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6 IEQ
6.4
V
ENTILATION
6.4.1 V
ENTILATION IN
A
IR
-
CONDITIONED
P
REMISES
E
XCLUSIONS
Residential and similar buildings using window units and/or split units.
O
BJECTIVE
Ensure that ventilation systems provide for effective delivery to support
the well being and comfort of occupants in normally occupied spaces.
C
REDITS ATTAINABLE
2
P
RE
-
REQUISITES
Compliance with CAP 123J Building (Ventilating Systems) Regulations.
C
REDIT REQUIREMENT
a) Outdoor air ventilation rate
1 credit for demonstrating that the corrected design ventilation rate
meets the design intent for normally occupied spaces, and the
corresponding outdoor air flow rate is achieved.
b) Air change effectiveness
1 credit for demonstrating that the air change effectiveness in normally
occupied areas meets the specified performance.
A
SSESSMENT
The Client shall provide evidence in the form of a report prepared by a
suitably qualified person detailing the design criteria that has been
adopted for each category of premises included in the development, and
the results of calculations, simulations and/or measurements in the
specified sample of premises to demonstrate compliance with the
assessment criteria.
a) Outdoor air ventilation rate
The calculations/simulations shall cover at least one representative
sample of each type of premises (normally occupied spaces).
Calculations should be based on guidance from recognised authorities
such as ASHRAE 62 [1] or equivalent that take into account ventilation
required to provide adequate indoor air quality for odour comfort.
The outcome of measurements shall demonstrate that the required
amount of outdoor air corresponding to the corrected design ventilation
rate is actually provided. Air flow measurements may be made using
conventional procedures, such as described in ASHRAE 111 [2], or by
tracer gas techniques in accordance with ASTM E 741 [3] or equivalent.
Where the corrected ventilation rate is achieved in a minimum of 90% of
premises, and the design ventilation rate is achieved in a sample of each
type of premises the credit shall be awarded
b) Air change effectiveness
Compliance may be demonstrated either through measurement of the
completed building in accordance with ASHRAE 129 (RA 2002) [4] or
equivalent, or in cases where measurement may be difficult using CFD
simulations produced by a suitable airflow model.
The measurement locations shall include at least one representative
sample of each type of premises (normally occupied spaces) as defined
1
American Society of Heating Refrigeration and Air Conditioning Engineers. ASHRAE Standard 62. Ventilation for
Acceptable Indoor Air Quality, Atlanta 2001.
2
American Society of Heating Refrigeration and Air Conditioning Engineers. ASHRAE Standard 111. Practices for
Measurement, Testing, Adjusting and Balancing of Building Heating, Ventilation, Air Conditioning and Refrigeration
Systems, Atlanta 1998
3
American Society for Testing and Materials. ASTM Standard E 741-00, Standard Test Methods for Determining Air
Change in a Single Zone by Means of a Tracer Gas Dilution. Philadelphia, 2000.
4
American Society of Heating Refrigeration and Air Conditioning Engineers. ANSI/ASHRAE Standard 129 (RA 2002).
Measuring Air-change Effectiveness. Atlanta 1997.
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by the type of HVAC system used, design occupancy density, nature of
usage, zoning, etc. Measurements are required at the occupied zone in
each representative test space in accordance with ASHRAE 129. The
measurements shall be undertaken under simulated full occupancy
conditions. All airstreams of the air-side system serving the test space
shall have a constant flow rate to the degree practical (e.g. the difference
between the maximum and minimum should be within 10%).
Where the air change effectiveness is demonstrated to be equal or
greater than 1, and matches the design intent in all sampled premises,
the credit shall be awarded.
B
ACKGROUND
The purpose of this assessment is to demonstrate the adequacy of
ventilation to provide for the control of odours, that is, the supply,
distribution and control of ventilation to maintain carbon dioxide (CO
2
)
levels within design targets in normally occupied spaces when fully
occupied. Design targets are set be the Client, but may take into account
the targets set in the IAQ Certification Scheme [5]. Contamination of
indoor air is dealt with under Indoor Air Quality. Air movement within
spaces is dealt with under Thermal Comfort criteria.
It should be noted that the key references for this section, namely
ASHRAE 62 and ASHRAE 129 use the terms ventilation effectiveness
and air change effectiveness, respectively, for the same quantity, i.e., the
ratio of the nominal time constant to the arithmetic mean of the ages of
air in the breathing zone.
The air change effectiveness (ACE) is a measurement based on a
comparison of the age of air in the occupied areas to the age of air that
would exist under ideal conditions of perfect mixing (effectiveness = 1).
ASHRAE 129 [4] provides a method for measuring ACE in mechanically
ventilated spaces, however, the standard places limitations on the
characteristics of the spaces that can be tested.
Different ventilation systems will generate different air flow patterns and
therefore deliver different proportions of the outdoor air to the occupants’
breathing zone. Approximate values of ACE that can be used at the
design stage can be found in CR1752 [6]. ACE has the value 1 for
complete mixing systems. A value higher than 1 indicates a system with
displacement ventilation characteristics, whereas a value less than 1
indicates ventilation short circuiting.
The design ventilation rate then needs to be corrected for the air change
effectiveness, where the corrected design ventilation rate = design
ventilation rate/(air change effectiveness). For displacement ventilation,
the outdoor air flow rate will be less than the design ventilation rate.
Conversely, for systems where some supplied air can bypass the
breathing zone, the corrected design ventilation rate will be higher than
the design ventilation rate.
For simple systems where the outdoor air serves only a single space, or
a small number of similar spaces, the outdoor air flow rate can be
measured either:
• using a pitot tube [7] and manometer to perform a pitot tube traverse
in accordance with ASHRAE 111-98 [2] or ASHRAE Handbook [8] or
5
Indoor Air Quality Management Group. A Guide on Indoor Air Quality Certification Scheme for Offices and Public Places.
http://www.iaq.gov.hk/cert/doc/CertGuide-eng.pdf
6
European Committee for standardization. CEN Report CR 1752. Ventilation for buildings – Design Criteria for the indoor
environment. Brussels. 1998.
7
International Organization for Standardization. ISO 3966-1977. Measurement of fluid flow in closed circuits - Velocity area
method using Pitot static tubes.
8
American Society of Heating Refrigeration and Air Conditioning Engineers. ASHRAE Handbook Fundamentals. Atlanta
2001.
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• using an installed Wilson Flow Grid with a manometer.
The majority of systems will be too complex for such measurements and
the supply of outdoor air should be verified using a tracer gas
measurement. ASTM 741-00 [3] describes the required properties of
tracer gases and discusses procedures for tracer decay, tracer step up
and constant concentration measurements.
A CFD (Computational Fluids Dynamics) model would typically be used
to perform the simulation and to compute the air change effectiveness
although alternative modelling methods may be proposed. Modelling
should take into account:
• HVAC system type, supply and exhaust dimensions, supply
temperature etc;
• climatic variations;
• air leakage of building envelope; and
• presence of expected furniture/fittings.