Table Of ContentMixed mode ventilation
CIBSE Applications Manual AM13: 2000
The Chartered Institution of Building Services Engineers
222 Balham High Road, London SW12 9BS
The rights of publication or translation are reserved.
No part of this publication may be reproduced, stored in a
retrieval system or transmitted in any form or by any means
without the prior permission of the Institution.
© 2000 The Chartered Institution of Building Services
Engineers London
Registered charity number 278104
ISBN1 903287 01 4
This document is based on the best knowledge available at
the time of publication. However no responsibility of any
kind for any injury, death, loss, damage or delay however
caused resulting from the use of these recommendations can
be accepted by the Chartered Institution of Building Services
Engineers, the authors or others involved in its publication.
In adopting these recommendations for use each adopter by
doing so agrees to accept full responsibility for any personal
injury, death, loss, damage or delay arising out of or in
connection with their use by or on behalf of such adopter
irrespective of the cause or reason therefore and agrees to
defend, indemnify and hold harmless the Chartered
Institution of Building Services Engineers, the authors and
others involved in their publication from any and all liability
arising out of or in connection with such use as aforesaid
and irrespective of any negligence on the part of those
indemnified.
Typeset by Florence Production Ltd, Stoodleigh, Devon
Printed in Great Britain by Page Brothers Ltd, Norwich,
Norfolk
Foreword
The successful design of mixed mode systems, or indeed of any other form of low energy
concept, involves collaboration between design team members. Integration, iteration and
co-operation lie at the heart of projects that deliver what the client wants, and with
minimum environmental penalty.
Although mixed mode is not a new approach, it is one that has many facets. This
Applications Manual is the result of much hard work and discussion on the part of a
multidisciplinary team of professionals working with BRE that reflects this commitment to
communication and mutual understanding. It delivers both the broad level of knowledge
required to make strategic decisions about mixed mode systems, and also the best of their
experience in overcoming the more specific pitfalls that await the inexperienced designer
of such systems. It contains a thought-provoking blend of the relatively well rehearsed and
the more aspirational.
We hope this Applications Manual will extend your understanding not only of building
services, but of buildings themselves, and of the people who occupy them.
D Jaunzens
AM 13 Task Group
D Jaunzens (BRE) (Chair)
J Deal
J Bailey (John Bailey Consulting)
R de Caux (Roger Preston and Partners)
M Buckley (BRE)
Contract Authors
F Alamdari (BRE)
D Arnold (Troupe Bywaters and Anders)
W Bordass (William Bordass Associates)
K Ho (Building Simulations Ltd)
R J Oughton (Consultant)
C Twinn (Ove Arup and Partners)
Contract Editor
L Browne (Sandhurst Editorial Consultants)
CIBSE Manager
A C Watson
Acknowledgements
The Chartered Institution of Building Services Engineers wishes to thank the Task Group
members, contract authors and reviewers for all their efforts. This work was funded by the
Department of the Environment, Transport and the Regions’ Construction Directorate.
However, the views expressed in the document are not necessarily those of the Secretary of
State for the Environment.
Contents
1 Mixed mode: an overview 1
1.1 What do we mean by mixed mode? 1
1.2 Why an Applications Manual? 1
1.3 Why consider mixed mode? Potential advantages and
disadvantages 1
1.4 Potential advantages of the mixed mode approach to individual
participants 2
References 3
2 Design principles for mixed mode buildings and systems 8
2.1 Physical mixed mode strategies 9
2.2 Operational strategies for complementary systems 9
2.3 Strategic assessment of possible options 10
2.4 Good practice design principles 10
References 12
3 Exploiting building fabric properties 14
3.1 Main design factors 14
3.2 Windows 15
3.3 Solar shading 15
3.4 Thermal inertia 18
3.5 Night ventilation 19
3.6 Cooling analysis 19
3.7 Self-heating buildings 21
3.8 Envelope airtightness 22
3.9 Trickle ventilation 22
References 22
4 Contingency mixed mode 23
4.1 Principles 23
4.2 Main plant 23
4.3 Distribution routes 24
4.4 Initial installation 24
4.5 Building fabric enhancement 25
4.6 Design process for contingency mixed mode 25
References 26
5 Complementary mixed mode 27
5.1 Design approach/methodology 27
5.2 Active thermal storage options 31
5.3 Complementary mixed mode options 32
5.4 Operating strategies 36
5.5 Plant and equipment sizing 38
References 38
6 Zoned mixed mode systems 39
6.1 Potential applications for zoned treatments 39
6.2 Design considerations for zoned systems 39
6.3 System choice 39
6.4 Cost consequences of zoned systems 40
7 Control of mixed mode systems 41
7.1 Controls design strategy 41
7.2 Controls concept description 41
7.3 Controls specification 41
7.4 Identification of control parameter priorities 42
7.5 Perceptibility of control responses 42
7.6 Expecting too much of occupants 42
7.7 Information for building users 42
7.8 Manual and automatic control interaction 43
7.9 Technology versus simplicity 43
7.10 Control of ventilation 43
7.11 Relative humidity levels 43
7.12 Night cooling: issues for consideration 44
7.13 Maintenance and servicing 44
References 44
8 Commissioning considerations for the designer 45
8.1 Importance 45
8.2 The commissioning specification 45
8.3 Commissioning management 46
8.4 Provision for revisits and fine-tuning provisions 47
References 47
9 Handover, management and operational considerations for 48
the designer
9.1 General 48
9.2 Handover duties 48
9.3 Handover documentation 48
References 49
10 Modelling and modelling techniques 50
10.1 Simulation objectives 50
10.2 Approach and interpretation 51
10.3 Incorporating control strategies 51
10.4 Occupant control 52
10.5 Selection and use of weather data 52
10.6 Modelling the effects of thermal mass 52
10.7 Modelling air movement 52
10.8 Design process example 53
10.9 Conclusions on the modelling process 53
References 53
11 Energy and environmental benefit 54
11.1 Energy consumption benchmarks 54
11.2 Energy consumption for heating 54
11.3 Energy use for humidity control 55
11.4 Energy consumption by fans 56
11.5 Common causes of energy wastage, and how to avoid them 56
11.6 Conclusions on energy and environmental benefits 57
References 57
12 Window design 58
12.1 Purposes and methods of ventilation 58
12.2 The appropriate number of window elements 58
12.3 Trickle ventilators 59
12.4 Windows and planning modules 59
12.5 Control, operation, management and maintenance 59
References 60
13 Thermal comfort issues 61
13.1 The theory 61
13.2 The practice 61
References 62
14 Terms of appointment 63
14.1 The existing situation 63
14.2 The building fabric: who should take responsibility? 63
14.3 Mixed mode feasibility studies 63
14.4 Defining the duties 64
14.5 Deciding the fee basis 64
14.6 Lifecycle considerations 65
References 66
15 Development of the brief 67
15.1 Defining the building fabric performance 67
References 67
16 Commissioning activities 68
16.1 Commissioning of passive systems 68
16.2 Subjective reactions 68
16.3 Commissioning of mechanical systems 69
16.4 Building airtightness checks 69
16.5 Witnessing arrangements 70
16.6 Commissioning return visits 70
17 Handover, management, operation and maintenance
considerations 71
17.1 Handover 71
17.2 Defects liability period 72
17.3 Management of mixed mode system premises 72
17.4 Operating a mixed mode building 72
17.5 Maintaining a mixed mode building 73
17.6 Managing mixed mode building adaptations 73
References 74
Bibliography 75
Index 76
1
1 Mixed mode: an overview
1.1 What do we mean by examination of these issues is beyond the scope of this
document, but points of interest are referred to.
mixed mode?
‘Mixed mode’ is a term used to describe servicing strategies
1.2 Why an Applications
that combine natural ventilation with mechanical venti-
lation and/or cooling in the most effective manner. It Manual?
involves maximising the use of the building fabric and
envelope to achieve indoor environmental conditions,
CIBSE has published this Applications Manual in order to
and then supplementing this with degrees of mechanical
explore the advantages of mixed mode design, and to warn
systems, in all or parts of the building. To date the approach
of some possible pitfalls. It will:
has been used most widely in offices; however, it is suitable
for a wide range of building types. (a) help all members of the design and construction
team and their clients to understand the issues
The construction of purpose-designed mixed mode involved throughout the process—from inception
buildings started in the UK after the oil crisis in the 1970s, and briefing to handover and operation
particularly for owner-occupied and public sector offices.
(b) identify desirable features, and alert designers to
However, increasing amounts of office-based information
potential problems that have been discovered in
technology equipment and the resultant assumptions with
surveys of completed buildings
respect to cooling load requirements, coupled with low fuel
prices, led to the rise of the sealed (that is, no openable (c) assist building services engineers in producing
windows), VAV-serviced institutional standard for offices. robust, effective and efficient solutions for the
Despite this, there are examples of major and influential passive and mechanical environmental engineering
mixed mode office buildings of the 1970s and 1980s that requirements and their associated control and
continue to perform successfully, and from which much can management systems
still be learnt. In the 1990s, concerns about the environ-
(d) improve professional and market awareness of
ment, value for money, occupant satisfaction and health
different variants of mixed mode design.
created a surge in interest in the mixed mode approach,
which is becomingly increasingly widespread. Pre-let
It includes guidance on the range of issues relating to mixed
mixed mode buildings began to appear during the late
mode design, operation and maintenance that are likely to
1980s, and speculative examples are now emerging.
affect the actions of the CIBSE member. It is structured—
where practicable—to follow the design process itself and
Mixed mode is not a technical fix. A successful design needs
to facilitate interactions with design team colleagues and,
robust integration of fabric, services and controls, with
most importantly, their clients.
features that add functionality without adding compli-
cation. Its engineering systems need to be intrinsically
The text includes a mixture of well-founded best practice
efficient, economically operated, and usable and under-
and the latest innovative thinking, with suitable caveats
standable by occupants and management. An enormous
where necessary. There is little about the engineering
range of systems may be installed and integrated with
services systems used that should be unfamiliar; it is the
natural ventilation in many ways—in space, over time,
integration of these systems with the passive building
working together, working alternately, and so on. At one
elements, and the control of both, that demands care.
end of the scale a mixed mode building may be entirely
naturally ventilated, with only strategic provision for
adding mechanical ventilation and cooling systems in part
or all of it. At the other end, a building may be fully air 1.3 Why consider mixed
conditioned, but planned so that at some time in the future,
mode? Potential
part or all of it could function with less (and possibly no)
mechanical assistance. In commercial terms mixed mode advantages and
buildings can potentially offer looser-fit, less specialised
disadvantages
and sometimes lower-cost buildings, with robust shells
and adaptable services that can be readily altered—
operationally or physically—to suit the needs of different Very early in many projects, irreversible decisions are often
occupiers, activities, and even sectors. However, such a made to seal and air condition buildings because natural
relatively new concept can create uncertainties for ventilation is (or is seen as) unable to meet all of the design
marketing, valuation and investment decisions. A detailed requirements or contingencies, or because air conditioning
2 Mixed mode ventilation
will be (or is predicted to be) a better investment for the site — increased costs if air conditioning is still required
regardless of technical need. Mixed mode concepts can after openable windows are added.
frequently provide more cost-effective, adaptable, lower-
Other points to consider are:
energy, comfortable, future-proof buildings, which rely
upon well-designed natural systems as far as is possible and — A form that helps to increase passive potential may
use mechanical systems efficiently, when and where reduce the building area that can be fitted on the
necessary, to deliver the necessary levels of comfort and site, but generally to a lesser degree than for fully
performance. naturally ventilated buildings.
— The need for access to windows may reduce the
This ‘bridge’ between natural ventilation and air
ratio of usable to total internal area.
conditioning avoids all-or-nothing choices, and offers
many advantages. Indeed, a blend of natural and mech- — The investment value may possibly be less certain
anical systems may be more appropriate to many than for fully air conditioned properties.
requirements than options at the extreme ends of the scale.
In suitable circumstances, mixed mode holds the promise
of:
1.4 Potential advantages of
(a) ‘tuneable buildings’ with longer life; greater
the mixed mode approach
adaptability of space and services; the ability to
meet occupiers’ real needs and changing priorities to individual participants
while avoiding wasteful over-provision, or
unnecessary capital and running costs (a) Developers: Mixed mode is a cost-effective (although
not necessarily the lowest cost) approach, which
(b) better occupant satisfaction, by combining the
can add value by appealing to a wide range of
perceived advantages of openable windows, assisted
customers. It could provide the flexibility that is
by any mechanical engineering services necessary
often an important selling point of full air
to provide suitable levels of performance, health,
conditioning—but is not always delivered in
safety and comfort
practice—at a lower cost and with less energy,
(c) buildings that are easier to manage, with HVAC management and maintenance requirements.
systems generally somewhat smaller and simpler
(b) Owners and investors: Mixed mode is a good short-
than in fully air conditioned buildings, and with
and long-term investment, offering a smooth
less exacting fine-tuning requirements
transition to more sustainable buildings. It is
(d) lower energy use and associated greenhouse gas and adaptable to a wider range of occupant require-
pollutant emissions, by avoiding the unnecessary ments and future scenarios; it has less risk of non-
provision and operation of mechanical systems at performance in terms of rental yield in today’s
times and in places where natural ventilation could harsh business environment; and, if necessary, it is
achieve the task more efficiently more readily sold or let than a fully naturally
ventilated property.
(e) lower capital costs.
(c) Designers: Mixed mode can make it easier to provide
The environmental imperative, trends in public attitudes, buildings to suit users with unknown needs. This
and good engineering practice make serious considera- applies not only to speculative buildings; many
tion of passive, low-energy solutions essential. CIBSE organisations and the uses they make of buildings
Applications Manual AM10: Natural ventilation in non- are changing rapidly. A mixed mode approach may
domestic buildings(1) recommends natural ventilation as the also be able to ‘rescue’ a building that cannot quite
default design strategy, followed by mixed mode, with work with natural ventilation alone—for example
sealed buildings with full air conditioning as the last resort. one with too deep a plan, or requiring some
There is also a view(2)that all buildings likely to need active localised additional cooling or heat removal; or
cooling should adopt mixed mode design guidelines. having traffic noise levels increasing at one façade
once built.
However, mixed mode design concepts can be unfamiliar (d) Occupiers: Mixed mode could potentially meet the
territory to many designers and their clients. A successful requirements of some, if not most, occupiers more
design may well require: effectively, more simply, at a lower cost than for air
conditioned buildings, and with less uncertainty
— more design effort to reduce subsequent inputs of
than with natural ventilation. It may also fit
materials, energy and management (as designers
strategic business thinking in an uncertain world,
become more familiar with the techniques
by hedging bets and demonstrating concern for
involved, this additional effort will reduce)
occupants, the environment and ‘no frills’ waste
— more investment in certain aspects of the fabric to avoidance.
save on services
(e) Building/facilities managers: Mixed mode buildings
— investment to improve the efficiency, responsive- may be potentially more robust, more adaptable
ness, control and adaptability of building services, and have lower costs, maintenance and manage-
even though they will usually cost less than air ment burdens. However, there are new skills to be
conditioning learned. In practice, time to manage buildings and
Mixed mode: an overview 3
their services tends to be scarce, so designers must (g) Environment: Mixed mode offers potentially more
aim to make systems simple to understand and easy durable buildings with lower energy consumption,
to manage. lower risk of premature obsolescence, and
sometimes, perhaps, including relocatable modular
(f) Individual occupants: In surveys, occupants often say
plant that could be sold and reused if it was no
that they like to have openable windows. Where
longer required.
they have them (and provided that the windows
open onto a reasonable environment, and are
Table 1.1 summarises the potential advantages of mixed
controllable by the occupant either manually or via
mode systems to the individual parties involved, and
responsive automatic controls) occupants also
identifies potential disadvantages.
appear to be less critical of numerous aspects of the
internal environment, including summertime
temperatures, noise and air quality. However, the
benefits of natural ventilation are often perceived References
primarily by those next to the windows. Mixed
mode designs can allow the internal environment 1 Natural ventilation in non-domestic buildings CIBSEApplications
to be objectively better than would be possible with Manual AM10 (London: Chartered Institution of Building
Service Engineers) (1997)
natural ventilation alone. However, it is important
that any local controls are well integrated and that 2 Liddament M A guide to energy efficient ventilation(Coventry:
their method of operation is intuitive to the user. Air Infiltration and Ventilation Centre) (1996)
