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Appendix A. Air Leakage from Ductwork

Appendix A. Air Leakage from Ductwork

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for the classification for the section of the ductwork that is to be tested.
The tests shall be carried out as the work proceeds
and prior to the application of thermal insulation.
In the event of test failure of the randomly selected
section, the designer shall have the right to select
two further sections at random for testing. Where
successive failures are identified there shall be a
right to require the contractor to apply remedial
attention to the complete ductwork system.
The contractor shall provide documented evidence
of the calculations used to arrive at the allowable
loss for the section to be tested and the client, or
his agent, shall witness and sign the results of the

Respecting both the cost and programme implications
associated with testing ducts for leakage, the designer
may, for example indicate that a particular system is
tested as follows:
a) High pressure ducts - all tested.
b) Medium pressure ducts - 10% of the ductwork
shall be selected at random and tested.
c) Low pressure - untested.
In the case where a random test is selected for medium
pressure ducts the following clause is suggested for
inclusion by the designer.
The designer shall select at random a maximum of
10% of the duct system to be tested for air leakage.
The duct shall be tested at the pressure recommended
in Table 17 of DW/144


There may be situations on a project where circumstances
dictate that special consideration be given to containing
air losses, e.g. a long run of ductwork may incur a
disproportionate level of air loss.
In cases such as this example the designer can specify an
improved standard of airtightness, i.e. 80% of allowable
loss for Class `B' ducts. The designer should not specify a
Class `C' test at Class `C' pressure for a Class `B' duct.
• High pressure ducts
100% test
• Medium pressure ducts
See A5
• Low pressure ducts
• Exposed extract systems
• Ceiling void extract
• Secondary ducts from
VAV or fan coil units
• Flexible ducts
• Final connections and
branches to grilles and
Items of inline plant (eg. Figs. 168 to 175) will not
normally be included in an air leakage test. The ductwork
contractor may include such items in the test if the
equipment has a certificate of


conformity for the pressure class and air leakage
classification for the system under test.
The designer can calculate with reasonable accuracy
the predicted total loss from a system by:
a) Calculating the operating pressure in each
section of the system.
b) Calculating the surface area of the ductwork
in each corresponding pressure section.
c) Calculating the allowable loss at the operating pressure for each section of the system
(see table 17 for allowable leakage figures).
Designers can achieve significant cost savings by
matching operating pressures throughout the system to
constructional standards and appropriate air leakage
testing, e.g. the practice of specifying construction
standards for whole duct systems based on fan
discharge pressures may incur unnecessary costs on a
For example, some large systems could well be
classified for leakage limits as follows:
Plant room risers
Class C
Main floor distribution
Class B
Low pressure outlets
Class A




The information given in this Appendix is for the
guidance of mechanical services contractors, consulting engineers, etc. The identification of ductwork does not form part of the work carried out
by the ductwork contractor unless required by the
designer in the job specification.
B.1.1 Introduction
With the increasing complexity of ventilation and air
conditioning systems, it is becoming more important
to ensure ready identification of ducts for the purposes
of commissioning, operation and maintenance of
systems. The purpose of these recommendations is to
lead towards the use and standardisation of a system
of identification for ducts for the benefit of designers,
contractors and clients.
B.1.2 Scope
B.1.2.1 These recommendations deal with the
identification of ducts for ventilation, air conditioning and simple industrial exhaust systems. They
do not include piped gas systems such as those dealt
with in BS 1710 1984, nor ductwork systems for
industrial processes, although the general
considerations and intentions could be extended with
the agreement of the client to cover such systems.
B.1.2.2 The method is designed to identify the air
being conveyed, the direction of flow, the
destination of the air and/or the location or
nomenclature of the plant where the air was treated.
With small or simple plants, it may not be strictly
necessary to provide identification because the
function is apparent, but it is considered advisable to
do so because this will increase familiarity with the
labelling system and also because the nature and
direction of air flow may not always be apparent.

possible, where there is adequate natural or artificial
B.2.2 Identification symbols will be needed in plant
rooms and remote areas. Symbols should occur
frequently enough to avoid the need for ducts to be
traced back. Symbols should be placed at any service
and access points to the distribution system, including
points where the distribution system has reduced to a
single duct.
B.2.3 Colour coding
The choice of colours has been based on the need to
B.2.3.1 Strong contrasting colours which are
recognisable even though covered with dust.
B.2.3.2 Contrast between the symbol colour and the
base colour of the duct. Usually the base colour
metallic grey of galvanized or aluminium sheet or foil
sheathing, or the white, pale grey, or buff paint on the
insulation is a neutral colour against which the
recommended symbol colours will stand out.
B.2.4 The recommended colours are given in Table 18.
The colour coding indicates the type of air being

B.2.1 Location
To be effective the identification must be placed
where it can be easily seen and at positions where
identification will be required. To ensure that the
symbols are seen, the following points should be
B.2.1.1 The symbols should be on the surfaces
which face the positions of normal access to the
completed installation.
B.2.1.2 The symbols should not be hidden from
view by structural members, other ducts, plant, or
other services distribution systems.
B.2.1.3 The symbols should be placed, where


B.2.5 For conditioned air, two symbols (one red, one
blue) may be used, or a single symbol coloured part
red, part blue.

of the plant. The plant itself must be clearly
numbered to correspond. Letters for Supply, Flow,
Extract, etc., should not be added because
identification will be clear from the colour symbol.
Thus confusion between `S' for Supply and `S' for
South will be avoided.

B.2.6 If a finer grading than that given in Table 18 is
required, as for instance in a laboratory with two
separate contaminated air exhaust systems, it is
recommended that the type colour be used with, say, a
stripe of a second colour. Where the duct contents
constitute a hazard, a symbol as given in BS 1710
1984 should be added to the type colour.
B.2.7 Direction of flow
B-.2.7.1 The form of symbol chosen indicates
direction. It is an equilateral triangle (see Fig. 179)
with one apex pointing in the direction of air flow.
Where the boundaries of the duct are not visible, two
triangles should be arranged in line ahead to indicate
direction of flow.
B.2.7.2 The size of the symbol will depend on the
size of the duct and the viewing distance. The
recommended minimum size for normal use is 150
mm length of side.

B.2.8.3 Where identification of the space is by
room number, this must be agreed with the user
who otherwise may have numbered the rooms
Some examples of further identification systems
are given in Table 19.
B.2.8.4 The letters and numbers should be in either
black or white, whichever gives the better contrast.
They should be marked on the colour symbol or
immediately adjacent to it. The size of the figures
will depend on how easily they can be seen, but
should not be less than 25 mm high.
B.2.9 Explanatory chart
An explanatory chart shall be included in the 0
(Operating) and M (Maintenance) manual and shall
also be kept in the plant room or other convenient
place. The chart should show and explain the colour
symbols used on the installation and where
appropriate the figure and letter codes used for
further identification.

B.2.8 Further identification
B.2.8.1 On small or simple installations where there
is one plant and one or two zones and therefore little
chance of confusing the ducts, it will not be
necessary to provide identification other than the
colour symbol. On large complex installations with
many zones, widely branched distribution systems or
several plants, further identification is necessary. In
this connection a plant refers to the ductwork and
equipment associated with one particular fan.

B.3.1 Several methods are available for applying the
symbols, the main factor being that the symbol is
permanently affixed. Suitable methods are:
B.3.1.1 Painting, using stencilled letters and
B.3.1.2 Self-adhesive plastics or transfers with
water soluble backing. (It is important to ensure
that the surface is smooth and clean and that the
adhesion will not deteriorate due to the surrounding atmosphere.)

B.2.8.2 The further information to be given will
normally be the space served by the duct and in
some cases the associated plant. The information
should be given as briefly as possible using
commonly accepted forms such as a number
indicating which floor of a building. The plant
identification should always be preceded by the
letter `P' to avoid confusion between the number of
the floor and the number

B.3.1.3 Purpose-made plastics or metal labels.

It is recommended that before a contract is finalised,
that consideration is given to the subject of site
access, material handling and storage as they have a
strong influence on the cost efficiency of the overall
activity of ductwork installation.

until they become an integral part of a completed
ductwork system. Whilst this may temporarily detract
from its intended appearance, this deformation will not
have any affect on the functionality of the finally
assembled system.
Installation of ductwork and associated plant items
will inevitably involve manual handling. The
responsibility of employers and employees to assess
the risk of personal injury during manual handling
operations is set out in the H.S.E. publication L23,
Guidance on Manual Handling Regulations 1992.

C.1 Transport
Large capacity vehicles with high-sided open or
closed-top bodies are the most suitable for the
transport of ductwork.
Careful consideration should be given to the
unloading of transport on site as not all sites benefit
from the material handling and access facilities that
exist in a manufacturing workshop such as cranes,
fork-lifts or loading bays. Site handling facilities
along with vehicular access restrictions may
influence the type and size of transport to be utilised.
Lengths of ductwork should be positioned so as to
avoid crushing. Lengths with projections, such as
branches and bends, flanges, girths, damper quadrants should be loaded so as to avoid damage to
adjacent duct panels. In some cases, particularly on
contracts calling for repetitive sizes, the use of
timber jigs and spacers may be justified.
Where reduced bulk and greater protection are major
factors, such as consignments for export,
transporting ductwork in `L' shape sections may
justify the increased site assembly costs.

C.3 Site storage
Adequate floor space must be provided within the
building for the site storage of ductwork. Such storage
shall make due allowance for the storage of ductwork
in stacks such that access between them is of sufficient
width to permit the removal of items without
interference to adjoining
components should be positioned so as to avoid
crushing. Ductwork of small panel size may be stored
horizontally; however care should be exercised to
ensure that stack sizes are limited to within the
structural strength of the duct sections to prevent
distortion of the lower sections within the stack.
C.4 Internal cleanliness of new ductwork
The site storage of ductwork introduces the important
consideration of maintaining the internal cleanliness of
the ductwork. Reference should be made to HVCA

C.2 Handling
To minimise the risk of damage, duct sections
should be clearly identified and deliveries to site
should be closely linked to the installation programme, so as to avoid accumulation of unfixed
ductwork and minimise double handling. It is
important to recognise that ductwork panels, joints
and corners are susceptible to damage and care must
be taken when handling such material through a site.
During handling, individual items of ductwork may
be liable to slight cross sectional deformation

• DW/TM2 Guide to Good Practice - Internal
Cleanliness of New Ductwork Installations.
If the above conditions can not be satisfied consideration should be given by the designer to
amending the specification to include for "Post
Installation Cleaning" as covered by the HVCA
• TR17 Guide to Good Practice - Cleanliness of
Ventilation Systems.


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.