensional Changes

In our opinion, the dimensional changes due to temperature

will be reflected qualitatively by the Deformation Test recommended as one of the criteria and the dimensional changes caused

by high humidity will be revealed adequately by the Rigidity

Test proposed as one of the criteria.

C. Damage from Dropping

Consideration of the types of materials likely to be offered as substitute materials for ducts in warm air heating systems

indicated that most of the materials, if damaged, could either be

restored to a usable condition by bending them back in shape or

the damaged portion could be cut out leaving a large portion of

the original piece undamaged. Very few materials would be shattered throughout by dropping from a height of 4 feet as from a truck

bed. Consequently, a criteria that would attempt to describe the

amount of damage permissible in a drop test was considered unnecessary.

D. Painting

Painting of warm air heating ducts will be desired in some

installations, but ducts would not ordinarily be painted in a crawl

space; they would not be painted if surrounded by concrete, and

they would frequently be enclosed in furring spaces in recreation

rooms. Therefore, ease of painting was not considered to be sufficiently important to warrant making it a requirement for acceptance.

- 9

-

E. Mold

The tests made of some commercial materials proposed for

warm air heating ducts showed that they would support mold growth

at a temperature of $5°F accompanied by a relative humidity of

90 percent and indicated that they might mold at the conditions

likely to be present at times around the heating ducts duringthe summer-season in crawl spaces, in basements, and in concrete

fl oors. However, extensive research would be required to correlate the quantitative results of mold tests in a laboratory

with the seasonal and variable conditions that would exist in

actual use of substitute duct materials in crawl spaces and

basements. Such a correlation has not been worked out. Therefore, it appears impractical to include a criteria for molding

of substitute duct materials at the present time. The lower

margin of temperature and humidity that will promote mold growth

is about 75°F dry bulb and 75 percent relative humidity. It is

probable that these conditions would be exceeded around warm

air heating ducts at times during the summer season.

THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

for making measurements consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials,

devices, and structures; advisory services to Government Agencies on scientific and technical

problems; invention and development of devices to serve special needs of the Government; and the

development of standard practices, codes, and specifications. The work includes basic and applied

research, development, engineering, instrumentation, testing, evaluation, calibration services, and

various consultation and information services. A major portion of the Bureau’s work is performed

for other Government Agencies, particularly the Department of Defense and the Atomic Energy

Commission. The scope of activities is suggested by the listing of divisions and sections on the

inside of the front cover.

Reports and Publications

The results of the Bureau’s work, take the form of either actual equipment and devices or published papers and reports. Reports are issued to the sponsoring agency of a particular project or program. Published papers appear either in the Bureau’s own series of publications or in the

journals of professional and scientific societies. The Bureau itself publishes three monthly periodicals, available from the Government Printing Office: The Journal of Research, which presents

complete papers reporting technical investigations; the Technical News Bulletin, which presents

summary and preliminary reports on work in progress; and Basic Radio Propagation Predictions,

which provides data for determining the best frequencies to use for radio communications throughout

the world. There are also five series of nonperiodical publications: The Applied Mathematics

Series, Circulars, Handbooks, Building Materials and Structures Reports, and Miscellaneous

Publications.

Information on the Bureau’s publications can be found in NBS Circular 460, Publications of

the National Bureau of Standards ($1.00). Information on calibration services and fees can be

found in NBS Circular 483, Testing by the National Bureau of Standards (25 cents). Both are

available from the Government Printing Office. Inquiries regarding the Bureau’s reports and

publications should be addressed to the Office of Scientific Publications, National Bureau of Standards, Washington 25, D. C.
NBS