DW 144

Specification for
Heating and
Ventilating
Contractors’
Association

Sheet
Metal
Ductwork

DW/144


DW/l44
Specification for
Sheet Metal Ductwork
Low, medium and high
pressure/velocity air systems
1998
Copyright © 1998 by the
Heating and Ventilating
Contractors’ Association
All rights reserved

ISBN 0-903783-27-4

Further copies of this publication are available from:
Publications Unit
Heating and Ventilating Contractors Association
Old Mansion House Eamont Bridge
Penrith Cumbria CA10 2BX

Tel: 01768 864771 Fax 01768 867138
e-mail:


THE INDUSTRY
STANDARD

Ken Parslow
Chairman
Executive Committee
Ductwork Group
1996-98

or more than a decade-and-a-half, the DW/142 Specification for Sheet Metal
Ductwork published by the Heating and Ventilating Contractors’ Association
has gained national and international recognition as the industry standard
against which the quality of ductwork manufacture and installation can be judged.
In recent years, however, it has become increasingly evident to the members of
the HVCA Ductwork Group that the developments in technology and working
practices which have taken place since the drafting of DW/142 have rendered
obsolete significant parts of the document.
It was an acknowledgement of this state of affairs which led the Technical SubCommittee of the Ductwork Group, ably chaired by Edgar Poppleton, to undertake
the task of producing a radically revised specification which would promote best
practice and quality standards well into the next Millennium.
This new publication — designated DW/144 — represents the direct result of
that initiative.
The new specification recognises the computer age — with special reference to
CAD/CAM procedures and techniques — and the international performance standards established by the Committee for European Normalisation (CEN), as well as
the need to update and consolidate much of the information contained in the original DW/142 publication and its Addendum A companion volume.
During the drafting process, the Technical Sub-Committee has consulted widely

with individuals and organisations throughout the building services and construction sectors in order to ensure that the new specification fully reflected the current
the “state-of-the-art” in terms both of technical expertise and industry best practice.
I firmly believe that this process has resulted in a publication which clearly
demonstrates the high level of professionalism which exists within the ductwork
community — and I take this opportunity of thanking all those who have contributed to its production.
In particular, my thanks go to Edgar Poppleton and his colleagues on the
Technical Sub-Committee, to Keith Elphick for the provision of invaluable technical consultancy, and to Ductwork Group secretary Gareth Keller for overseeing the
project as a whole.

F

3


MAINTAINING QUALITY

L

ike most industries, the ductwork sector

Class A, B and C air leakage characteristics,

must be prepared continually to innovate in

mandatory testing Class C only;

•

order to survive and prosper.

A key element in that innovation process is the

updated

appendices

on

galvanising

after

manufacture, stainless steel, pre-coated steel,

timely review and updating of quality standards to

aluminium, Eurovent and galvanised material,

ensure that they continue to offer realistic bench-

plus a bibliography;

•

marks to which all professional individuals and
organisations can perform.
The development of this new Specification for
Sheet Metal Ductwork — designated DW/144 —
has been carried out with that objective in mind.
In the 16 years since the publication of its

predecessor, DW/142 — and in the ten years since
the supplementary volume Addendum A appeared

Edgar Poppleton
Chairman

•
•

Technical
Sub-Committee
Ductwork Group

— many technical advances, changes in working
practices and regulatory introductions and amend-

•

transport, handling, storage and interface with
DW/TM2 Guide to Good Practice — Internal
Cleanliness of New Ductwork Installations;
an overview of fire-rated ductwork;
a new appendix on inspection, servicing and
cleaning access openings (the default inclusion
of Level 1 should be noted);
a new section on standard component drawings
— incorporating a framework of nomenclature,
and a description of drawing symbols, abbreviations and rules — which is intended to reduce

ments have taken place.

The common performance standards for duct-

•

work being developed by the Committee for
European Normalisation (CEN), for example, had

ambiguity and promote common understanding;
a rewritten description of all forms of dampers,
for which I am indebted to Bill Clark and John
Mawdsley of the HEVAC Association.

to be taken fully into account during the drafting
process. Similarly, notice had to be given to the

I take this opportunity to acknowledge the per-

provisions of the Control of Substances Hazardous

mission granted by the Sheet Metal and Air

to Health (COSHH) and Construction (Design and

Conditioning

Management) Regulations, neither of which had

(SMACNA) of the USA for the use of its tie rod

been issued when DW/142 was published.

specification (designer approval required).

Contractors’

National

Association

It is not possible — nor, I think, desirable — to

And I also include a plea on behalf of ductwork

include in this foreword an exhaustive catalogue of

constructors to be allowed to make the final choice

the points of difference between this specification
and its predecessor. These will clearly emerge

of components and techniques within the parame-

from a detailed reading of the text.

specification to satisfy performance characteristics.

ters set by the designer, and allowed within this

I should, however, like to take the opportunity

It will, of course, be clear to anyone who has

to highlight a few topics which I believe to be of

ever taken on such a task that the production of this

particular significance. They are:

specification has involved a colossal input in terms

•
•
•
•
•
•
•
•
•

the omission of high-pressure Class D (in order
to conform to European practice);

of industry consultation and from a wide variety of

the highlighting of information to be provided
by the designer;

identify for special mention.

the end-sealing of ducts and explosion risks;

members Keith Waldron and the late Keith Angood;

the removal of standard sizes of rectangular

current members Chris Collins, Stuart Howard,

ducts;

Brian James and — last but by no means least —

the omission of cleated joints;

Jim Murray; technical consultant Keith Elphick; and

the acceptance of proprietary flanges certificated
to DW/TM1 no longer illustrated in detail;

Ductwork Group secretary Gareth Keller.

the consolidation into the document of coverage
of hangers and supports;

importance of ensuring that all ductwork is manu-

the addition of a note on linings, along with their
cleaning considerations;

efficient, effective and free of risk.

the consolidated graphical representation of

significantly in the achievement of this objective.

individuals, a number of whom I should like to
They

are:

former

Technical

Sub-Committee

Finally, may I remind readers of the crucial
factured and installed in a manner which is safe,
The publication of DW/144 is intended to assist

5


Acknowledgements
The HVCA wishes to record its sincere thanks to the following
members — past and present — of the Technical Sub-Committee
of the Ductwork Group, who contributed their time, knowledge
and experience to the production of this document
Edgar Poppleton (chairman)
Keith Angood

Chris Collins
Stuart Howard
Brian James
Jim Murray
Keith Waldron
Technical Consultant:
Keith Elphick

Ductwork Group Secretary:
Gareth Keller

6


Other Ductwork-Related Publications
DW/143
DW/151
DW/171
DW/191
DW/TM1
DW/TM2
DW/TM3
TR/17

A Practical Guide to Ductwork Leakage Testing
Specification for Plastic Ductwork
Guide to Good Practice for Kitchen Ventilation Systems
Guide to Good Practice: Glass Fibre Ductwork
Acceptance Scheme for New Products: Rectangular Cross Joint Classification
Guide to Good Practice: Internal Cleanliness of New Ductwork Installations

Guide to Good Practice for the Design for the Installation of Fire and Smoke Dampers
Guide to Good Practice: Cleanliness of Ventilation Systems
Copies of the above publications are available from:
Publications Unit
Heating and Ventilating Contractors Association
Old Mansion House Eamont Bridge
Penrith Cumbria CA10 2BX
Tel: 01768 864771 Fax 01768 867138
e-mail:

–
–
DW/131
DW/121
DW/122B
DW/112
DW/132

DW/141

DW/142
DW/l42

Previous Sheet Metal Ductwork Specifications
Ductwork Specification for High-Velocity Air Systems (Circular)
Standard Range of Rectangular Ducting
Sheet Metal Ductwork Specification for High-Velocity Air
Systems (Rectangular)
Specification for Sheet Metal Ductwork (Low-Velocity
Low-Pressure Air Systems) (Rectangular and Circular) — Metric

Specification for Sheet Metal Ductwork (Low-Velocity
Low-Pressure Air Systems (Rectangular and Circular) — British
Standard Range of Rectangular Ducts and Fittings — Metric and
British Units
Specification for Sheet Metal Ductwork (High-Velocity
High-Pressure Air Systems) (Rectangular, Circular and Flat
Oval) — Metric
Specification for Sheet Metal Ductwork (Low and High-Velocity/
Pressure Air Systems) (Rectangular, Circular and Flat Oval)
— Metric
Specification for Sheet Metal Ductwork (Low, Medium and High
Pressure/Velocity Air Systems)
Specification for Sheet Metal Ductwork Addendum A (Low,
Medium and High Pressure/Velocity Air Systems)
7

1963
1967
1968
1969
1969
1970

1970

1977
1982
1988

Contents

Page
Notes

10

Part One - Technical Information to be
provided by the designer
1. Introduction
2. Standards
3. Components
4. Particular Requirements

11
11
11
11

Part Two - Standards
5. Application
6. Ductwork Classification and Air Leakage
7. Materials
8. Ductwork Construction and Joint Sealing

13
13
13
14

Part Three - Rectangular Ducts
9. Rectangular Duct Sizes
10. Construction
10.1 General
10.2 Steel Thicknesses
10.3 Longitudinal Seams
10.4 Cross Joints
10.5 Stiffeners
10.6 Ductwork Galvanised After
Manufacture
10.7 Fastenings
11. Fittings
11.1 Standardisation of Fittings
11.2 Stiffeners
11.3 Splitters
11.4 Turning Vanes
11.5 Branches
11.6 Change Shapes
11.7 Expansions and Contractions
11.8 Sealant

16
16
16
16
16
16
16
16
16

17
17
27
27
27
27
27
29
29
29
29
29

Part Five - Flat Oval Ducts
15. Standard Sizes and Sheet Thicknesses
16. Construction (Spirally wound)
16.1 General
16.2 Longitudinal Seams
16.3 Cross Joints

35
35
35
35
35

35
35
35
35

35
35

Part Six - Hangers and Supports
19. General

43

Part Seven - General
20. Access/Inspection Openings
21. Regulating Dampers
22. Fire Dampers
23. Smoke Dampers
24. Combination Smoke and Fire Dampers
25. Flexible Ducts
26. Flexible Joint/Connections
27. Protective Finishes
28. Connections to Building Openings
29. Internal Duct Linings
30. Thermal Insulation
31. Kitchen Ventilation
32. Fire Rated Ductwork
33. Standard Component Drawings
and Abbreviations

15
15
15
15
15

15
15

Part Four - Circular Ducts
12. Standard Sizes
13. Construction
13.1 Longitudinal Seams
13.2 Cross Joints
13.3 Fastenings
14. Fittings
14.1 Standardisation of Fittings
14.2 Nominal Diameters
14.3 Sheet Thickness
14.4 Sealing of Joints

16.4 Fastenings
16.5 Stiffening
17. Construction (Straight Seamed)
18. Fittings
18.1 General Construction Requirements
18.2 Standardisation of fittings

Part Eight - Appendices
Appendix A.
Air Leakage from
Ductwork
Appendix B.
Identification of
Ductwork
Appendix C.

Guidance Notes for the
Transport, Handling and
Storage of Ductwork
Appendix D.
Ductwork Systems and
Fire Hazards
Appendix E.
Hot Dip Galvanizing after
Manufacture
Appendix F.
Stainless Steel for Ductwork
Appendix G.
Pre-Coated Steel
Appendix H.
Aluminium Ductwork
Appendix J.
Eurovent
Appendix K.
Summary of BS.EN10142:
1991 Continuously Hot-Dip
Zinc Coated Mild Steel Strip
and Sheet for Cold Forming
Appendix L.
‘Design Notes for Ductwork’
(CIBSE Technical
Memorandum No. 8)
Appendix M. Guidance Notes For Inspection,
Servicing and Cleaning Access
Openings
Appendix N.

Bibliography
Appendix P.
Conversion Tables

8

47
48
49
50
51
51
52
53
53
54
54
54
54
54

75
80

82
83
85
86
89
90

91

92

93

94
95
97


List of Tables
Table

Page

Part Two - Standards
1. Ductwork Classification and Air
Leakage Limits
2.
3.
4.
5.

6.
7.
8.
9.
10.

Part Three - Rectangular Ducts
Constructional Requirements
Low Pressure up to 500Pa
Constructional Requirements
Medium Pressure up to 1000Pa
Constructional Requirements
High Pressure up to 2000Pa
Fastening Centres
Part Four - Circular Ducts
Standard Sizes
Spirally-Wound Ducts
Straight-Seamed Ducts
Permitted fastenings and maximum
spacings
Fittings Sheet Thicknesses

Part Five - Flat Oval Ducts
11. Standard sizes and sheet thicknesses
12. Stiffening requirements
low and medium pressures
13. Stiffening requirements
high pressure
14. Permitted fastenings and maximum
spacings
Part Six - Hangers and Supports
15. Supports for horizontal ducts - rectangular,
flat oval and circular
Part Seven - General
16. Standard Abbreviations
17.

18.
19.
20.
21.
22.
23.
24.
25.

Part Eight - Appendices
Air Leakage Rates
Recommended duct identification colours
Examples of further identification symbols
Ductwork galvanized after manufacture rectangular
Compositions of the commonly used
Stainless Steel grades
Rectangular aluminium ducts low pressure constructional requirements
Circular aluminium ducts low pressure constructional requirements
Zinc coating mass (weight)
Access requirements for inspection,
servicing and cleaning

13

18

1-8
9
10-12

19
24

31
32-38
39-45

Part Four - Circular Ducts
Spiral and straight seams
Cross joints spirally wound ducts
Cross joints straight seamed ducts

29
30-31
32-33

53-58
59-63

Part Five - Flat Oval Ducts
Cross joints spirally wound ducts
Cross joints straight seamed ducts

39-40
41-42

Part Six - Hangers and Supports
Horizontal ducts
bearers and hangers
Vertical ducts supports

45-46
46

64-75
27
28
28

76-77

Part Seven - General
Fire barrier/fire damper expansion
Flexible joint connections
Standard component drawings Rectangular
125-152 Standard component drawings Circular
153-167 Standard component drawings Flat Oval
168-177 Plant/equipment/miscellaneous
78-79
80
81-124

29
29
36
37
38
40

178

179

44

72-73
76
80
81
85
88
90
91
93
94

Pages
Part Three - Rectangular Ducts
Longitudinal Seams
Illustrations of panel stiffening
Flanged cross joints

Socket and spigot cross joints
Stiffeners
Tie rod assembly
Hard and Easy bends
Turning Vanes

19

List of Illustrations

Figs

13-17
18-24
25-28
29
30

20
20
21

9

Part Eight - Appendices
Permitted leakage at various
pressures
Example of duct identification symbol

22
23
24
25
25

50
52
55-61
62-67
68-70

71

78
81


Notes
In this document:
(1) Even where a ductwork job specification calls for the system to be
wholly in accordance with DW/144, it will still be necessary for the
designer, in addition to providing drawings showing details and
dimensions of the ductwork, to identify specific requirements, particular to his or her design.
The technical information to be provided by the designer is therefore set out in detail on page 11.
(2) All dimensions quoted in this specification refer to the nominal
sizes, which are subject to the normal relevant commercial and
published tolerances.
(3) Manufacturing techniques are continually subject to change and
improvements and in respect of proprietary methods and devices
this specification does not preclude their use if they can be demonstrated to the designer to be equally satisfactory. Where there is
divergence between the requirements of DW/144 and the
manufacturer’s recommendations for proprietary methods and
devices, the latter shall take precedence.
(4) The expressions ‘low-pressure,’ ‘medium-pressure’ and ‘highpressure’ relate to the pressure/velocity classes set out in Table 1.
(5) ‘Mean air velocity’ means the design volume flow rate related to
the cross-sectional area.
(6) Reference to the air distribution system pressure relate to the static pressure of the relevant part of the ductwork system and not to
the fan static pressure.
(7) The symbol for litres is ‘L’: 1000 litres per second is equivalent to
1 cubic metre per second.
(8) The Pascal (Pa) is the internationally agreed unit of pressure. The

relationship of the Pascal to other units of pressure is: 500 pascals
= 500 Newtons per square metre = 5 millibars = approximately 2
inches water gauge.
(9)
Duct pressure classification
As the static pressure in a duct system progressively changes from the fan,
economic advantage can be obtained by changing the duct pressure
classification to match more closely the duct distribution static pressure.
For example, some large systems could well be classified for leakage
limits as follows:
Plant rooms and risers

Class C

Main floor distribution

Class B

Low-pressure outlets

Class A

10


Part One – Technical information to be
provided by the designer to the ductwork contractor
1 INTRODUCTION
The selection of constructional methods is the
decision of the Manufacturer to conform with the

performance
requirements
of
the
specified
ductwork classification. Sections 2-4 below
define the information that is to be provided by
the Designer.

4.2 Protective finishes (Section 27)
Details and specification of any
finishes.

2 STANDARDS

4.4 Internal thermal/acoustic lining
(Section 29)
The extent of any ductwork requiring internal
acoustic/thermal lining is to be clearly identified.
A detailed specification of materials and method
of application is required. The practical aspects
of cleaning or maintenance must be addressed by
the designer before deciding to internally line
ductwork.

2.1
2.2
2.3
2.4
2.5

4.3 Fire rated and smoke extract ductwork
(Appendix D)
The extent and limits of protection for any fire
resisting ductwork.

Pressure classification (Table 1)
Leakage classification (Table 1)
Positive and Negative pressures (Table 1)
Materials (Section 7)
Any special system requirements

3 COMPONENTS
3.1 Inspection/servicing access openings
(Section 20 and Appendix M)
Number and location of all panels and covers
for inspection and/or servicing access other than
those covered in Section 20 and summarised as
Level 1 requirements in table 25 of Appendix
M. Number and location of test holes,
instrument connections and hinged doors as
defined in Section 20.

4.5
External
thermal/acoustic
insulation
(Section 30)
The extent and thickness of insulation to be
provided by others should be stated.

4.6 Special supports (Section 19)
Details of any spanning steel or special support
requirements not covered by Section 19
4.7 Attachment to building structure (Section 28)
Specific requirements for the junction of
ductwork
and
associated
components
to
openings should be detailed and specified and
the limits of responsibility defined.

3.2 Cleaning access
(Section 20.8 and Appendix M)
Designers shall stipulate their requirements for
periodic internal cleaning of ductwork and for
the consequent need for adequate access for
specialist cleaning equipment.

The provision of penetrations and associated
framings are outside the scope of this
specification.

3.3 Regulating dampers (Section 21)
Specification, location and mode of operation of
all regulating dampers.

4.8 Air terminal units
Detail and specifications of all Air Terminal

Units. It is expected that all Air Terminal Units
and their Plenums (See Figures 120 to 124) will
be supported by the Ceiling Grids unless the
designer indicates an independent method of
support.

3.4 Fire dampers (Section 22)
Specification and location of all fire dampers to
meet the requirements of the Authority directly
concerned with fire protection.
3.5 Smoke dampers (Section 23)/Combination
smoke and fire dampers (Section 24)
Specification and location of all smoke dampers
to meet the requirements of the Authority
directly concerned with fire protection.
3.6 Flexible ducts (Section 25)
Specification and location of
ductwork.

any

protective

4.9 Ductwork layout drawings
Details of any special requirements relating to
CAD, scales, etc. It is common practice and cost
effective for ductwork manufacturers to utilise
their approved ductwork layout drawings as a
basis
of

their
manufacturing/installation
information by adding the necessary details to
the same drawing. Scales of 1:50 or smaller may
preclude this practice, therefore, larger scales
might be more appropriate. The final choice of
manufacturing/installation scales shall be left to
the ductwork contractor.

flexible

3.7 Flexible joint connections (Section 26)
Specification and location of any flexible connections eg. plant or building expansion joints.
4. PARTICULAR REQUIREMENTS
4.1 Air leakage testing (Section 6 and Appendix A)
The extent of any air leakage testing. While it
shall be mandatory for high-pressure ductwork
(as defined in this specification) to be tested for
air leakage in accordance with the procedure set
out in DW/143, A practical guide to Ductwork
Leakage Testing, no such testing of low- or
medium-pressure ductwork is required.

4.10 Other requirements
Details of any requirements for the ductwork
not in accordance with the provisions of this
specification, including any modified construction required to conform with any
requirements
concerning
external

ductwork
(See 5.3) or to meet the regulations of a local
authority or other controlling body.
11


4.11 Reference to the designer
In consideration of the foregoing, reference is also
made to the designer in the following clauses:Clause
5.3
7.4, 7.5, 7.6
10.5.2
11.1
14.1
16.3.1
19.1, 19.4
19.6, 19.7
20.1, 20.1.1.1, 20.6, 20.8
20.9
21.1, 21.3.1
21.3.4
22.3, 22.7
24.3
25.1
26.1
27, 27.3.4
29.1, 29.4, 30.2, 30.3, 33.2
Fig. 176
Appendix A
Appendix B

Appendix C
Appendix D
Appendix E
Appendix F
Appendix L
Appendix M

Page
13
14
16
16
29
35
43
44
47
48
48
49
50
51
51
52
53
54
71
75-79
80, 81
82

83, 84
85
86-88
93
94

12


Part Two - Standards
6.3 Leakage at various pressures; and other
relationships
Applying the limits specified in Table 1, Appendix
A (Table 17) sets out the permitted leakage at each
of a series of pressures up to a maximum for each
class. Included in that appendix is a graphical presentation of the pressure/leakage relationship.

5 APPLICATION
5.1 This specification sets out minimum requirements for the manufacture and installation of ductwork for commercial and industrial air distribution systems, made from any of the materials listed
in Section 7 and being within the limits of size
and/or metal thicknesses specified in the relevant
tables.
Normal
operating
temperatures
are
assumed within the pressure/velocity limits and
the limits of air leakage for the various pressure
classes prescribed in Table 1.

DW/143 A practical guide to Ductwork Leakage
Testing, also gives details of the basis for the
leakage limits specified in Table 1.
6.4 Air leakage testing

5.2 This specification is not intended to apply to
ductwork handling air which is polluted or is otherwise exceptional in respect of temperature or
humidity (including saturated air); nor is it suitable
for ductwork exposed to a hostile environment,
e.g. contaminated air, off-shore oil rigs, etc. The
design, construction, installation, supports and
finishes in such cases should be given special consideration in relation to the circumstances of each
case.

Air leakage testing of low and medium pressure
ductwork is not mandatory under this specification.
Air leakage testing of high pressure ductwork is
mandatory under the specification and for details
of testing procedure refer to DW/143 A practical
guide to Ductwork Leakage Testing.

5.3 This specification is not suitable for ductwork
exposed to external atmosphere and the Designer
will need to give specific details of any special
finishes/construction (See Section 27).

7 MATERIALS
7.1 Application
This specification applies to ductwork constructed
from materials as defined below, or equal.

Minimum steel thickness is to be taken as a nominal
thickness
within
the
tolerances
to
BS.EN10143:1993. (See Appendix K)

6 DUCTWORK CLASSIFICATION AND
AIR LEAKAGE
6.1 Classification and air leakage limits
Ductwork classification and air leakage limits are
set out in Table 1.

7.2 Zinc-coated steel
Ductwork will normally be constructed from
hot-dip galvanized steel to BS.EN10142:1991,
Grade DX51D+Z, coating type Z275.

6.2 Compatibility with CEN
The leakage factors used in Table 1 for Classes A,
B and C are the same as those for the classes
similarly designated in the CEN Document
Pr EN12237/Pr EN1507.

Table 1 Ductwork Classification and Air Leakage Limits
Static pressure limit
Duct pressure class
Positive

Negative

Maximum air
velocity

Air leakage limits
litres per second per square
metre of duct surface area

2

3

4

5

Pa
500

Pa
500

m/s
10

0.027 x p0.65

Medium-pressure – Class B

1000

750

20

0.009 x p0.65

High-pressure – Class C

2000

750

40

0.003 x p0.65

1
Low-pressure – Class A

Where p is the differential, pressure in pascals.

13


8.2.2 Liquid and mastic sealants
These are typically applied to a longitudinal
seam formed between two sheets of metal, a
socket and spigot, cleated or flanged cross joints.

Particular care is needed when sealing of “corner
pieces” on the proprietary ‘slide-on’ type flange
and reference should be made to the manufacturer’s assembly and sealing instructions.

7.3 Mild steel
Where mild steel is specified, it shall be coldreduced steel to BS.EN10130:1991, Grade FEP
01A.
7.4 Stainless steel
Where stainless steel is specified. it will be the
responsibility of the designer to indicate the type
most suitable for the conditions to which the ductwork will be exposed. In doing so, it is recommended that the factors set out in Appendix F
should be taken into account. In this connection,
reference must be made to BS 1449: Part 2, which
includes stainless steel.

8.2.3 Gaskets
These can be of various materials in the form of
a preformed roll, sheet or strip, applied between
opposing faces of flanged cross joints. In the
case of proprietary ‘slide-on’ type flanges, it is
advisable to use the gasket strip recommended
by the manufacturer.

7.5 Pre-coated steel
Pre-coated steel may be specified for aesthetic or
other reasons. The designer must then consider
the availability of suitable materials and the
restriction on fabrication methods. Guidance
notes are available in Appendix G.

Factory-fitted proprietary synthetic rubber ‘O’ring type gaskets are also acceptable for socket
and spigot joints on circular duct systems.
8.2.4 Tapes
8.2.4.1 The application of tapes – Best
suited, but not limited, to cross joints on
circular or flat oval ductwork. Where chemical reaction tape, heat shrinkable tape or other
approved material is used on flat oval ductwork care should be taken to maintain close
contact between the material and the flat sides
of the duct until the joint is completed.

7.6 Aluminium
Where aluminium is specified, it will be the
responsibility of the designer to define the type
most suitable for the conditions to which the ductwork will be exposed. Reference must be made to
BS.EN485,
BS.EN515
and
BS.EN573
for
aluminium sheet and BS.EN755 Parts 3-6 for
aluminium section. (Constructional requirements
for ductwork made from aluminium sheet and
general notes on the material are set out in
Appendix H.)

8.2.4.2 Chemical reaction tape – An
impregnated woven fibre tape and a resin type
activator/adhesive. On application of the
activator/adhesive the tape becomes pliable
and can then be applied to any surface shape.

The liquid reacts with the tape, causing the 2part system to ‘set’.

8 DUCTWORK CONSTRUCTION AND
JOINT SEALING

8.2.4.3 Heat shrinkable band/tape – A thermoplastic material, coated on the inside with
hot metal adhesive. The band (or an appropriate length of tape) is cut from the roll and
wrapped around the joint. When heated the
tape shrinks tightly around the joint thus providing a seal.

8.1 Ductwork construction
The selection of longitudinal, cross joint and stiffener types within the criteria laid down in the
tables should be the responsibility of the manufacturer.
8.2 Joint sealing and sealants
8.2.1 General
The integrity of the ductwork depends on the
successful application of the correct sealant,
gaskets or tape. The materials used should be
suitable for the purpose intended and satisfy the
specified pressure classification.

8.2.4.4 Self adhesive tape – Manufactured
from various materials including cloth based,
PVC and aluminium foil. Typically applied
externally to socket and spigot cross joints.
However, it is difficult to provide the dry, dust
and grease free surface that is required for a
successful application and this method is
therefore not recommended as a primary
source of sealant.

Illustrations indicating sealant locations will be
found in the following sections dealing with the
construction of rectangular, circular and flat
oval duct sections.

NB! Risk of explosions
Where ductwork is blanked off prior to leakage
testing or to prevent the ingress of contamination,
care should be taken to ensure that all joint sealing
solvent vapours are dispersed from the ductwork
systems.

IN ALL CASES, SEALANT MATERIALS
MUST
BE
APPLIED
STRICTLY
IN
ACCORDANCE
WITH
THE
MANUFACTURER’S
INSTRUCTIONS
AND
COSHH
ASSESSMENT.

14

Part Three - Rectangular Ducts

duct size longer side and maximum spacing, are
given in Tables 2 to 4. Other limits on use are
given with the individual drawings.

9 RECTANGULAR DUCT SIZES
This specification covers duct sizes up to a maximum longer side of 3,000 mm. Duct sizes with an
aspect ratio greater than 4:1 are not recommended. Although they offer no problems of construction, they increase frictional resistance and the
possibility of noise.

Note: Proprietary products used in the construction of cross joints should be approved by an
independent test house following tests defined
in DW/TM1 “Acceptance scheme for new products – Rectangular cross joint classification.”
Figures Nos 10 and 13 to 17 illustrate non proprietary joints that have an established rating.

10 CONSTRUCTION
10.1 General
The minimum constructional requirements for
rectangular ductwork depend upon the pressure
classification as set out in Tables 2 to 4. The ductwork construction and joint sealing standards are
set out in section 8.

10.4.2 Sealant in cross joints
Sealant shall be used between sheet and section
in all cross joint assemblies. (see section 8)
With socket and spigot joints made on site,
sealant shall be applied during or after assembly
of the joint. It is permissible to use chemicalreaction tape or heat-shrink strip as alternative

methods of sealing, provided that close contact
is maintained over the whole perimeter of the
joint until the joint is completed.

10.2 Steel thicknesses
Minimum steel thicknesses related to duct longer
side to pressure classification are given in
Tables 2 to 4.
10.3 Longitudinal Seams
Longitudinal seams are illustrated in Figs. 1 to 8.
The limits of use, if any, are given with the individual illustrations.

With all flanged joints, the sealant between
sheet and section should preferably be incorporated during construction at works, but site
applied sealant is acceptable. The joint between
sections of ductwork is then made, using
approved type of sealant or gasket. With
proprietary flanging systems particular attention
should be paid to the sealing of corner pieces
and flanges, reference should be made to the
manufacturer’s assembly and sealing instructions.

10.3.1 Sealing of Longitudinal Seams
Sealant will be applied using one of the following methods:
a) As an edge sealant on the external seam surface.
b) As an edge sealant on the internal seam surface.
c) Internal to the joint seam itself.
The most appropriate method will be determined by the manufacturer relative to their
product and will be associated with either traditional fabrication/assembly methods, factory or
site based, and/or proprietary methods. The ultimate proof of a seal is that the ductwork system

meets the pressure classification specified. For
details of sealant see section 8.

10.4.3 Adjustable/slip joints
In order to accommodate manufacturing/building tolerances, site modifications etc, it is
accepted practice to use an adjustable joint as
illustrated in Fig. 14.
10.5 Stiffeners
10.5.1 External stiffeners
The sections (including proprietary flanges)
suitable for use as single stiffeners have been
given a rating from S1 to S6 in terms of duct
size longer side and maximum spacing. The ratings are specified with the illustrations of the
stiffeners, Figs. 18 to 23, and the limits of use
are given in Tables 2 to 4. The stiffeners for
socket and spigot joints covered in Figs. 15, 16
and 17 are also applicable to stiffeners in general.

10.3.2 Welded seams
A welded seam is acceptable without sealant,
provided that the welding is continuous.
10.4 Cross joints
10.4.1 Cross joint ratings
For cross joints, a system of rating has been
used to define the limits of use. The rating for
each cross joint is given with its drawing, and
the limits applying to that rating, in terms of

15

a minimum.

10.52 Internal stiffeners
Tie bars connecting the flanges of cross joints
illustrated in Figs 11 and 12, are the only form
of internal stiffening for rectangular ductwork
recognised by this specification and reference
should
be
made
to
HVCA
publication
DW/TM1.

Areas where the galvanizing has been damaged or
destroyed by welding or brazing shall be suitably
prepared and painted internally and externally with
zinc-rich or aluminium paint as defined in Section
27.3.2.

Alternative methods for the attachment of tie
bars are shown in Figs. 25 to 28.

11 FITTINGS
11.1 Standardisation of fittings
The terminology and descriptions of rectangular
duct fittings as set out in Section 33 are recommended for adoption as standard practice to provide common terms of reference for designers,
quantity surveyors and ductwork contractors, and

of those using computers in ductwork design and
fabrication.

The use of tie bars or other forms of internal
stiffening or bracing shall be acceptable if
proved to the designer to be equally satisfactory.
SMACNA (Sheet Metal and Air Conditioning
Contractors’ National Association), which is the
American equivalent to the HVCA Ductwork
Group, have produced an Addendum No.1
(November 1997) to their publication “HVAC
Duct Construction Standards, Second Edition 1995”. The addendum contains the extensive
technical information and data on the subject of
mid panel tie rods and SMACNA have given
their kind permission for this specification to
make reference to this fact. Designers and manufacturers who wish to incorporate this form of
internal stiffening into a ductwork system
should contact SMACNA direct to obtain copies
of their publications (See Appendix N,
Bibliography).

Bends are designated as ‘hard’ or ‘easy’, and these
terms as used herein have the following meanings:
‘Hard’ signifies rotation in the plane of the
longer side of the cross section.
‘Easy’ signifies rotation in the plane of the
shorter side of the cross section.
An example illustrating these terms is given in
Fig. 29.
11.2 Stiffeners

The flat sides of fittings shall be stiffened in accordance with the construction Tables 2 to 4. On the
flat sides of bends, stiffeners shall be arranged in
a radial pattern, with the spacing measured along
the centre of the bend.

10.6 Ductwork galvanized after manufacture
Appendix E sets out the recommended sheet
thicknesses and stiffening for ductwork galvanized after manufacture.

10.7.1 Permitted types and maximum centres
Table 5 sets out the permitted fastenings and the
maximum spacings for all ductwork classifications. All duct penetrations shall be sealed.

11.3 Splitters
If the leading edge of the splitters exceeds 1250
mm fit central tie bars at both ends to support the
splitters. Leading and trailing edges of splitters
must be edge folded and flattened and be parallel
to the duct axis.

10.7.2 Rivets
Manufacturers’ recommendations as to use,
size and drill size are to be followed. Rivets resulting in an unsealed aperture shall not be used.

Splitters shall be attached to the duct by bolts or
mechanically-closed rivets at 100 mm maximum
spacing (or by such other fixing as can be shown
to be equally satisfactory e.g proprietary sealed
splitter pins).

10.7.3 Set screws, nuts and lock bolts
Materials shall be of mild steel, protected by
electro-galvanizing, sherardizing, zinc-plating,
or other equal and approved corrosion resistant
finish.

11.4 Turning vanes
Where specified, or shown on drawings, square
throat bends with either duct dimension greater
than 200 mm shall be fitted with turning vanes
which are illustrated in Figures 30a and 30b.

10.7.4 Self tapping and piercing screws
Providing an adequate seal can be achieved, and
the protrusions into the ductwork are unlikely to
cause injury, then self-tapping or piercing
screws may be used.

Turning vanes at 60 mm maximum centres shall
be fixed at both ends either to the duct or compatible mounting tracks in accordance with manufacturer’s instructions, the whole bank being fixed
inside the duct with bolts or mechanically closed
rivets at 150 mm maximum spacing.

10.7 Fastenings

The maximum length of turning vane between
duct walls or intermediate support shall be 615
mm for single skin vanes and 1250 mm for double
skin vanes.

10.7.5 Welding of sheet
The suitability of welding for sheet-to-sheet
fastening will be governed by the sheet thickness, the size and shape of the duct or fitting and
the need to ensure airtighteness. Welded joints
shall provide a smooth internal surface and shall
be free from porosity. Distortion shall be kept to

Typical examples of fitting turning vanes when
the maximum permitted vane lengths are
exceeded are shown in Fig. 30c.
16


11.7 Expansions and contractions

11.5 Branches
When fitting branch ducts to a main duct, care
should be taken to ensure that the rigidity of the
duct panel is maintained in terms of the stiffening
criteria.

Where these are required, an expansion shall be
made upstream of a branch connection and a contraction downstream of a branch connection. The
slope of either an expansion or a contraction
should not exceed 22½° on any side. Where this
angle is not practicable, the slope may be increased, providing that splitters are positioned to bisect
the angle between any side and the centre line of
the duct (See Figs 99 to 101).

11.6 Change shapes

Where a change shape is necessary to accommodate the duct and the cross-sectional area is to be
maintained, the slope shall not exceed 22½° on
any side (See Figs 99 to 103). Where a change in
shape includes a local reduction in duct crosssectional area, the slope should not exceed 15° on
any side and the reduction in area should not
exceed 20 per cent.

11.8 Sealant
Sealant shall be used in all longitudinal seams and
cross joints of fittings. Sealant shall be to the
options listed in Section 8.

17


Constructional Requirements – Rectangular Ducts
Table 2 LOW PRESSURE (limited to 500 Pa positive and 500 Pa negative)
Dimensions in mm

Maximum duct size
(longer slide)

400

Minimum sheet
thickness

0.6

Type Rating

Socket & Spigot Joints

1

2
A1

A2

A3

Flanged Joints & Stiffeners

J1/S1

J2/S2
J3/S3

J4/S4
J5/S5

J6/S6

sheet
3

600

800

1000

1250

0.8

1600

2000

2500

1.0

3000
1.2

Maximum spacing between joints and stiffeners
4

5

6

7

8

9

10

11

12

PS

3000

SS

3000

PS

3000

2000

1600

1250

SS

3000

3000

1600

1250

PS

3000

2000

1600

1250

1000

800

SS

3000

3000

2000

1600

1250

800

PS

3000

1600

1250

625

SS

3000

3000

1250

625

PS

3000

2000

1600

1250

625

SS

3000

3000

1600

1250

625

PS

3000

2000

1600

1250

1000

800

SS

3000

3000

2000

1600

1250

800

PS

3000

2000

1600

1250

1000

800

800

SS

3000

3000

2000

1600

1250

1000

800

PS

3000

2000

1600

1250

1000

800

800

800

625

SS

3000

3000

2000

1600

1250

1000

800

800

800

PS

3000

2000

1600

1250

1000

800

800

800

800

SS

3000

3000

2000

1600

1250

1000

800

800

800

Notes (applicable to Tables 2 to 4)
(1) The joints and stiffeners have been rated in terms of duct longer side and maximum spacing – see 10.4
for joints and 10.5 for stiffeners.
(2) In Col. 3:
‘PS’ = plain sheet
‘SS’ = stiffened sheet, by means of
(a) beading at 400 mm maximum centres: or (b) cross-breaking within the frame formed by joints and/or
stiffeners: or (c) pleating at 150 mm maximum centres.
(3) Stiffened panels may limit the choice of insulation materials.
(4) For ductwork galvanized after manufacture, see 10.6 and Appendix E.
(5) For aluminium ductwork, see Appendix H.
(6) For constructional constraints of stainless steel ductwork see Appendix F.
(7) Although not covered in this specification, due to their relatively infrequent use, cleated cross joints are
an accepted constructional practice and the HVCA Ductwork Group should be contacted if details of
their ratings and limitations are required.
(8) Intermediate stiffeners using rolled sheet angle profiles, illustrated in Figs. 19 to 23 of the appropriate
rating may also be utilised ensuring that rigid corners are achieved.

18


Constructional Requirements – Rectangular Ducts
Table 3 MEDIUM PRESSURE (limited to 1000 Pa positive and 750 Pa negative
Dimensions in mm

Maximum duct size
(longer slide)

400

Minimum sheet
thickness

0.6

Type Rating

A2

A3

Flanged Joints & Stiffeners

J1/S1

J2/S2
J3/S3

J4/S4
J5/S5

J6/S6

1000

1250

1600

2000

2500

3000

1.0

1.2

Maximum spacing between joints and stiffeners

3

A1

800
0.8

sheet

2

Socket & Spigot Joints

1

600

4

5

6

7

8

PS

3000

SS

3000

PS

3000

SS

3000

PS

3000

1600

1250

1000

800

1250

800

SS

3000

3000

1600

PS

3000

1250

625

SS

3000

1250

625

PS

3000

1250

1250

9

10

11

12

625

SS

3000

1600

1250

625

PS

3000

1600

1250

1000

800

SS

3000

3000

1600

1250

800

PS

3000

1600

1250

1000

800

800

SS

3000

3000

1600

1250

1000

800

PS

3000

1600

1250

1000

800

800

800

625

SS

3000

3000

1600

1250

1000

800

800

800

PS

3000

1600

1250

1000

800

800

800

800

625

SS

3000

3000

1600

1250

1000

800

800

800

625

Constructional Requirements – Rectangular Ducts
Table 4 HIGH PRESSURE (limited to 2000 Pa positive and 750 Pa negative)
Dimensions in mm

Maximum duct size
(longer side)

400

600

Minimum sheet
thickness

800

1250

0.8

1600

2000

Rating

Sheet

1

2

3

4

Socket &
Spigot Joint s

A1

PS/SS

3000

A2

PS/SS

3000

A3

PS/SS

3000

J1/S1

PS/SS

3000

625

J2/S2

PS/SS

3000

1250

800

2500
1.2

1.0

Type

Flanged Joints & Stiffeners

1000

Maximum space between joints and stiffeners
5

6

7

8

9

10

J3/S3

PS/SS

3000

1250

1250

800

J4/S4

PS/SS

3000

1250

1250

1000

800

J5/S5

PS/SS

3000

1250

1250

1000

800

800

625

J6/S6

PS/SS

3000

1250

1250

1000

800

800

800

Notes on page 18 also apply to Tables 3 and 4

19

11

625


Longitudinal seams
For permitted fastenings (types and spacing), see Table 5
Fig. 1 Grooved seam

Fig. 5 Returned standing seam (internal or
external)

Alternative sealant locations

alternative
sealant
locations

Fastening

Fig. 6 Capped standing seam (internal or
external)

Fig. 2 Grooved corner seam

Fastening
Alternative
sealant
locations

Alternative sealant
locations

Fig. 3 Pittsburgh lock seam

Fig. 7 Tray standing seam (internal or
external)
Fastening

Alternative
sealant
locations

Alternative
sealant locations
Fig. 8 Lap seam

Fig. 4 Button punch snap lock seam
Alternative
sealant locations
Note. – This
seam is acceptable for use
on low and
medium-pressure
ducts only

Fastening

Alternative

sealant locations

Fig. 9 Illustrations of panel stiffening
Cross breaking between
joints or stiffeners

Pleating (may also
be along the duct)

Beading (may also
be along the
duct)

Examples of Cross sections

20


Flanged cross joints
Minimum
Dimen- Rating
sions

Type
Fig. 10 Rolled steel angleflanged joint, with welded
corners
Fixing
Bolt

Gasket or

sealant

Fastening

Sealant if
turn up not
used

Fastening

mm
25 x
30 x
40 x
50 x

Pressure
classes

Duct ends turned up 8 mm
minimum
3
4
4
5

J3
J4
J5
J6

Low
Medium
High

Fixing Bolts
25
30
40
50

x
x
x
x

Notes/corner treatments

3
4
4
5

6 mm
8 mm
8 mm
10 mm

Fixing bolts required at
each corner and at 300 mm

centres.
Angle flanged joint does NOT
require DW/TM1 certification

A turn up as illustrated is NOT
mandatory. If not used, the toe
of the angle is to be sealed.

Fig. 11 Examples of typical rollformed sheet metal profiles

Sealant, clamps/cleats and fixings omitted for clarity

Fig. 12 Examples of typical cross joint flanges formed from the duct wall

NOTE: The above illustrations are typical examples of cross joint profiles that are in common use for connecting
rectangular sheet metal ducts.
There are no set dimensions for these profiles shown in Figs. 11 and 12 provided they are certified under the HVCA
testing scheme DW/TM1 “Acceptance Scheme for new products – Rectangular cross joint classification” and are
appropriate to the duct application. The manufacturer’s technical data should be followed with respect to:
Connections to duct wall
Corner treatment
Addition of cleats
Application of sealants
Strength ratings
Application of tie bars
A list of manufacturers and profiles that are covered by current DW/TM1 certificate is available from the Ductwork
Group Secretary at HVCA.

21

Socket and spigot cross joints
Note – Particular care must be taken in the sealing of these joints. The ratings stated for cross joints
in Figs. 13 to 17 inclusive do not require DW/TM1 certification
For permitted fastenings (types and spacing), see Table 5
Angle
size mm

Type

Rating

Pressure
classes

A1

Low
Medium
High

A1

Low
Medium
High

This joint can be used
on any ducts subject to
the addition of an

adjacent stiffener with a
rating appropriate to the
duct size

A2

Low

Locate stiffener back
from end of spigot
joint to allow access
for sealing joint

Notes

Fig. 13 Plain

Alternative sealant locations

Fastening
–

Fig. 14 Adjustable

Alternative sealant locations

Fastening

Fig. 15 Angle reinforced (Ducts with
shorter side 400 mm and less)

25 x 3

Corners can be mitred
30 x 4

Fig. 16 Back to back stiffeners (Ducts with
both sides greater than 400 mm)

25 x 3

30 x 4

Fig. 17 Full girth welded stiffeners
(Ducts with both sides greater
than 400 mm)

25 x 3

30 x 4

22

A2

Low
Medium

A2

Low

A3

A2

A3

Low
Medium

Low

Low
Medium

Locate stiffener back
from end of spigot joint
to allow access for
sealing joint.
Stiffeners shown in Figs.
19 to 23 are permissible
if provided with rigid
corners.
Corners can be mitred

Locate stiffener back
from end of spigot joint
to allow access for
sealing joint.
Stiffeners shown in Figs.

19 to 23 are permissible
if provided with rigid
corners.
Corners can be
mitred


Single stiffeners
Dimensions and ratings
For permitted fastenings (types and spacings), see Table 5

Section

H
mm

Thickness
mm

Rating

25
30
40
50
60

3
4
4

5
5

S2
S3
S4
S5
S6

H

25
30
40
50

1.6
1.6
1.6
2.0

S1
S2
S3
S4

H

20
25

1.6
1.6

35
40

1.6
2.0

S1
S2
S3

Fig. 18
H

H

Fig. 19
H

2

Fig. 20

40

Fig. 21
H

25

25

25

S4

15

1.2

20
25
40

1.2
1.6
1.6

S1
S2
S3
S4

50

2.0

S5

20
30

0.8
1.0

S1
S2

40

1.2

S3

25

0.8

S1

Fig. 22
H

Fig. 23
H

Note – Other profiles may be used providing the duct deflection is limited to a maximum of 1/250 of the duct

side under operating pressure.

Intermediate

Stiffeners

Fig. 24
For permitted fastenings (types and
spacings) see Table 5

Full girth stiffener with
welded corners

Illustrations show rolled steel angle
stiffeners. Stiffeners shown in Figs. 19 to
23 are permitted. If used as full girth
stiffeners rigid corners are required.

Back to back stiffener with
bolted corners
Longest side stiffener
Ductwork with short side 400 mm and less

23


Tie rod assembly – alternative arrangement

Fig. 26 With tubing or
conduit and

threaded inserts
Fig. 25 With internal and
external nuts

Fig. 27 With spacers

Fig. 28 With shouldered rod

Table 5 Fastening Centres

Dimensions in mm

Sheet to sheet
Type
of
fastening

Longitudinal
seams

Sheet to section (1)
Cross
joints

Stiffeners

Cross joints

Lap

Standing
& Capped
standing

Socket
&
spigot

RSA

Slide
on
flanges (4)

Mechanically
closed rivets

60

300

60

150

300

150

Self piercing screws

60

–

60

–

300

150

Lock bolts

60

300

–

150

300

300

Set screws & nuts

–

300

–

150

300

300

Spot welds

30

150

–

75

300

150

Dimpling

–

150

–

–

1.50

(2)

(1) A minimum of 2 fixings per side, with a maximum distance from
(2) Except when pierced dimpling is used, one of the other types of
addition to dimpling
(3) In addition to dimpling, one of the other types of fastening must
cases not less than 1 per side
(4) Where manufacturers have specific recommendations, then these
in the Table above

(3)

(5)

–

the corner to the first fixing of 50 mm
fastening must be used at each end in
be used at 450 mm centres, and in all
shall take precedence over the centres

(5) Mechanically closed rivets are not recommended for fixing external stiffeners to ductwork exceeding
500pa negative.

24


Fig. 29 Bends - examples of ‘hard’
and ‘easy’

Hard

Easy

Fig. 30 Turning Vanes

Example of bracing when vane length
exceeds max. permitted.
Alternatively use two banks of vanes
fixing the centre rails together at
150 mm centres.

Fig. 30c

Fig. 30a Single skin
vane
Max. vane length
Single skin – 615 mm
Double skin – 1250 mm

50 mm
90°
ELEVATION

Fig. 30b Double skin
vane

50 mm
25 mm

60

m

m

pi

tc

h

25 x 50 x 1 mm
Channel or angle
fixed to duct
wall both ends

90°
10

Fig. 30a and 30b
Maximum distance between
centres of turning vanes should
not exceed 60 mm pitch.

00

M.10 clamping bolt
at 1000 mm centres

SECTIONAL PLAN SHOWING TYPICAL EXAMPLE OF FIXING

25


26