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AS 1668.2 Supp1—1991 THE USE OF MECHANICAL VENTILATION AND AIR-CONDITIONING IN BUILDINGS - MECHANICAL VENTILATION FOR ACCEPTABLE

AS 1668.2 Supp1—1991 THE USE OF MECHANICAL VENTILATION AND AIR-CONDITIONING IN BUILDINGS - MECHANICAL VENTILATION FOR ACCEPTABLE

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This Supplement was prepared by Committee ME/62, Mechanical Ventilation and Air
Conditioning. It was approved on behalf of the Council of Standards Australia on
20 May 1991 and published on 1 July 1991.

The following interests are reprsented on Committee ME/62:
Association of Consulting Engineers, Australia
Australian Assembly of Fire Authorities
Department of Administrative Services
Australian Construction Services

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Australian Institute of Environment Health
Australian Institute of Refrigeration Air Conditioning and Heating
Australian Uniform Building Regulations Coordinating Council
Building Owners and Managers Association of Australia
Council of Air Conditioning and Mechanical Contractors Associations of Australia
Council of the City of Sydney
Confederation of Australian Industry
Fire Protection Industry Associations of Australia
Insurance Council of Australia
Metal Trades Industry Association of Australia
Public Works Department, N.S.W.

Review of Australian Standards. To keep abreast of progress in industry, Australian Standards are subject to
periodic review and are kept up to date by the issue of amendments or new editions as necessary. It is important
therefore that Standards users ensure that they are in possession of the latest edition, and any amendments thereto.
Full details of all Australian Standards and related publications will be found in the Standards Australia Catalogue
of Publications; this information is supplemented each month by the magazine ‘The Australian Standard’, which
subscribing members receive, and which gives details of new publications, new editions and amendments, and of
withdrawn Standards.
Suggestions for improvements to Australian Standards, addressed to the head office of Standards Australia, are
welcomed. Notification of any inaccuracy or ambiguity found in an Australian Standard should be made without
delay in order that the matter may be investigated and appropriate action taken.

AS 1668.2 Supp1—1991

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Australian StandardR

The use of mechanical ventilation
and air–conditioning in buildings
Part 2: Mechanical ventilation for
acceptable indoor–air quality—
Commentary
(Supplement to AS 1668.2 – 1991)

First published as SAA MP47.C2–1980.
Revised and redesignated AS 1668.2 Supp1–1991.

PUBLISHED BY STANDARDS AUSTRALIA
(STANDARDS ASSOCIATION OF AUSTRALIA)
1 THE CRESCENT, HOMEBUSH, NSW 2140
ISBN 0 7262 6981 6

PREFACE

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This Commentary on AS 1668, The use of mechanical ventilation and air–conditioning in
buildings, Part 2: Mechanical ventilation for acceptable indoor–air quality, was prepared by
the Standards Australia Committee on Mechanical Ventilation and Air Conditioning to
provide guidance on the application of the Code by explaining the intent of those clauses
which could be the subject of requests for interpretation.
This publication supersedes SAA MP 47, Part C2: Commentary on AS 1668,
SAA Mechanical Ventilation and Airconditioning Code, Part C2: Ventilation requirements.
The contents have been revised to incorporate all alterations included in AS 1668,
Part 2–1991.
The clause references in the Commentary are those for the corresponding clauses in AS 1668,
Part 2–1991, prefixed by the letter ‘C’.

E Copyright — STANDARDS AUSTRALIA
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Care should be taken to ensure that material used is from the current edition of the Standard and that it is updated whenever the Standard is amended or
revised. The number and date of the Standard should therefore be clearly identified.
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subject to the payment of a royalty. This policy may be varied by Standards Australia at any time.

CONTENTS
Page
SECTION C1
C1.1
C1.2

SCOPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APPLICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION C2
C2.1
C2.2
C2.3
C2.5
C2.6

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C3.5
C3.6
C3.7

C4.2
C4.3
C4.4
C4.5
C4.6
C4.8
C4.10
C4.11
C4.13

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5
5
6
7

EXHAUST DILUTION PROCEDURES

SCOPE OF SECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
GENERAL EXHAUST VENTILATION . . . . . . . . . . . . . . . . . . . . . . . . . . .
LOCAL EXHAUST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AIR FROM ENCLOSURES HAVING EXHAUST AIR
REQUIREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REPLENISHMENT OF EXHAUST AIR . . . . . . . . . . . . . . . . . . . . . . . . . . .
COMBINATION OF EXHAUST SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . .
AIR DISCHARGES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION C4

4
4

SUPPLY AIR DILUTION PROCEDURE

SCOPE OF SECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
OUTDOOR AIR INTAKES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
OUTDOOR AIR FLOW RATES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
OUTDOOR AIR MIXING AND DISTRIBUTION . . . . . . . . . . . . . . . . . . .
OUTDOOR AIR FLOW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

SECTION C3
C3.1
C3.2
C3.3
C3.4

SCOPE AND GENERAL

8
8
8
8
9
9
9

VENTILATION OF ENCLOSURES USED BY VEHICLES
WITH INTERNAL COMBUSTION ENGINES

APPLICATION OF SECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
GENERAL CASE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CARPARKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ENCLOSURES OTHER THAN CARPARKS . . . . . . . . . . . . . . . . . . . . . . .
QUEUING AREA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REPLENISHMENT OF EXHAUST AIR . . . . . . . . . . . . . . . . . . . . . . . . . . .
STAFF—VENTILATION RATE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LOCATION OF EXHAUSTS IN BELOW GROUND ENCLOSURES . . . .
MONITORING OF ATMOSPHERIC CONTAMINANTS . . . . . . . . . . . . . .

10
10
10
11
11
11
11
11
12

APPENDICES
CA DERIVATION OF THE AIR CLEANING EFFICIENCY EQUATIONS USED
IN APPENDIX D, AS 1668 PART 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CB BASIS OF AIR FLOW RATES FORMULAE FOR CARPARKS . . . . . . . . . .
CC DERIVATION OF AIR FLOW RATES FOR QUEUING AREAS . . . . . . . . . .

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AS 1668.2 Supp1—1991

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STANDARDS AUSTRALIA
Australian Standard
The use of mechanical ventilation and air-conditioning in buildings
Part 2: Mechanical ventilation for acceptable indoor-air quality—Commentary
(Supplement 1 to AS 1668.2–1991)

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SECTION C1

SCOPE AND GENERAL

C1.1 SCOPE Present building regulations (and the Australian Model Uniform Building Code) are based on the
assumption that natural ventilation will be the first ‘natural’ choice and arbitrary minimum criteria for such
‘natural’ ventilation are prescribed. Presumably the occupants will have control over this aspect of their
environment in the type of space where natural ventilation is employed.
In the event of the criteria not being met, e.g. percentage of floor area as openable windows or fixed openings
and/or distance from openings are insufficient, then mechanical ventilation or air-conditioning would be required,
as a permissible substitute. Where the criteria were met, then mechanical ventilation or air-conditioning would
be a permissible substitute at the constructor’s option – generally at a higher cost.
AS 1668, Parts 1 and 2 are references in both State and Territory building regulations and in the recently
published Building Code of Australia. Where mechanical ventilation is required by such regulations and codes,
compliance with AS 1668 is prescribed. This therefore places constraints on the wording of the Standard that
would not otherwise be necessary.
AS 1668.2–1980 provided some interpretations of the application of natural ventilation to carparks as prescribed
in building regulations. Since AS 1668.2 is only referenced in building regulations in respect to mechanical
ventilation requirements, consideration was given to the deletion of these natural ventilation interpretations. The
Australian Uniform Building Regulations Interstate Committee deemed it appropriate that such interpretations
remain in AS 1668.2 at this time. They have therefore been developed in more detail in the revision for further
clarification. The wording of the Standard in respect of the use of natural ventilation has also been changed to
provide a more logical basis for its inclusion.
Although AS 1668.2 has been incorporated in State and Territory building regulations, its use has been
supplemented or replaced in specific areas by local codes. The revision is expected to achieve greater acceptance
of AS 1668.2 except where a local code also addresses air-conditioning. Although the title of AS 1668.2 has been
changed from ‘Ventilation requirements’ to ‘Mechanical ventilation for acceptable indoor-air quality’ the
extension of the Standard to include comfort in terms of the control of temperature, humidity, air-movement or
noise was still deemed to be inappropriate at this time. These elements are therefore only addressed in terms of
the definement of limits beyond which health or safety may be impaired.
C1.2 APPLICATION The applications of the Standard are now more precisely described by direct reference
to the specific application of each section.
To provide greater assistance in the use of the Standard, a logic diagram has also been included.

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SECTION C2

AS 1668.2 Supp1—1991

SUPPLY AIR DILUTION PROCEDURE

C2.1 SCOPE OF SECTION The term ‘fresh air’ has been deleted entirely from the Standard and term ‘outdoor air’ used in place of ‘outside air’.
This has been done to remove possible misunderstandings and to achieve conformity with American Society of
Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) and other overseas Standards.

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C2.2 OUTDOOR AIR INTAKES
C2.2.1 Location The work of Wilson, Halitsky and others indicates the complicated wake effects and the
interaction between adjacent obstructions and the intakes and discharges of a building.
Chapter 14 of the 1989 ASHRAE Fundamentals Handbook contains a comprehensive coverage of air flow around
buildings, dispersion of building exhaust gases and design to minimize re-entry.
Particularly critical cases may warrant wind tunnel testing of models.
C2.3 OUTDOOR AIR FLOW RATES
C2.3.2 Occupancy In some cases, the occupancies used for the purposes of determination of outdoor air flow
rates may differ in aggregate for a given floor from those used for egress provisions in the Building Code of
Australia.
This can arise as a result of the use of the maximum occupancy for each area with no account taken of movement
of people from one area to another at different times during the period of occupancy. An example would be a
conference room used substantially by the occupants of adjacent areas on the same floor.
Table A1 and the associated notes comprising Appendix A need to be read together.
C2.3.3 to C2.3.4 Minimum outdoor air flow rates (Q f ) The minimum outdoor air flow rates required to
maintain acceptable air quality depend on the definition of ‘acceptability’. The rates specified in the Standard
are consensus values, and have been arrived at after extensive consideration of published research, rates used in
the past, changing patterns of occupant behaviour, and changing patterns of pollution emissions within the
building.
The Standard requires outdoor air flow rates in accordance with Appendix A, but allows reductions if a sufficient
quantity of recycle air is treated to remove contaminants at an efficiency that meets the requirements of the
appropriate formula from Appendix D. It should be noted that:
(a) The total supply air quantity (i.e. recycle air + outdoor air) must exceed the air quantity required by Appendix
A, otherwise the formula will require an efficiency greater than 100%.
(b) The efficiency for removal of particulate contaminants must be as determined using one of the specified test
procedures.
(c) No test procedure for determining efficiency of removal of odours and gaseous contaminants is nominated
as there is no recognized procedure. It will be necessary for designers proposing such air treatment to justify
the method used to determine the efficiency.
If air treatment is provided that removes particulates, tobacco smoke odours, and body odours, the Standard
requires a minimum air flow rate for dilution of other gaseous contaminants that are generated in buildings such
as CO2, ozone, formaldehyde and volatile organic hydrocarbons.
If air treatment is provided that removes only particulates, the Standard requires a minimum air flow rate for
dilution of gaseous contaminants and odours, a 0 , that is based on the air quantity necessary to dilute body odours.
In enclosures where heavy smoking occurs, such as bars, this will not control odours from tobacco smoking, but
this is considered acceptable on the basis that occupants have a choice of whether they enter and remain for an
extended period in these enclosures.
For control of body odours, recent studies by Bouwan, Cain, Fanger and others show an outdoor air flow rate of
7.5 to 10 L/s/person to be appropriate and to be a consensus value of minimum ventilation rates in the US,
Germany and the UK. The Standard requires a rate of 7.5 L/s/person, increasing to 15 L/s/person for enclosures
in which temperatures are not kept below 27°C by air-conditioning or other means, as the generation and/or the
perception of body odours increase with temperature.
It was additionally recognized that children are more odorous than adults; the tabulated ventilation rates for some
occupancies reflect this.
For special enclosures, such as operating theatres, a high value of a 0 is specified, so that no reduction in outdoor
air flow rates is allowed if recycle air treatment is provided.

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If no air treatment is provided, the Standard requires the minimum outdoor air flow rates to be in accordance with
Appendix A. The rates in Appendix A take account of contaminants from smoking. Based on the work of Wanner,
Weber and Johansson, concentration of carbon monoxide may be taken as an indicator of the concentration of
contaminants from tobacco smoke. A limit of 1 p.p.m. of CO from tobacco smoke is considered necessary to
maintain acceptable indoor air quality.
The rates listed in Appendix A, other than those for special enclosures, can be divided into those applying to
areas subject to moderate, medium or heavy smoking, for which the rates are 10, 15 and 20 L/s/person
respectively.
For an enclosure in which an excess of environmental tobacco smoke load is anticipated, it is recommended that
outdoor air quantities should be increased to an appropriate value. Where occupants are required by their
employers to enter and remain for an extended period in such enclosures, it is recommended that ventilation
systems should be designed so that the employees receive an appropriately higher amount of outdoor air.
Building ‘leakage’ or a requirement for make-up air may result in a need for higher levels of outdoor air flow
than the prescribed minimum.
C2.3.6 Minimum flow rate of outdoor air into a system serving a group of enclosures (Q F ) The concept of
unused ‘outdoor air’ with ‘return air’ is well developed in Appendix D of the Standard. Derivation of the air
cleaning efficiency equations used in that appendix are included as Appendix CA to this commentary.
For single enclosures the formulae in Appendix D used to determine the required efficiencies are straightforward
(Clauses D2.2.1. and D.2.2.2). For a group of enclosures for which a multiple enclosure factor is applicable, the
appropriate formula depends on the location of the air treatment in the system, and the formula can be
complicated. Clause D2.3 of Appendix D provides formulae for some configurations, for which the derivations
are set out in Clause CA2.3 of this Commentary. For any other configuration proposed, the appropriate formula
would have to be derived by the designer.
C2.3.7 Variable air volume systems Clauses 2.3.6 and 2.3.7 are both designed to take account of variations
in air flows in variable air volume systems.
Table A1 in Appendix A of the Standard specifies the minimum outdoor air flow rates that must be supplied to
one enclosure to maintain an indoor air quality that, in the present state of knowledge, should eliminate possible
adverse effects of the indoor air on healthy adult occupants remaining in this enclosure for unrestricted periods.
Quite often one air-handling plant supplies air to more than one such enclosure. The question then arises: How
much outdoor air has to be introduced to the air-handling plant so that the minimum air quality required by this
Standard will be maintained in all enclosures served by this air-handling plant?
The formula given in Clause 2.3.6(a) provides the answer to this question and is known as the Multiple Enclosure
Formula, and yields the Multiple Enclosure Factor ‘M’. The derivation of this formula can be found in Appendix
D of the Standard and a nomogram is provided (Figure 2.2 of the Standard) to simplify the computation of M.
The factor M is usually greater than one. The reason for this is obvious from the derivation and Example D1 in
Appendix D.
One way of satisfying the requirements of this Standard, would be to introduce into the air-handling plant,
sufficient outdoor air, so that the outdoor air content in the air supplied by the air-handling plant, would be
determined by the use of the highest ratio of outdoor air to total supply air for any enclosure, applied to the total
of air supplied to all enclosures. This would certainly satisfy the Standard but would not be economical since all
enclosures with lower ratios would be oversupplied with outdoor air.
The Multiple Enclosure Formula is based on the fact that the air returned from enclosures oversupplied with
outdoor air still has some potential to dilute pollutants to satisfactory levels when introduced into the supply duct
from the air-handling plant to enclosures.
It should be evident that further economies can be achieved by the grouping of enclosures with like ratios of
outdoor air to total air on the same air-handling plant. This of course may not be possible or appropriate in many
cases for a variety of reasons.
C2.5 OUTDOOR AIR MIXING AND DISTRIBUTION The purpose of this Clause is to ensure that all
enclosures supplied with air by an air-handling plant receive at all times the minimum amount of outdoor air
required by the Standard.
As it is not practicable to measure the outdoor air content and its distribution in the air supply duct, the
requirements of this Clause will be satisfied if the system design provides for –
(a) minimum outdoor air to be carried at all times by the supply ducts from the air-handling plant to all
enclosures; and
(b) sufficient mixing of outdoor air with the recycle air in the air-handling plant to supply all enclosures with
air of about the same ratio of outdoor to recycle air.
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A well designed mixing chamber before the main fan in a single supply duct system would satisfy this Clause.
It would also satisfy the clause for other systems if the design provides for the minimum outdoor air to be
delivered to each enclosure under all possible conditions of operation, i.e. from full heating to full cooling. The
Standard accepts that this condition will be met if all supply ducts carry the same fraction of outdoor air.
The other requirement of this Clause of the Standard that outdoor air be well distributed within the occupied zone
of an enclosure is to ensure the elimination of pockets of stagnant air where pollutants could accumulate.

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C2.6 OUTDOOR AIR FLOW
C2.6.1 Systems serving enclosures in which the temperature may exceed 27°C under normal operation
It is well recognized that additional body odour is generated at elevated temperatures. The determination of
outdoor air flow rates in Table A1 of Appendix A of the Standard for the ‘no smoking’ requirement is usually
based on odour control. Recognize that, in the majority of applications, air-conditioning would be used.
Where this is not the case, or for some other reason, temperatures in excess of 27°C occur under normal
operation, rates must be adjusted upwards to compensate for increase in body odour generation by occupants.
C2.6.2 Systems serving enclosures with transient or variable occupancy The intent of this Clause is to
conserve energy use in terms of the hours of operation of mechanical ventilation systems related to periods of
occupancy of the enclosures served.
This provision was introduced by ASHRAE into its Standard on Ventilation For Acceptable Indoor Air Quality
in 1979 and is included in ASHRAE 62–1989.
The rationale provided in the ASHRAE Standard is included as Appendix J in AS 1668.2–1991.
It should be recognized that although a system may be turned off during periods of non-occupancy a period of
post-purging will be required after departure of occupants to clear pollutants generated by the occupants. Also
a period of pre-purging may be required before entry of occupants to clear accumulation of pollutants generated
by furnishings and other materials, etc., within the space.

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AS 1668.2 Supp1—1991

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SECTION C3

EXHAUST DILUTION PROCEDURES

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C3.1 SCOPE OF SECTION The supply air dilution procedure prescribed in Section 2 of the Standard is adequate
in most circumstances to control the level of contaminants in the indoor air.
Where enclosures contain processes which generate contaminants of a type or concentration deemed in the Standard to
be objectionable or dangerous, it is required that these be removed directly from the enclosure by a separate exhaust
system and not be recirculated through the supply air system.
It is also required that the exhaust air flow rate be adequate to dilute the contaminants and be arranged to discharge them
outside the building in such a manner that no harm or offence results to people outside the building.
Section 3 of the Standard defines enclosure and effluent types requiring exhaust and prescribes minimum exhaust air
flow rates and methods of exhaust.
Enclosures used by vehicles with internal combustion engines are dealt with in a separate section. Refer to Section 4
of the Standard.
C3.2 GENERAL EXHAUST VENTILATION This refers to the removal of effluents which are generally spread
throughout the enclosure by the ‘dilution principle’. The concern here is to keep the concentration of effluent down to
an acceptable level.
Table B1 of Appendix B of the Standard specifies rates for ‘general exhaust ventilation’. A rate of 5 L/s.m 2 of floor area
of enclosure is equal to six air changes per hour based on a ceiling height of 3 m. This is still deemed adequate except
for such applications as bathrooms, laundries and hospital sterilizing rooms where excessive heat or steam is generated
or for sanitary compartments where excess odours are generated.
In kitchens, local exhaust must be used in addition to the specified general exhaust, where the size of cooking apparatus
exceeds the limits stated in Clause 3.3.1(b) of the Standard.
The requirement for the general exhaust rate to exceed the supply rate (where used) by 10 percent is simply to ensure
that effluent cannot flow to adjacent enclosures.
Where mechanical ventilation is used to produce pressure differentials between different enclosures, the requirement for
the general exhaust to exceed the supply may be waived if a general exhaust system is not required by the Standard,
or where a required general exhaust can be replaced by a local exhaust system to approval.
Air locks may not need to be separately ventilated, and reference should be made to the Note under Table 3.2 in the
Standard for the relevant requirements.
C3.3 LOCAL EXHAUST
C3.3.1 Types of effluents requiring local exhaust In the current Standard, the types of effluents have now been
grouped into two categories instead of the five categories used in the previous Standard. This is intended to simplify the
application of the Standard and is based on experience gained since this Standard was first published.
Type A effluents which are dangerous are required to have the point of generation enclosed and this enclosure exhausted.
Type B effluents are more of a nuisance than a danger and the most common application would relate to kitchen exhaust
hoods.
C3.3.2 Effluent removal Apart from establishing requirements for local exhaust, the Standard does not attempt to
prescribe the form, capture velocities or air flow rates of hoods or other means used for local exhaust of effluents.
One exception to this in the previous Standard was kitchen hoods for which the Standard provided design guidance,
owing to a perceived absence of authoritative publications. Although this situation has been improved to a large extent
and some States have industry standards covering this subject, the Committee believed that the best interests of all parties
who use this Standard nationally would be served by the continued inclusion of design guidelines on kitchen hoods.
Having made this decision, work was done on revising and expanding this material to improve its usefulness. This now
comprises Appendix E of the Standard. An additional Appendix F has been included to provide some design guidelines
but these do not constitute part of the Standard in terms of building regulations.
For other applications, references are included in the Standard to other Australian Standards and to the Industrial
Ventilation Manual published and regularly revised by the American Conference of Governmental Industrial Hygienists.
Australian Standards now exist for the design of laboratories and should be referred to in regard to local exhaust and
make-up air provisions in such applications.
C3.4 AIR FROM ENCLOSURES HAVING EXHAUST AIR REQUIREMENTS The intent of this Clause is to
require the regulatory authority to allow return air to be recycled from enclosures in which local exhaust air collection
is used, provided that there is no reason against doing so. Approval, of course, would be conditional upon evidence that
the design of the local exhaust will prevent effluent being carried into the enclosure from which air was to be recycled.
Clause 3.4.3 essentially re-states the requirements of Clause 3.2.3 of the Standard.
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C3.5 REPLENISHMENT OF EXHAUST AIR It is often overlooked in design that the performance of a
mechanical ventilation system is dependent on achieving a balanced flow of supply and exhaust air.
This Clause in the Standard makes provision for the make-up supply air for a mechanical exhaust system, whether
general or local, to be naturally or mechanically provided.
Conditions under which either would be approved are provided. The quality of the make-up air may not be so
critical where there is only short-term occupancy involved. An example is given of make-up air from a carpark
being acceptable for a garbage room exhaust system. Another example would be exhaust air from a kitchen being
replenished from an adjacent eating area or office but not from an adjacent toilet.
Make-up air from an enclosure served by a non-required exhaust system in conjunction with a mechanical supply
or natural ventilation system should be considered. Also make-up air from an enclosure ventilated by a required
exhaust system may in some cases be acceptable, as indicated in the above examples.
In some applications, air change rates are likely to be high in an enclosure due to local exhaust air flow
requirements. Where this could result in severe discomfort and the risk of such systems being turned off by
occupants, heating or cooling of the make-up air may be desirable.
Clause 3.5.4 of the Standard is intended to cover the situation where a local exhaust is used intermittently but
requires a mechanical supply air system for make-up. In such circumstances, control of the systems would be
normally located adjacent to the local exhaust and not integrated into the total building or floor mechanical
ventilation system.
C3.6 COMBINATION OF EXHAUST SYSTEMS The Committee gave consideration to the reduction of
groups in Table 3.2 of the Standard but concluded that there was no identifiable benefit and possible increased
system costs that would result from such a change.
It should be understood that duct connections between enclosures in different groupings present a problem when
the exhaust system is not in operation rather than when in use.
It may also be unacceptable to combine the process exhausts or exhausts from laboratory fume hoods during
system operation. Such applications are not included in this Standard because they are, or will be, covered by
other Australian Standards, e.g. Australian Standard relating to laboratory planning.
C3.7 AIR DISCHARGES Although the objective of this Standard is achievement of acceptable indoor air
quality, the implication of this in terms of any impact on the surrounding environment of air discharges from the
building is directly related to this objective. It was therefore necessary to prescribe requirements to control such
air discharges.
As in the previous Standard, this Clause attempts to cover most situations that are likely to occur and again, as
not all combinations of circumstances can be foreshadowed, guidelines have been laid down to provide for
unusual or novel situations. The inference is still maintained that some exhaust discharges may be found to be
objectionable to the extent that treatment may be required before final discharge into the environment.
The minimum separation distance of 6 m given in Clauses 3.7.2(b), 3.7.2(e) and 3.7.3(c) of the Standard
is intended to convey an awareness of the significant distances necessary to prevent cross-contamination. It was
not determined by any rigorous computation and reference to ASHRAE Handbook–1989 Fundamentals, Page
14.12 will provide further guidance.

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AS 1668.2 Supp1—1991

10

Licensed to LUU MINH LUAN on 25 Feb 2002. Single user licence only. Storage, distribution or use on network prohibited.

SECTION C4 VENTILATION OF ENCLOSURES
USED BY VEHICLES WITH INTERNAL
COMBUSTION ENGINES
C4.2 APPLICATION OF SECTION Ventilation of enclosures used by vehicles with internal combustion
engines has now been established as a separate section from other exhaust ventilation requirements.
Generally, there has been an expansion of applications but vehicular tunnels have been deleted since these are
not directly related to buildings and are more adequately covered in such internationally recognized documents
as prepared by the Permanent International Association of Road Congresses (PIARC) Technical Committee on
Road Tunnels.
Provision is now made to use a mechanical supply air system as an alternative to a mechanical exhaust air system
where natural means of make-up (or air relief) are deemed acceptable.
As stated in C1.1 Scope, the treatment of natural ventilation provisions has been revised and expanded to improve
interpretation.
C4.3 GENERAL CASE Two categories of occupants must be taken into account in such enclosures; these are
the occupants of vehicles using the enclosure and those who work in the enclosure. Standards of exposure are
different for each category and the difference is mainly related to the period of exposure and frequency of
exposure.
C4.4 CARPARKS
C4.4.1 General This Clause now includes a logical progression from full mechanical supply and exhaust
ventilation to full natural ventilation. The conditions under which concessions may be made to omit one or both
mechanical ventilation systems are listed sequentially to assist in the determination of requirements for a
proposal.
Areas of carparks where natural ventilation may be used are now also clearly defined together with conditions
under which natural ventilation of such areas may be approved. The use of diagrams to assist in clarification has
also been expanded.
Unfortunately, the Committee was unable to locate any substantial research on systematic assessment of
performance with respect to natural ventilation. In many cases, regulatory authorities continue to require design
parameters on the basis that the observance of such parameters has not resulted in any complaints from users of
such carparks.
There are significant differences between the natural ventilation parameters established by regulatory authorities
in each State and Territory. The Committee was unable to achieve agreement between even two of the States and,
where unavoidable, adopted a compromise in this revision of the Standard. It is hoped that funds will be made
available for research in terms of levels of carbon monoxide found in naturally ventilated carparks so that
parameters adopted in the next revision of this Standard can be more soundly based.
It should however be clearly understood that Clause 4.3 of the Standard places responsibility on owners to ensure
that concentrations of atmospheric contaminants within the carpark do not exceed exposure standards listed by
Worksafe Australia.
A breeze blowing across a carpark having opposite sides reasonably open will generally be more effective in
removing car exhaust fumes than a mechanical ventilation system. The unpredictability of wind strength and
frequency must be recognized and will limit the effectiveness of natural ventilation provisions under still air
conditions.
C4.4.2 Air flow rates The formulae in the previous Standard which utilized a ‘simultaneous operation factor
C’ to allow reductions in air flow rates for multiple level carparks has been replaced with formulae that can be
used for all carpark applications.
Experience with the use of the formulae in the previous Standard revealed shortcomings which included the
application to carparks with large areas but few levels. Problems in application also arose where exits and entries
were on different levels or in several locations on one level.
The formulae in the new Standard utilize a ‘parking usage factor’ which takes account of the different usage
patterns of carparks associated with various building projects, e.g. shopping centre and office building carparks.
The basis of the air flow rates formulae are provided in Appendix CB of this Commentary. This also contains
the basis of the ‘parking usage factors’ in Table 4.1 of the Standard.
C4.4.2.2 Small carparks For small carparks with 50 or less car spaces, simplified formulae have been included
in this Clause of the Standard.
C4.4.2.3 Number of car spaces The figure of one twenty-third (1/23) of floor area has been arrived at through
surveys conducted of existing carparks. It is a density that can be reached in certain circumstances. The most
satisfactory approach is to show car spaces on the drawings of the proposed carpark.

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