NATIONAL BUREAU OF STANDARDS REPORT

3396

GIASS FIBER DUCTS IN COLEMAN BLENDER UNITS

Selden D. Cole

Paul R. Achenbach

Report to

Federal Housing Administration

Washington, D. C»

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Mi'eeks, Secretary NATIONAL BUREAU OF STANDARDS

A. \ . Asiin, Director

THE NATIONAL BUREAU OF STANDARDS

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^Office of Basic Instrumentation ^Office of Weights and Measures.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1003-30-4830 June 28, 1954 3396

GIASS FIBER DUCTS IN COLEMAN BLENDER UNITS

Selden D. Cole

Paul R. AchenUach

Heating and Air Conditioning Section

Building Technology Division

to

Office of the Commissioner, Underwriting

Federal Housing Administration

Washington, D. C.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, reprinting, c

unless permission Is obtained I 25,D.C. Such permission Is

cally prepared If that agency

Approved for public release by the prohibited Director of the National Institute of

Standards and Technology (NIST)

Is own use. on October 9, 2015.
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GIASS FIBER DUCTS IN COIEMAN BLENDER UNITS

by

Selden D. Cole and Paul R. Achehbach

ABSTRACT

At the request of the Federal Housing Administration a study was made of glass fiber ducts submitted by Coleman Co., Inc. to

determine their suitability as a substitute for metal ducts in

the blender \uiits manxifactured by this con5)any for warm air

heating systems. Such flexible ducts appear to have advantages

in remodelling the heating systems in existing houses because

less alteration and repair of the wall is needed than for metal

ducts. The tests showed that the fiber glass ducts could readily

be installed in an existing dry wall partition as a part of the

Coleman air blending units, but that they were not well-suited to

existing wet wall construction where extruded plaster keys reduce

the cross section area of the stud space. Air leakage throu^

the duct material might cause an appreciable loss of heat in a

poorly-constructed wall but would probably be unimportant in

tight construction, especially on inside partitions. The fiber

glass material showed good resistance to mold growth and flame

spread, but it was attacked readily by rats without a hunger

motive. For the type of installation and rate of air delivery

recommended by the manufacturer the fiber glass ducts do not

appear to transmit less noise to the room than metal ducts.

I. Introduction

At the request of the Office of Assistant Commissioner, Underwriting,

Federal Housing Administration, in a letter dated October 22, 1952 studies

were made of glass fiber ducts and glass fiber material submitted by the

Coleman Co., Inc. to determine their suitability as substitutes for metal

ducts in warm air heating systems, and especially as ducts in the stud

spaces of residential construction. The duct specimens and the sheet

material were examined for ease of installation in existing walls, air leakage,

sound absorption, insulating value, flame spread, mold resistance, and rodent

resistance.

II. Description of Duct Application

The glass fiber ducts were designed to be used as a part of air-blending

units manufaxitured and marketed by Coleman Co., Inc. The blenders are a wall

stack, grille combination for introducing a mixture of warm air from a furnace
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and recirculated air into a room by providing for the mixing within the wall

stack.

The conqplete blender as submitted by the manufacturer consisted of

three parts — the inlet stack head, the outlet stack head and the glass fiber

duct. In addition two steel straps with attached seals were included in the

package. As used in a house, the mixture of warm air and recirculated air

may be introduced into the room either at baseboard level or above head level

through one grille with the alternate grille being used as the opening throu^

which the recirc\ilated air enters the blender.

The two stack heads were of metal and were essentially short square elbows

with overall dimensions of 12 in^x 8 in.x in. with the grille opening being

12 in.x 6 in. and the other opening being 12 in.x 3"^ in. for attachment of the

glass fiber duct. One stack head was fitted with tiirning vanes to direct the

air at the outlet grille, whereas the other contained a short piece of 3? inpipe which conveyed the stream of warm furnace air into the fiber glass duct.

This piece of 3'i’ in. pipe was located in the stack head so the stream of warm

air aspirated room air into the blender. The stack head with the turning

vanes is shown in the upper part of Fig.l,

The glass fiber duct was prefabricated from a in, thick semirigid sheet

of treated glass fibers. The rectangular shape was maintained because the

corners were partially cut and mechanically creased and the one inch overlap

of the material at the edges was stapled together. The manufacturer also

provided 20 flat sanqples 8 in.x 8 in. and 20 boxes 8 in.x 8 in.x 6 in. hi^ of

the same duct material for closure to mold spores and rodents,

III. Test Procedure .

To evaluate the ease of installation of the fiber glass blenders, air

leakage and soxmd characteristics, two sections of interior partition wall

were constructed. Both wall sections were alike in that they were constructed

of nominal 2x4 in. lumber spaced l6 in. on centers with three stud spaces and

were 8 ft in hei^t. They were different in that one was of dry wall con- struction and the other of wet wall construction. The wet wall panel was lathed on both sides with esqpanded metal lathe and plastered with a scratch

coat, brown coat, and a white finish coat between 3/4 in, grounds. The dry wall

panel was covered on both sides with 4 ft x 8 ft sheets of ^ in. plaster board.

A blender unit was installed in the center stud space of each wall panel following the manufacturers' instructions. To install the blender in an existing

dry wall i>artition using a duct system under the floor, holes were located and

cut in the plasterboard wall as follows: (l) a vertical line was drawn about

15 in. upward from the floor at the inside edge of one of the studs in the

center of the panel; (2) at a distance of 2 in, from the floor aJ.ong this line,

a line was drawn parallel to the floor to the approximate location of the

other stud; (3) at a distance of 7^ in, up from the point selected in step #2

a line was drawn parallel to the floor to the approximate location of the

other stud; (4) measuring along the two lines parallel to the floor 12 3/8 in.
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from the line on the inside of the stud, marks were made. After connecting

the two marks, a 22 3/8 in.x in. rectangle was cut out for the inlet

stack. ( 5 ) the bottom line of the hole for the outlet stack was located

6 ft T'f in. above the floor and the same size hold - 12 3/8 in. wide and T?

in. hi^ - was laid out for the upper stack.

Since the stack heads were not as wide as the space between studs,

blocks approximately 6 in. long were fastened to the studs at top and bottom

to provide a solid member for securing the blender heads and mounting the

grilles. These blocks are visible in Figure 1.

Removing the dan5)er assembly from the blender stack head, the bottom

of the blender head was placed on the sole of the partition with one side

flush against the stud and the location of the hole for the warm air supply

pipe was marked. after which the stack head was removed. The opening for

the 3? in. pipe was cut out using a brace and bit and key hole saw.

The glass fiber duct was fitted over the 3^^ in. by 12 in. collar of the

outlet stack and securely fastened with the metal strap provided for this

purpose. The open end of the duct was placed in the top opening and fed

downward into the stud space until most of it was in the partition. By

reaching upward through the lower hole, the duct was piilled into position

while being pushed in at the top. The upper stack head was centered in the

opening so the grille would cover the hole, and there nailed to the stud on

one side and the block on the other. The glass fiber duct hanging in the

partition covered the lower opening in the wall. The front side of the

duct was cut off so the lower end extended about one inch below the upper

edge of the hole. The collar of the lower stack head was then inserted

into the glass fiber duct and pushed up as far as possible. The stack head

was centered both ways so the grille would cover the opening and the stack

head was nailed to the stud on one side and the block on the other. A rear view of this installation is shown in Fig. 2 with the plasterboard removed.

The same procedure was followed with the section of wet wall partition

as with the dry wall partition with the following additional operations;

( 1 ) the plaster keys that were extruded through the metal lath on the wall

opposite the two openings had to be removed so the stack heads could fit

into position, (2) tbe glass fiber duct could not be pushed downward far

enough to be reached from the lower hole because of Idle plaster keys so a

chalk line was fastened to the open end of the duct and it was pulled downward while being pushed from the top until it was in place. The plaster

keys had extended through the metal lath in some places far enough to practically close the opening in the fiber glass duct after it was installed in

the partition.

The static pressure in the blender was determined at three places when

normally installed in the stud space and with approximately 75 cfm of air
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at a tenqoeratiire of 140®F being introduced through the 3^ in. inlet pipe.

A warm air supply was provided by means of a small blower and electric

heaters. The air leed^age through the fiber glass duct was determined by

direct measurement with a gas meter for a range of static pressures inside

the duct, with the duct lying in an open room having both ends blocked

except for the supply pipe.

Sound readings were taken for similar installations using the fiber

glass duct in the stud space in one case and a metal duct in the other

case. The sound meter was located one foot from the face of the partition

and midway between the upper and lower grilles. Readings were taken with

the blower running and with it shut off.

The flame spread test was performed by the Fire Protection Section of

the National Bureau of Standards under the conditions proposed as Criteria

for Sibstitute Duct Materials in National Bureau of Standards Report 2540.

The test consisted of applying a continuous flame of specified characteristics

to the inside surface of the duct, measuring the rapidity of flame spread,

and observing the extent of surface involvement during a 40-minute test

period.

The rodent repellency tests were conducted by the Fish and Wildlife

Service of the Department of the Interior, Samples were placed between

laboratory rats and their food supplies and the time that elapsed before

the samples were gnawed through was observed. Other san5>les were placed

on a floor of a room housing wild rats with no motivation for attack and

the damage over night was observed.

The mold test was conducted by the Testing and Specifications Section

of the Organic and Fibrous Materials Division of the National Bureau of

Standards. The test was conducted in accordance with Federal Specification

CCC-T-191b Methods 5751 »1 and 5756 for mildew susceptibility. Two sets of

ten specimens were prepared from the sample. One set was tested in accordance

with Method 5751 •! and -Oie other in accordance with Method 5756. Two controls were employed for each set to test the viability of the organism.

IV, Test Results

The Coleman air blender with a fiber glass duct can be installed neatly

in a relatively short time in an existing partition with dry wall construction

\dien the detailed instructions of the manufacturer are followed. Considerable

time was needed for installation in the wet wall construction because it was

necessary to break off the plaster extruded through the lathe before the stack

heads would fit in place. No way was devised for breaking off the plaster

keys in the stud spaoe between the stack heads. For this reason the air

passage in the fiber glass duct was seriously restricted after the installation

was completed. The plaster keys are visible in Fig 1 as well as the areas

opposite the grilles where they were removed.
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static pressiires meas\ired within the duct at a station ^4- in. from the

top, midway between top and bottom, and at a station k in from the bottom

of the fiber glass duct were 0.01, 0.017, and - 0.001 in,W.G. respectively

when 75 cfm of air at a ten5)erature of li+2°F was introduced at the lower

end through the 3a in,pipe. These pressure readings indicate air leakage

from the stud space into the duct near the bottom and outward leakage from

the duct in the upper half of the stud space. Smoke measurements revealed

a flow of air into the stud space around the supply pipe at the bottom.

There would probably be an exchange of air between the room and the stud

space around both the upper and lower grilles unless they were fitted with

gaskets.

Fig. 3 shows graphically the relation between static pressure and air

leakage throu^ the walls of a duct section placed in an open room. This

graph indicates the ma.xi.ini.im air leakage that mi^t occur in an installation

where the stud space provided no resistance to loss of air. The air leakage

from the glass fiber duct in an actual installation in a partition would be

variable depending on the tightness of the particiilar stud space. In many

cases the warm air that leaked through the fiber glass would still enter

the room to be heated at cracks around the supply grille. If the 3'i’ in.

warm pipe was not properly caulked where it passed throu^ the plate, some basement or attic air would probably be drawn into the stud space and

possibly into the blender.

The insulating value of the glass fiber duct is unimportant when the

duct is installed in an inside partition. In an installation in an outside

partition the instilating value of the fiber glass is probably voided by the

leakage of warm air through the material into the stud space around the

duct. A con?)utation of the probable increased heat loss from a stud space

in an outside wall caused by the installation of such a blender indicated

that the 500 Btu/hr reduction in heat delivery# as recommended by the manufacturer for such an installation, is an ample allowance.

The sound level measurements indicated that the increase in noise

level one foot from the partition containing the blender, as a result of

operating a given blower and distribution system,was the same whether the

blender used a metal stack or a fiber glass stack in the stud space. Since

the air velocity was so low that no air noise was audible in the duct, it was throught that most of the sound was transmitted to the rigid members

of the partitions throu^ the metal stack heads which were nailed to the

studs.

The flame spread tests showed that at no time was there any flame

issuing from the specimen, or spread of flame on the surface of the specimen.

In one of the tests, the flame in5>inged on a double thickness of the material,

as representative of the lapped Joint, and one layer of the material, in this case melted at the point of flame application. No melting was observed

when the flame impinged on a single thickness of the material.
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I GLASS FIBER DUCT AIR LOSS VS STATIC PRESSURE

- 6 -

The Rodent repellency test showed that hungry rats gnawed through the

fiber glass material in an average of 22 minutes to reach a food supply.

This is rou£pily equivalent to the time required for penetration of solid

fiber box board. For individual specimens the time for penetration ranged

from 3 to 92 minutes.

The panels placed on the floor of a room housing wild rats^with no motivation for attack^ were badly damaged over ni^t.

These tests results indicate that this material does not offer appreciable resistance to rodent attack, and that its use is not to be recommended

in areas where it mi^t be e^qposed to rodents.

The mold growth tests indicated that none of the test specimens showed

visual evidence of mildew growth upon conpletion of the test. The viability

controls were heavily covered with reproductive growth. The sample complied

with the requirement of the tests in Federal pacification CC-T-191b.

V. Conclusions

The test results and experience obtained with the specimens of glass

fiber duct indicate that they are well-suited to ajod easily installed in

an existing dry wall partition, but not well-suited to existing wet wall

partitions where extruded plaster keys reduce the cross section area of

the stud space. The protruding keys make it very difficult to insert the

duct in the stud space and deform the duct sufficiently to seriously reduce

the air passage inside the duct.

Air leakage from the duct can be considerable, depending on partition

construction, but in a tight stud spaoe the leakage would be tiniii5)ortant

except that it probably voids the thermal insulation value of the fiber

glass. In many cases the air that leaked from the duct into the stud space

would be partially or wholly recovered by leakage into the room around the

grille. In a poorly-constructed partition the wall stack might promote

exchange of air between attic and basement or vice versa.

The duct material has good resistance to mold growth and it neither

spreads flame nor was it penetrated by flame under the test conditions.

At the air velocities recommended by the manufacturers there was no measurable difference in noise transmission with fiber glass and metal

stacks in the stud spaces.

The material does not appear to offer appreciable resistance to rodent

attack.
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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

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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.
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