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Ducts of combustible materials for heating systems
CANADA
DIVISION OF BUILDING RESEARCH
DUCTS OF CObIDUSTIBLE IdATERIALS FOR HEATING SYSTEMS
by
N. B. Hutcheon
A study prepared for the NRC Associate Committee on the
National BUilding Code
Internal Report No. 197 of the
Division of BUilding Research
OTTAWA
series of research Building Research Buildin gOode of this important The Associate Oommittee on the National Building Oode and its Advisory Fire Group have been giving very careful con-sideration for some time to questions raised by a proposal to relax a restriction in the
1953
Edition of its Code on the use of combustible linings in heating ducts.The Associate Oommittee requested the Oouncil's Division of Building Research to review the information on this subject and to report the results so that the Oommittee might have conveniently available the best possible back-ground for its own deliberations and ultimate decision.
Dr. N.B. Hutcheon, Assistant Director of the Division (and a mechanical engineer) undertook this survey and has pre-pared this Report. It is made available in this way for the use of the Associate Oommittee in the first instance but also so that the results of
Dr.
HutCheon's careful study may be used by others who wish to have an objective over-all view of this difficult detailed technical problem in the field Of house design and construction.The ｾ･ｰｯｲｴ is one of a continuing studies being undertaken by the Division of for the Associate Oommittee on the National as a contribution to the steady improvement national document.
Ottawa Tray 1960
Robert F. Legget Director
Requirements of the National Building Code 1953 Comment on National Building Code (1953)
ReqUirements
Requirements of Central Mortgage and Housing Corporation
Comment on Requirements of the Housing Standards 1958
ReqUirements of Federal Housing Administration National Warm Air Heating and Air-Conditioning
Associa tion
Requirements of the National Fire Protection Association
Fire Hazard Considerations
Test Methods and Criteria for Combustibility Test Methods and Criteria for Flame Spread Durability Considerations
Conclusion
Appendix A. Extract from Appendix 4.1.B National Building Code 1953
Appendix B. Ilefinition of "Noncombustiblell
Appendix C. Ilefinition of "NoncombustibleII for the
National Building Code (Canada) 1960
2 3 4 5 6 8 9 10
14
1617
20by
N. B. Hutcheon
Sheet metal has been and still is by far the most common material used in the fabrication of ducts for the handling of air in heating and ventilating systems. Other non-combustible materials such as concrete, asbestos-cement, and ceramic tile which compete successfully with metal for some fluid piping applications have been used to a limited extent in duct construction. They may have advantages over metal in special cases, notably where corrosion is an important consideration. In times of war emergency, when metals are in short supply for civilian purposes, alternatives to metal find increased use and tend to remain in competition with metal subsequently to the extent that they have be8n found to be technically acceptable and competitive in over-all costo
Organic materials such as wood, plywood, and paper are potentially competitive with metals on a cost basis for many purposes but have tended to be restricted for duct use by
technical reqUirements in respect of durability and fire safety. Tubes laminated from paper were developed and extensively used during World War II for the forming of ducts in concrete slabs, in place of metal ducts and conduits. After World War II sheet metal continued at times to be in short supply and this, coupled with the availability of production capacity for laminated paper tUbes, led to the promotion of such products for use in forming heating ducts in basementless house slabso The acceptance of paper products in such applications has grown but there is still a reluctance on the part of many agencies to accept combustible materials for duct construction of any kind o
The National Building Code of Canada (1953) has not allowed the use of ducts of other than non-combustible materials.
This question has come up for reconsideration in the 1960
revision of the Code. The available evidence and the considerations involved in the acceptance of combustible materials for ducts
are now discussed. This discussion relates mainly to the use of combustible materials as duct liners in slab on ground
residential construction but a possible extension of use beyond this limited application must also be kept in mind since there may well be requests for such consideration in the future. REQUIREMENTS OF THE NATIONAL BUILDING CODE 1953
6.2.3010.3(b) Warm Air Supply Ducts for One- and ｾｙｯＭｆ｡ｭｩｬｺ Dwellings "Eve.ry warm air supply duct for one- and two-family
dwellings shall be constructed of non-combustible corrosion-resistive materials of adequate strength and durability such as aluminum, galvanized iron, and asbestos-cement. II • • • "Linings used inside of any
duct shall be non-combustible."
6.2.3.10.3(c) Cold or Return Air Ducts for One- or Two-Familz Dwellings "Any such duct, except as required by the following
paragraph may be constructed of metal, or I-inch (nominal) wood boards, or other suitable material, provided that no material more flammable than I-inch wood boards shall be used."
"Portions of such a duct within 6 feet of the heater
shall be constructed in accordance with the provisions of Table 6.8 for warm air ducts. 11 • • •
"The interior of any combustible duct shall be lined with non-combustible material at points where there might be a danger from incandescent particles dropped through the grilles or heater, such as directly under a floor register and at the bottom of a vertical duct, or directly under a heater having a bottom return."
Comment on National Building Code (1953) Requiroments
It seems qUite clear that the requirements of the NBC 1953 envisaged mainly metal and asbestos-cement as materials for supply ducts, but the stated requirement was for non-combustible materials. The definitions of combustible and non-combustible were never clearly settled. The main guidance given in this was
the statement contained in Appendix 4.1.B (see Appendix A) which left room for argument when applied to the paper products used as forms and liners for ducts formed in slab floors of dwellings.
Any argument that ducts formed in slabs are essentially of concrete and therefore non-combustible under the 4.1.B definition may be countered by the clause in the Code which states that linings used inside of any duct shall be non-combustible. This raises
again the question of the definition of non-combustible, but it fixes attention fairly on the liner.
Some agencies have accepted paper tube products as forms and liners in slabs but have required that metal be used within a certain distance of furnace plenums and below registers and risers. It is interesting in connection with requirements for return ducts in the NBC 1953 to note that they may be made of I-in. wood but must be metal within 6 ft of the heater and must be lined with non-combustible material (amount not specified)
at points where there might be a danger from incandescent particles dropped into registers.
Any
argument that organic liners should withstand a fire such as might be kindled in the debris in such a duct should also recognize that the same standard is not required for return ducts which might be regarded as more likely to experience this condition, but which are permitted to be of wood within anunspecified distance of a register or riser. Both might have forced air flow, but the supply duct might be warm, up to 200 or 250°F. When in use, a return duct is more likely to collect debris than a supply duct, since the air flow through the latter must normally
pass through filters and the furnace before passing out of the system through the registers.
Requirements of Central Mortgage and Housing Corporation
The current requirements of CMRC for slab on ground con-struction are contained in Housing Standards 1958. Pertinent extracts
follow:-ttThis compacted sub-soil surface shall be at least 2 inches higher than finished grade at the exterior walls of the house."
itA base of granular material compacted and levelled shall
be placed over the sub-soil. The base shall consist of at least 6 in. of crushed rock, natural gravel or other aggregate
acceptable to the authority having jurisdiction."
"A vapour barrier shall then be placed over the base prior to placing of the concrete. It shall consist of a 4 mil polyethylene film placed over a layer of building paper, or 45 or 55 pound roll roofing, or other material accepted by the authority having jurisdiction."
ltMonolithic concrete shall be poured to a depth of not less than
3-b
in. 1I"Rigid insulating material shall be placed at the edge of the slab and shall extend at least 6 in. below finished grade. It
HAll natural or manufactured gas burning appliances and their installation shall at least be in accordance with the Canadian Gas Association Stan(lards. II
"Desi gn and installation of water or steam heat distribution equipment shall at least conform to the published bulletins of the Institute of Boiler and Radiator Manufacturers.It
"Design and installation of gravity and forced warm air heat distribution systems shall at least conform to the published bulletins of the National Warm Air Heating and Air Condition-ing Association. It
Comment on ReqUirements of the Housing Standards 1958
Elevation of the rough slab is required to be ｬｬｾ in. above finished grade outside the house. An additional! in. topping is called for, making a total of 12-a- in. Nothing more is said about ducts in slabs than that heating systems shall con-form to the reqUirements of the National Warm Air Heating and Air Conditioning Association. Presumably a thickened edge will be permitted in which any warm air perimeter dvct will be placed. Assuming 2 in. of cover over an 8-in. diameter duct, the bottom of the duct would be approximately 2 in. above finished grade. Grading of the ductwork to a central plenum would reduce this unless the slab were thickened locally.
It is proposed that the Standards be amended to require that sidings be at least 12 in. above grade if of wood. This could be met by starting siding at slab level as called for under present reqUirements.
One organic duct material, Sonoairduct regular, is covered in the ｃｾｬｬｩｃ Acceptable Building Materials, Systems and Equipment list. Reference to it is as
follows:-"Description - Laminated fibre duct provided in sizes 2' to 24Y in length and 2" to 241! in diameter.I!
ffConditions
10 Sonoairduct regular may be used only in the concrete portion of slab on ground construction providing it is not connected directly to the heating unit. Such ductwork shall be separated by at least 6' of galvanized iron, aluminum or asbestos
cement piping.
2. Sonoairduct regular may only be used as in (1) above when the heating stystem is fired by a gas or oil burning applicance.
3.
Sonoairduct XP is not accepted at this time." (Accepted 4.8.54 - Revised 6.7.59)Prior to about 1956, Sonoairduct was a vrax-finished product. About 1956, an improved product was produced, using resin-bonding of paper and an aluminum foil liner.
Requirements of Federal Housing Administration
It is believed that at least one type of combustible duct has been permitted by Federal Housing Administration in the United States in basementless house slabs since 1950. The current reqUirements for such materials are given in an ｆｦｾ
publication dated April 1956 entitled "Criteria and Test Procedures for Combustible Materials used for Warm Air Ducts Encased in Concrete Slab Floors". This covers flame spread, crushing strength, bending strength, deterioration and odour, delamination, and hydrogen ion concentration of papero
Further requirements covering installation permit the use of combustible ducts only when cast integrally with concrete slab floors, with automatically fired oil or gas units equipped
with 200°F temperature limit control and located above floor
level o A continuous vapour barrier is required and complete encasement in not less than 2 ina of concrete is called foro Ducts are required to slope toward the plenum chamber and the bottom of all ducts shall not be ｬｾｬ･ｲ than finished grade
exterior to the slab. Galvanized metal joints and fittings are required and the combustible duct material is not permitted within 2 diameters of a plenum, or for use as risers, or within 2 duct diameters of a riser.
Recent activities under the Building Research Advisory Board are pertinent to the FHA picture. BRAE Publication 651 dated 1959 is entitled "Criteria for Ducts to be used in
Hesidential Warm Air Heating and Air Conditioning Systems"
(Technical Study for Federal Housing Administration). 1bis report covers the deliberations of a BRAE committee regarding the criteria to be used for ducts in residential work but excludes ducts
encased in concrete. The availability of tests and criteria in respect of 12 essential duct properties are reviewed. Some are considered acceptable, some are recommended for laboratory
validation before use o The report contains the following recommenda-tion regarding one important test:
"Flame Spread Currently available test methods for flame spread treat the material to be tested as a flat specimen, and do not consider the duct in its fabricated form, nor
do they recognize the possible deleterious effects of ageing upon the material o The Advisory Committee believes, therefore, that performance criteria for ducts are needed and recommends that they be establishedo Accordingly, recommendations for research leading to the establishment of such criteria have been made under 100, po 120
"Until such time as the above can be accomplished it is recommended that the material from which the duct is to be fabricated, including facings or treatments if any, have a flame spread classification of not over 50 as
determined in accordance with ａｓｾｔ Designation E-84-50T (NFPA 255 or UL 723). Pending development of a suitable ageing test, it is also recommended that the provisions
of paragraph 121, d(l) of NBFU-NFPA Standard No. 90-B be applied, i.e., 'They shall be made from a base material of metal or mineral'."
These recommendations are not binding upon FHA but are of great interest to the extent that they reflect the
thinking of an able committee in the light of present knowledge. It may be inferred from what is said about tests and criteria for flame spread and ageing that generally acceptable tests for ducts to be used in slabs are also difficult to establish. National Warm Air Heating and Air-Conditioning Association
The latest edition (1959) of Manual 4 of lITvAHACA permits the use of combustible ducts in loop and
perimeter-radial systems. Such ducts must satisfy the requirements of the FHA Criteria and Test Procedures outlined above. A vapour barrier under the slab and a mlnlmum of 2 in. concrete encasement are
called for. There must be a minimum of Ｒｾ in. of concrete cover over ducts. The slab is required to be at least 8 in. above
finished grade so that the bottoms of ducts could be below grade, but it is also noted that local reqUirements may call for ducts to be at least 2 inches above finished grade. The necessity for a well-drained site is emphasized. Combustible duct is not to be used within 2 ft. of the furnace plenum nor within 2 ft. of a vertical connection to a riser or register. All ducts are to be
graded toward the plenum. Nothing is said about the materials permitted for joints and fittings.
RegUirements of the National Fire Protection Association These are outlined in NFPA No. gOB, Standards for the Installation of Residence TYpe Warm Air Heating and Air-Conditioning Systems July 1956. (The Standards of the Dominion Board of Insurance Underwriters are almost identical. See
DBIU No. gOB August 1955.)
Supply ducts are required to be constructed entirely of non-combustible material, equivalent in structural strength and durability to certain gauges of metal ducts specified. An
exception is made for supply ducts in slabs as follows: "Supply ducts that are completely encased in not less than 2 in. of concrete in a floor slab need not meet the requirements of para. 121 (as given above) except within 2 ft. of the furnace supply plenum, and within
2 ft. of a vertical connection to a riser or register. II
No further requirements are set down for ducts in
slabs. Down Flow Furnace Systems are required to have automatically operated oil,gas,or electric furnace equipped with a control that will limit outlet air temperatures to 200°F. When an under floor
space is used as a supply plenum the enclosing material is re-quired only to be no more flammable than I-in. wood boards.
Return ducts are,with certain limitations,permitted to be of I-in. wood boards, and the interior of combustible returns ducts must be lined with non-combustible material (not specified) under registers and risers.
Fire Hazard Considerations
There is little reason to suppose that warm air heating ducts of cellulosic or other combustible materials encased in concrete slabs will become a fire hazard solely as a result of heating. Temperatures much higher than the 200°F or 250°F
produced by the heating system will be required to produce com-bustible gases. These will only be ignited if there is pilot ignition from a spark, a flame or incandescent material, unless the temperature is raised to a high level sufficient to produce spontaneous ignition. It is unlikely that the duct material will be set on fire by itself without the aid of an initiating fire to produce heat and pilot ignition, but on the other hand,
am
rcombustible material can be ignited if the initiating fire is large enough.
The main hazard from combustible duct linings encased in concrete will arise from the support which they give to the spread of flame thrOUgh the duct system under the influence of an initiating fire. The extent of the hazard will depend not only upon the flame spread characteristics of the material but also upon the likelihood that an initiating fire of sufficient size will occur. It is reasonable to suppose that the worst case to be considered is one in which the air system blower continues to operate after a fire has started either in the
furnace, the plenum,or in the duct itself, producing air velocities of several hundred feet per minute.
A plenum fire in a basementless house, if large enough, might well be projected through a non-combustible duct system under the action of the air system blower. But a smaller plenum fire incapable of reaching a register with non-combustible duct
Debris falling through a register at times when the air system
blower is shut off might then be blown along the duct to accumulate at points away from the registers. Debris entering the system
during construction could conceivably lodge in the duct between registers. As in the case of the combustible duct itself, this debris is unlikely to be set on fire by temperature alone but might be ignited by a pilot ｩｧｮｩｾｩｯｮ source such as a glowing
cigarette. The incandescent material for ignition would have to enter a register, in the case being considered, and be moved along by the air stream until it came in contact with combustible debris. A fire of sufficient size to ignite the duct lining might then
result. If the duct material had a sufficiently high flame spread tendency, flame might develop along the duct and issue from a
register to create a serious hazard of ignition of the building. It may be concluded that the increased fire hazard associated with the use of combustible ducts encased in concrete slabs lies primarily with their capability to support flame
spread, under the action of an initiating fire. Initiating fires could arise in a plenum, beneath a register, or in a duct itself. The possibility that an initiating fire will develop will be
largely independent of the duct lining, and will not be greatly increased by the use of moderately combustible duct linings. It is reasonable to expect that the use of combustible ducts in con-crete slabs will increase the fire hazard, but it may be that this increase is very small.
lni tiating fires capable of involving cotibustLbLe ducts in a fire, in the ways already discussed, are observed so rarely that there is no general awareness even in the minds of fire officials that they do occur. Consider, for example, that there are many tens of thousands of warm-air installations in Canada in
which wood construction is allowed for part or all of the return duct. Return ducts, furthermore, present the greatest possibility for accumulation of debris, ignition, and spread of fire back
towards the furnace under the action of the return air so as to create the greatest probability that the duct lining materials could become involved. Yet this occurs in so few cases that it is not recognized as a reasonable probability, and wood-lined return ducts, and wood-lined crawl space plenums continue to be allowed under almost all codes.
Field experience is ultimately the best indtcator of probabili41 of occurrence. Al though statistics of in-duct
fires have not been collected in any systematic way, there is no real indication from the field that initiating fires in ducts capable of involving combustible duct linings are likely to occur. At the same time, both reason and demonstration indicate that
they can develop.
All of the foregoing discussion relates to combustible duct materials encased in concrete for use as warm air supply ducts. The case of similar materials used as non-encased ducts may be similarly argued, so far as fires initiated within
the
ducts are concerned. There is, however, a further major hazard
in such cases, that of a fire outside the ducts which may destroy the duct wall and then spread through the duct system to other parts
of a building. It is interesting that the only documented case
of a dwelling fire involving ducts in recent years in Canada occurred in a basement fire which destroyed the aluminum duct and then spread to upper stories through the opened duct system. This led to a
Test Methods and Criteria for Combustibility
Since the National Building Code 1953 called for non-combustible materials for duct construction, the only possibility for acceptance of a duct material under the Code was to show that it was non-combustible. Unfortunately, no generally acceptable test methods and criteria for ､･ｴ･｝ｾｩｮ｡ｴｩｯｮ of combustibility existed at that time. Subsequent to the appearance of the Code, Appendix 4.1.B was produced. It contained a series of guiding notes for use in the interim period until suitable standards
could be developed in Canada.
SpecificLtion B54.1, "De'termt.na tf.on of Non-Combustibility in Building Materials" has now been produced by the Canadian Standards Association. Having this, the Associate Committee on the National BUilding Code has n ow approved a definition of "non-combustible", based on the definition given by the NBFU National Building Code 1955 but modified to include the new CSA Specification (see
Appendices B and C).
CSA Specification B54.1 reads in part as follows:
"This specification provides a means of determining the non-combustible nature of elementary building materials. It
shall be applied to materials used in the construction of buildings. It does not apply to materials with a decorative or protective
coating, or impregnation, or built up of laminations of dissimilar materials. (Adhesives betvjeen laminations shall be considered laminations.)"
It will be seen that CSA B54.1 deals only With the elementary material and intentionally excludes laminates. ｾｉｄｴ･ｲｩ｡ｬｳ
group (b) of the llew NBO definition, while other laminates of
various kinds must conform to group (c) requirements. It is likely that most ducts containing organic materials will be of such con-struction as to be admissible as non-combustible only if they meet the group (c) requirements.
Unfortunately, the basis for evaluation of ducts
containing combustible materials is still not made clear. The new lIDC definition of non-combustible states at the outset that the
definition does not apply to the detennination of whether a material is non-combustible from the standpoint of clearances to heating
appliances, flues, or other sources of high temperature. It has, however, already been argued for the case of ducts encased in concrete that the hazard exists, not from high temperature, but from flame spread and tt follows that they need not be excepted on the basis of the above limitation, but can be considered under the appropriate section of the definition.
Laminated paper ducts, if permitted to be considered under the IillC definition of non-combustible, would have to meet
the reqUirements of group (c). Here again there are two difficulties. The specified test method is the ａｓｾｪ E84 tunnel test which applies to materials in sheet form and not as formed into ducts. Objection may well 1Je raised that specimens f'orme d flat will not be repre-sentative af the materials as formed into ducts, and that ducts which are e pl.Lt and flattened for 'test s will differ in performance from the uriginal shape. There is also a further diffiCUlty since the materials are reqUired to be "••• of such composition that surfaces that would be exposed by cutting through the material in any way •••11 must meet the same requirements as to flame spread
rating and absence of continued progressive combustion as the original form. This is obviously open to differences in inter-pretation which could become of major concern, for example, in
the case of a paper laminate having a foil liner. It is not clear whether it is intended that the laminated paper without the foil must meet the requirement or only that the foil should be cut for testing to expose, in some way representative of service conditions, the paper laminate beneath. It may well be that such cases will not arise in practice, if, as is not un-likely, the laminates containing combustible material which are proposed for ducts will fail the test in the original form, with-out cutting.
These difficulties which appear to be compounded for the case of ducts containing some combustible material are in-dicative of the very great problem of defining "non-combustiblefl for code purposes. It may be noted that in dealing with compound materials containing combustible surfacings or laminae, it has
been necessary to resort to flame spread tests, which are normally considered as distinct from combustibility tests, in order to
determine how much "combustible" material may be permitted before the combination becomes unacceptable.
Test Methods and Oriteria for Flame Spread
The only flame spread test method having any broad acceptance for a wide range of materials is the Underwriters
Tunnel Test adopted for ASTM E84. The difficulty in establishing a suitable flame spread test is indicated by the existence of a large number of other flame spread tests, many of which have been developed for use with specific materials. Further, the ASTM E84 test has only recently received SUfficient support to permit it to be moved from the tentative status which it has held before ASTM since 1950. Nevertheless, it remains at present the only generally acceptable test and has been adopted for Oanadian use in the corresponding CSA specification.
The difficulties in applying ASTM E84 to ducts have already been discussed under the previous section on combustibility. That this is of concern in the United States also is confirmed by the quotation on page 8 from BRAE Publication 651, which, although it refers to ducts which are not encased, is still pertinent.
The FHA criteria for combustible duct materials (see page 6) call for a flame spread test. It involves a duct specimen 8 in. in diameter and
4
ft long set at 20° to the horizontal. A gas-fed bunsen burner is placed at the lower end so that flame impinges onthe interior surface of the duct. There is a requirement for minimum time: for flaming from the upper end of the specimen, and for
self-Gxtinguishment after removal of the burner. This test is the only one specifically devised for ducts and has no other official status than that given by its use by FHA and as accepted by WwAHACA.
Should it be decided that duct materials which cannot meet the requirements for non-combustible may still be acceptable for some uses, i t will be necessary to establish some upper limit on combustibility, or flame spread, or both, for such "combustible" ducts. As previously argued, such ducts when encased in concrete might be adequately assessed as to fire hazard by a suitable flame
spread test alone. However, if to be used as supported, non-enclosed ducts, they should probably also be required to meet some further criterion as to combustibility.
Durability Oonsiderations
The second major matter of concern over combustible duct performance in slabs is that of degradation with time, not only with respect to ability to remain in place and to function as a
duct but also in relation to any increase in the fire hazard which it may present. This matter is complicated by the inaccessibility of ducts in slabs for examination, and replacement in the event of failure. Degradation will be related mainly to incidence of wetting and to repeated cycling from damp to dry conditions.
It is a common requirement, where the use of com-bustible tubes is permitted in the forming of ducts in slabs, that there shall be complete encasement in not less than 2 in. of concrete and that a vapour barrier shall be used. Ducts so formed are also in most cases required to be 2 in. above finished exterior grade. In some cases well-drained sites are called for, and ducts are required to be graded towards the
plenum so that no water can lie in them. When these requirements are met, the duct lining material in a high percentage of the installations will never be subjected to more serious wetting conditions than those provided by the wet concrete in which it is placed. The duct liner will be subjected to liquid water conditions for an hour or uvo, until the concrete sets, and
following that to a moisture condition corresponding to a humidity quite close to 100 per cent while the concrete dries. Drying
of the slab may take several months unless accelerated by operation of the heating system at an early age. Following this, the
moisture condition will be caused to cycle ｢･ｾｎ･･ｮ moderately
high humidities in summer and very Ion humidities with temperatures up to 2000 P
in the heating season.
Under the worst possible conditions, the vapour barrier may be omitted, the encasement in concrete may be incomplete, the duct liner may be in contact with SOil, and the site may be SUbject to periodic flooding. WIlen water can enter a duct system and remain until the furnace is operated, very serious condensation is usually produced in the house, and a most unsatisfactory con-dition results. It is difficult to decide, in establishing the environmental conditions that a duct liner should be able to with-stand, whether the conditions produced by such serious design
cannot be decided by laboratory studies, which at best can only provide some improved basis for making jUdgments. There is, inevitably, room for much argument over the sUitability of any such performance requirements which may be established, since these must always be arbitrary to some degree until they can finally be validated through widespread field experience.
The duct liner should be able to withstand at least one or more wetting cycles and some substantial number of cycles
｢･ｾＷ･･ｮ a high humidity condition at room temperature and a dry, heated condition. The performance requirements set out by
FHA include tests of this kind which cannot be considered part-icularly severe. A more severe test would be provided by
requiring that the material pass a number of cycles from a thoroughly saturated condition such as might be produced by
flooding of the system to a completely dry condition correspond-ing to that of winter operation under heatcorrespond-ing conditions.
Accelerated laboratory tests ought to provide for severe wetting and drying conditions in order to simulate properly, on a short time basis, the conditions produced in practice under much longer cycling times.
The most serious effect of degradation is likely to arise through delamination or disintegration of the liner which might conceivably cause blocking of the duct and render it unservice-able. It is possible also that ducts which just pass a non-combustibility or a limiting non-combustibility requirement when new might well fail such a test after degradation.
Degradation by rotting is not likely to be as serious as disintegration or delamination from wetting and drying. The duct will, if completely encased in concrete, be protected from soil bacteria and the operation of the heating system will, by
both heating and dryint, periodically discourage the growth of organisms.
Conclusion
This review and i lscussion of some of the major
considerations involved in ｾｬ･ acceptability of ducts of com-bustible materials shows that the greatest difficulty in such acceptance is in the establishment of suitable test methods and criteria by which their use may be regulated in the interests of safetyo It is not evident that the use of combustible ducts encased in concrete for basementless house construction will necessarily add appreciably to the hazard of fire or of failure of the system to function.
NATIONAL BUILDING CODB 1953
Combustible Olassification
7v11en the National Building Oode (1953) was issued, corrunittees both in the United States and Oanada were con-sidering mat er l.a Ls and assemblies with a view to classifying them accoYding to their combustibility. No firm conclusions have yet t.e en :,:'cc;ached. One of the difficulties in forming this classification is that many assemblies which have been regarded as non-combustible contain some combustible materials.
Comm0n sense and judgment must be combined with the best technical information. The following notes are included as a
guide:-(a) m1Y ｡ｳｾ･ｭ｢ｬｹ should be regarded as combustible in which the frame, or structural parts) are made of wood or
otter organic material, or by-products of these;
(b) Any assembly which is wholly made of inorganic materials should be regarded as non-combustible;
(c) Jeuveen the above ｾｊｏ groups is a wide range of assemblies which. ｭ｡ｾｔ leave doubt as to their classification. Any
assembly which is constructed essentially of non-combustible materials but which has some combustible material forming a part of the construction is usually regarded as non-combustible construction;
(d) Gypsum concrete and cinder blocks which incorporate an amount of combustible aggregate as limited in the material specification in Part
5:
Materials are considered non-combustible construction.DEFINITION OE' !lHOlTOONffiUSTIBLEII
(Extracted from The National Building Code
1955
Golden Anniversary EditionNational Board of Fire Undenvriters)
Noncombustible as applied to a building construction material means a material which, in the fo:rm in which it is used, falls in one of the following groups (a) through (c). It does not apply to surface finish materials nor to the determination of whether a material is noncombustible from the standpoint of clearances to heating appliances, flues or other sources of
high temperature. No material shall be classed as noncombustible wht.ch is subject to increase in combustibility or flame spread rating beyond the limits herein established, through the effects of age, moisture or other atmospheric condition. Flame spread rating as used herein refers to ratings obtained according to the Standard Test Methods for Fire Hazard Classification of Building Materials of Underwr-i ters' Laboratories, .Inc , , ASTIYl
E84.
(a) Ｑｾｴ･ｲｩ｡ｬｳ no part of which will ignite and burn when subjected to fire. Any material which liberates
flammable gas when heated to a temperature of 1,380 F., for five minutes shall not be considered noncombustible within the meaning of this paragraph.
(b) Materials having a structural base of noncombustible material, as defined in (a), with a surfacing not over
1/8-inch -Chicle wht ch has a flame spread rating not higher than
50.
(c) Materials, Jther than as described in (a) or (b), having a surface flame spread rating not higher than
25
withoutevidence of continued progressive combustion and of such composition that surfaces that would be exposed by cutting through the material in any way would not have a flame spread rating higher than 25 without
Definition of "Honcombustiblelt
for the
National Building Code (Canada) 1960
The definition of tlnoncombustible" adopted in January 1960 by the Associate Committee on the National Building Code is identical with that of the NBFU National Building Code 1955 as shovvn in AppendiX B, except that the whole of item (a) is replaced with the following:
"(a) Materials which are classed as non-combustible when tested in accordance with CSA specification B54.1-1960, Specification for Determination of Non-combustibility of Building Materials."
