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Appendix D. Ductwork Systems and Fire Hazards

Appendix D. Ductwork Systems and Fire Hazards

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D.1 Fire and smoke containment/hazards are factors
which influence the design and installation of
ductwork systems.
Information concerning fire protection systems is laid
down in BS 5588, Fire Precautions in the design and
construction of Building Part 9 (1989) Code of
Practice for Ventilation and Air Conditioning
Ductwork and tested in accordance with BS 476 Part
20 (1987) and BS 476 Part 22 (1987) for Fire and
Smoke Dampers and British Standard 476 Part 24
(1987) - ISO 6944 - (1985) for Fire Rated Ductwork.

Method 3 - Protection using Fire Resisting
The ductwork itself forms a protected shaft. The fire
resistance may be achieved by the ductwork material
itself or through the application of a protective
material provided that the ductwork has been tested
and/or assessed to BS476 Part 24 with a fire
resistance, when tested from either side that should
not be less than the fire resistance required for the
elements of construction in the area through which it
passes. It should also be noted that the fire resisting
ductwork must be supported with suitably sized and
designed hangers, which reflect the reduction in
tensile strength of steel in a fire condition i.e:
Fire resisting ductwork rated at 60 minutes (945°C),
reduces the tensile strength from 430
N/mm2 to 15 N/mm2.
Fire resisting ductwork rated at 120 minutes (1,049°C)
tensile strength reduced to 10 N/mm2.
Fire resisting ductwork rated at 240 minutes (1,153°C)
tensile strength reduced to 6 N/mm2.
Where the fire resisting ductwork passes through a
fire compartment wall or floor, a penetration seal must
be provided which has been tested and/or assessed
with the ductwork to BS476 Part 24, to the same fire
rating as the compartment wall through which the fire
resisting ductwork passes. It should also be noted that
where the fire resisting ductwork passes through the
fire compartment wall or floor, the ductwork itself
must be stiffened to prevent deformation of the duct in
a fire to:
a) maintain the cross-sectional area of the duct
b) ensure that the fire rated penetration seal around
the duct is not compromised.

D.2 Building Regulations in the United Kingdom
require that new buildings be divided into fire
compartments in order that the spread of smoke and
fire in the building is inhibited, and to stop the spread
of smoke and fire from one compartment to another,
for given periods of time as specified by the Building
Regulations 1991 (Approved Document B).
D.2.1 There are three methods of fire protection,
related to ductwork systems as given in BS 5588 Part
9 (1989).
Method 1 - Protection using Fire Dampers
The fire is isolated in the compartment of origin by
the automatic or manual actuation of closures within
the system. Fire dampers should, therefore, be sited at
the point of penetration of a compartment wall or
floor, or at the point of penetration of the enclosure of
a protected escape route.
Fire dampers should be framed in such a way as to
allow for thermal expansion in the event of fire, and
the design must provide for the protection of any
packing material included.
Standard types of fire dampers and frames are
described in Section 22 of this specification.
For further information refer to the impending HVCA
publication DW/TM3, `Guide to Good Practice for
the Design for the Installation of Fire
and Smoke Dampers'.

D.2.2 - Main areas within building where
Ductwork should be fire protected
The following notes are for guidance only, and it
should be noted that authority rests with the Building
Control Officer and/or the Fire Officer responsible for
the building. Reference on the folowing systems
should also be made to the current Building
a. Smoke Extract Systems:
If the ductwork incorporated in a smoke extract
system is wholly contained within the fire
compartment, it must be capable of resisting the
anticipated temperatures generated through the
development of a fire. BS 476 Part 24 also requires
ductwork, which is intended as a smoke extract,
must retain at least 75% of its cross-sectional area
within the fire compartment. If the ductwork
penetrates a fire resisting barrier, it must also be
capable of providing the same period of fire
b. Escape Routes covering Stairways, Lobbies and
All escape routes must be designed so that the
building occupants can evacuate the building

Method 2 - Protection using Fire Resisting
Where a building services shaft is provided through
which the ventilation ductwork passes and if the shaft
is constructed to the highest standard of fire resistance
of the structure which it penetrates, it forms a
compartment known as a protected shaft. This allows
a complicated multiplicity of services to be
transferred together through a shaft transversing a
number of compartments and reaching remote parts of
the building, without requiring further internal
divisions along its length. The provision of fire
dampers is then required only at points where the
ventilation duct leaves the confines of the protected
However, if there is only one ventilation duct and
there are no other services within the protected shaft,
between the fire compartment and the outside of the
building, no fire dampers will be required.

safely in the case of fire. Ductwork which passes
through a protected escape route must have a fire
resistance at least equal to the fire compartment
through which the ductwork passes, either by the use
of fire dampers or fire resisting ductwork.
c. Non Domestic Kitchen Extract Systems
Where there is no immediate discharge to
atmosphere, i.e. the ductwork passes to atmosphere
via another fire compartment, fire resistant ductwork
must be used. Kitchen extract ductwork presents a
particular hazard as combustible deposits such as
grease are likely to accumulate on internal surfaces,
therefore, all internal surfaces of the ductwork must
be smooth. A fire in an adjacent compartment,
through which the ductwork passes, could lead to
ignition of the grease deposits, which may continue
through the ductwork system, possibly prejudicing
the safety of the kitchen occupants. For this reason
consideration must be given to the stability, integrity
and insulation performance of the kitchen extract
duct which should be specifically tested to BS 476
Part 24 for a kitchen extract rating.
• Access doors for cleaning must be provided at
distances not exceeding 3 metres.
• Fire dampers must not be used.
• Use of volume control dampers and turning
vanes are not recommended.
Further information on kitchen extract systems will be
found in the HVCA publication DW/171 Specification
for Kitchen Ventilation Systems.
d. Enclosed Car Parks - which are mechanically
Car Parks must have separate and independent
extract systems, because of the polluted nature of the
extract air. Due to the fire risk associated with car
parks, these systems should be treated as smoke
extract systems and therefore maintain a minimum
of 75% cross-sectional area under fire conditions in
accordance with BS 476 Part 24. Fire dampers must
not be installed in extract ductwork serving car
e. Basements - Ductwork from Basements must be
Fire Rated
If basements are compartmented, each separate
compartment must have a separate outlet and have
access to ventilation without having to gain access
(i.e. open a door to another

compartment). Basements with natural ventilation
should have permanent openings, not less than 2.5%
of the floor area and be arranged to provide a
through draft with separate fire ducts for each
f. Pressurisation Systems
Pressurisation is a method of restricting the
penetration of smoke into certain critical areas of a
building by maintaining the air at higher pressures
than those in adjacent areas. It applies particularly to
protect stairways, lobbies, corridors and fire fighting
shafts serving deep basements as smoke penetration
to these areas would inhibit escape.
As the air supply creating the pressurisation must be
maintained for the duration of a fire, fire dampers
cannot be used within the ductwork to prevent the
spread of fire. Any ductwork penetrating fire
resisting barriers must be capable of providing the
same period of fire resistance.
g. Hazardous Areas
There are other areas within the building where the
Building Control Officer or the Fire Officer could
state a requirement for fire resisting ductwork, eg.
areas of high risk, Boiler Houses, Plantrooms,
Transformer Rooms etc.
D.2.3 Cautionary note to all Ductwork Designers/
Ductwork constructed to DW/144 Standard has no
tested fire resistance. General purpose ventilation/air
conditioning ductwork and its ancillary items do not
have a fire rating and cannot be either utilised as or
converted into a fire rated ductwork system unless the
construction materials of the whole system including
supports and penetration seals are proven by test and
assessment in accordance with BS 476 Part 24.
In the case where galvanised sheet steel ductwork is
clad by the application of a protective material, the
ductwork construction must be as type tested and
comply with the protective material manufacturers
recommendations, eg. gauge of ductwork, frequency of
stiffeners and non-use of low melting point fasteners or
rivets. Sealants, gaskets and flexible joints should be as
tested and certificated in accordance with BS 476 Part
Careful consideration must also be given to the
maximum certificated size tested to BS 476 Part 24 and
the manufacturers recommendations should always be

This appendix incorporates information given in the A.S.F.P publication `Fire Rated and Smoke Outlet Ductwork:
An Industry Guide to Design and Installation' available from Association for Specialist Fire Protection, Association
House, 235 Ash Road, Aldershot, Hampshire GU 12 4DD (Telephone: 01252 21322 Fax: 01252 333901)


E.1 General
E.1.1 For Hot Dip galvanizing after the fabrication of
any article it is necessary to appreciate the nature of the
process, including the surface preparation of the object
to be treated and the precautions to be taken in design,
fabrication and handling.
E.1.2 Hot Dip galvanizing involves dipping the object
into a bath of molten zinc (at a temperature of between
445° and 465° C), and it is necessary for the zinc to
cover the whole of the surface leaving no gaps in the
E.2 Design and fabrication
E.2.1 Rectangular ductwork must be fabricated using all
welded construction techniques with vented flanges and
stiffening frames (see E.2.3) as mechanical fixing and
lock-forming techniques are not compatible with the
galvanising process. In the course of dipping into the
molten zinc bath, unsightly panel distortion will occur
due to the relief of inherent stress in the steel sheet or of
any stresses that may have been built into the item during fabrication, or indeed of any stresses introduced
during the handling, loading or unloading of the item.
Table 20 indicates the minimum requirements for the
construction of rectangular ductwork.
E.2.2 It is essential to have a free flow of the molten
zinc over the object to be galvanized, together with
quick and complete drainage of the molten metal.
Because of the high temperature involved, the object to
be galvanized should be as rigid as possible, either by
the use of sufficiently heavy sheet or by stiffening or
bracing, or both.
E.2.3 Any sealed hollow section or cavity must be
adequately vented in order to obviate any possibility of
explosion. Holes of sufficient size (See E.2.4) in vertical
members must be provided diagonally opposite each
other, top and bottom of the member.
E.2.4 Vent holes should be of sizes as follows:
Size of
diameter of
vent end
(dia. or side)
drainage holes
Up to 25
50 to 100
100 to 150
Over 150

ever, the pickling process does not generally remove
grease, oil or oil-based paint, and such substances
should be removed by the fabricator by the use of
suitable solvents before the object to be treated is
delivered to the galvanizing works. Any surface rust
that develops on the object between the time of
treatment by the fabricator and delivery to the
galvanizing works is not important, as this is cleaned
off by the acid pickling process.
E.4 Handling and storage after galvanizing
E.4.1 While a galvanized surface will not develop rust
in the ordinary sense as long as the zinc coating is
undamaged, zinc is subject to what is known as `wet
storage stain,' which is a white powdery deposit on the
zinc surface. Wet storage stain can arise from the
stacking of articles when wet, acid vapours, the effect
of salt spray, the reaction of rain with flux residues, etc.
The damage to the zinc coating is negligible in most
cases. When the deposits are heavy, these should be
removed by brushing with a stiff bristle or wire brush.
E.4.2 Galvanized articles should therefore not be
stacked or loaded when wet; they should preferably be
transported under cover or shipped in dry, well
ventilated conditions, inserting spacers (but not
resinous wood) between the galvanized articles.

E.2.5 Stiffeners should desirably have their corners
cropped so as to allow a free flow of zinc. Stiffeners
should be rolled steel angle, uncoated.
E.3 Surface preparation before galvanizing
E.3.1 The steel surface to be galvanized must be
chemically clean before dipping to ensure a continuous
coating. This is mainly achieved at the galvanizer's
works by pickling in an acid bath and fluxing before the
article goes into the zinc bath. How

E.4.3 When stored on site or elsewhere, care should be
taken to avoid resting the galvanized article on cinders
or clinker, as the acid content of these substances will
attack the zinc surface.
E.5 Subsequent finishing
E.5.1 Paint finishing subsequent to galvanizing is
sometimes required either for additional protection or
for decorative reasons. Galvanised surfaces require
chemical pretreatment prior to painting. Examples of
such a treatment are T-Wash and Etch Primer Types.
Advice should be sought from the paint manufacturer.

This Appendix incorporates information given in publications available from the
Galvanizers' Association, 6 Wrens Court, 56 Victoria Road, Sutton Coldfield, West Midlands B72 1SY

usually adjusted by the manufacturer to balance
forming response, weldability and corrosion
resistance. It is readily welded in thin sheet form and,
since it does not form a hardened weld HAZ, no postweld heat treatment is required. It is widely used for
automotive exhaust system parts and is suitable for a
range of ducting and structural applications in mildly
corrosive applications.

F.1 General
F.1.1 Stainless steel is not a single specific material:
There is a large family of stainless steels with varying
compositions to suit specific applications, but all
contain at least 11 % of chromium as an alloying
F.1.2 Modern stainless steels have a combination of
good formability and weldability, and can be supplied
with a variety of surface finishes (see E4.1 below) They
have been developed to cover a wide range of structural
uses where high resistance to corrosion and low
maintenance costs are demanded.

F.2.2.2 17% chromium ferritic steel, 430S17, New
Designation 1.4016, x6Cr 17.
Forming and general characteristics are similar to the
409 grade, but the higher chromium level confers
better general corrosion resistance.

F.1.3 Ductwork applications for which stainless steels
are particularly suited include those where a high
integrity inert material is essential; where a high degree
of hygiene is required; in the chemical industries where
toxic or hazardous materials may be contained; in
nuclear and marine applications (e.g. on offshore
platforms). Stainless steels also find application in
exposed ductwork where their finish can be used to
aesthetic advantage.
F.2 Grades of stainless steel
F.2.1 The grades of stainless steel most commonly used
for ductwork applications are among those covered
currently by BS 1449, Part 2. However, a European
Standard, will supersede this British Standard. New
designations of the most common steel grades are given
in Table 21.
In some cases there are minor differences in chemical
composition between the BS and EN grades.
Before a grade is specified, the nature of the interior and
exterior environments of the ductwork system should be
taken into account. The steels described below cover
most normal applications. However, advice on specific
corrosion risks should be taken if the ductwork is to be
installed in a chemically contaminated atmosphere, or is
to be used to transport contaminated air, particularly if
there is a risk of internal condensation. More highly
alloyed grades of stainless steel with enhanced corrosion
resistance are available if required.
The commonly used steels divide into two main
families; the lower alloy ferritic 11-18% chromium
stainless steels are magnetic. The austenitic, 18%
chromium, 9% nickel steels have generally better
corrosion resistance and are non- or only slightly
F.2.2 The more commonly used stainless steels and
their characteristics are described below.
F.2.2.1 11.5% chromium ferritic steel with a titanium
addition, 409S 19, New designation 1.4512,
This, and related grades, are among the leanest alloyed
of the stainless steels. Forming characteristics are
similar to those of mild steel, so it can be worked
using conventional practices. The composition and
processing of the steel is

F.2.2.3 18% chromium, 9% nickel austenitic stainless
A widely used grade is 304S15, New Designation
1.4301, x5CrNil8-10. There are compositional variants
within this family, designed to give specific
formability and welding characteristics. All are
weldable and have good general corrosion resistance
to normal and mildly corrosive atmospheres. They are
ductile and formable, but forming loads are higher
than for mild steels and suitable, robust equipment is
F.2.2.4 17% chromium, 11% nickel, 2% molybdenum
austenitic steel, a widely used grade of this type is
316S31, New Designation 1.4401, x5CrNiMo17-11-2.
This steel has a significantly higher corrosion
resistance than the standard 18% chromium, 9% nickel
steels and is suitable for use in more aggressive
environments such as are met in ductwork in process
plants. However, more highly alloyed stainless steels
with better corrosion resistance are also available and
the advice concerning aggressive environments given
under section F.2.1 above should be noted.
F.3 Availability
Stainless steel is supplied in a wide range of thicknesses, from 0.4 mm for cold-rolled sheet and coil, and
from 0.075 mm for precision rolled strip. It is supplied
in slit widths as specified by the customer, up to a
maximum width of 2030 mm, depending on thickness.
Material compatability of sheet, section and fixings is
not always assured in practice due to commercial
F 4 Surface finishes
F4.1 Stainless steel is available in a wide selection of
finishes, varying from fine matt to mirror polished, as
defined in BS 1449: Part 2: and in EN 10088: Part 2.
Mill finishes
Type 2D


Cold finished softened and descaled.
A uniform matt finish.