Understanding controls

37 Pages·2005·0.461 MB·English

by  Butcher, Ken, Race, Gay Lawrence, Roebuck, Justin

Understanding controls
CIBSE Knowledge Series: KS4
Principal author
Gay Lawrence Race
Editors
Justin Roebuck
Ken Butcher
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.
© September 2005 The Chartered Institution of Building Services Engineers London
Registered charity number 278104
ISBN-10:1-903287-63-4
ISBN-13: 978-1-903287-63-7
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 CIBSEPublications
Printed in Great Britain by Latimer Trend &Co. Ltd., Plymouth PL6 7PY
The Institution gratefully acknowledges contributions to this publication from
John Armstrong, James Ferguson (AEC), David Lush, Julian Miller (AEC)
8 Selected bibliography
BS EN ISO 16484-2: 2004: Building automation and control systems. Hardware;
BS EN ISO 16484-3: 2005: Building automation and control systems. Function
(London:British Standards Institution) (2004/2005)
BS IEC 60050-351: 1998: International electrotechnical vocabulary. Automatic
control (London:British Standards Institution) (1998)
Building Controls Group Control sensor installation pocket book(Benfleet:
Energy Systems Trade Association) (www.esta.kiwi.co.uk)
Building Performance Group Building services component life manual(Oxford:
Blackwell Science) (2001)
Building Regulations 2000 Approved Document L Conservation of fuel and
power(London:The Stationery Office) (2002)
Carbon Trust Energy use in officesEnergy Consumption Guide ECG019 (The
Carbon Trust) (www.thecarbontrust.co.uk)
CIBSE Building control systemsCIBSE Guide H (London:Chartered Institution
of Building Services Engineers) (2000)
CIBSE Energy efficiency in buildingsCIBSE Guide F (London:Chartered
Institution of Building Services Engineers) (2004)
CIBSE Environmental designCIBSE Guide A (London:Chartered Institution of
Building Services Engineers) (2000)
CIBSE Guide to ownership, operation and maintenance of building services
(London:Chartered Institution of Building Services Engineers) (2004)
de Saulles T Illustrated guide to mechanical services BSRIA AG 15/02
(Bracknell: BSRIA) (2002)
Martin A and Banyard C Library of system control strategiesBSRIA AG 7/98
(Bracknell:BSRIA)(1998)
Pennycook K The Effective BMSBSRIA TN 10/2001(Bracknell:BSRIA)(2001)
Pennycook K Rules of ThumbBSRIA BG 14/2003 (Bracknell:BSRIA)(2003)
Pennycook K and Hamilton G Specifying building management systemsBSRIA
TN 6/98 (Bracknell:BSRIA)(1998)
SLL Code for lighting(London:Society of Light and Lighting) (2004)
34 CIBSEKnowledge Series — Understanding controls
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
1.1 Use of this guidance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
2 Controls and control systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
2.1 What do control systems do? . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
2.2 What are controls and control systems? . . . . . . . . . . . . . . . . . . .6
2.3 The main control system elements . . . . . . . . . . . . . . . . . . . . . . .7
2.4 Types of control system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
3 Control components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
3.1 Control components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
3.2 Control reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
3.3 Control component life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
4 Control modes and functions — jargon buster . . . . . . . . . . . . .17
5 What do I want to control? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
6 Developing a control strategy — What are the choices? . . . . .25
6.1 Control strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
6.2 Limitations of controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
6.3 Making a choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29
7 Key questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30
8 Selected bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34
Note from the publisher
This publication is primarily intended to provide guidance to those responsible for
the design, installation, commissioning, operation and maintenance of building
services. It is not intended to be exhaustive or definitive and it will be necessary for
users of the guidance given to exercise their own professional judgement when
deciding whether to abide by or depart from it.
1 Introduction
Building Regulations
Controls and control systems are an essential part of buildings, from the
simple switching on and off of equipment to sophisticated building
System controls are required by the
management systems that monitor and optimise plant performance to meet
Building Regulations to ensure that
building needs. Nowadays it is impossible to avoid the use of control systems, systems operate efficiently and safely and
make effective and efficient use of energy.
which necessitates some knowledge of what they do, and how they function,
in order to be able to ask for the right level of control in the first place and to
operate the controls successfully.
For example, domestic heating systems commonly use time clocks and
temperature controls, often electronic ones which will require programming
and adjustment by the user to meet their specific needs. Many people are
unaware of basic control functions and, in consequence, can use controls
incorrectly — for example by using a thermostat (automatic temperature
control) as an on/off control for their systems. Who has not come into the
house when it is cold and turned the thermostat up ‘till it clicks’? The problem
for building users and operators is more acute as they will have much more
complex systems to operate and, for new projects, they will have to explain, or
even specify, their operating needs and requirements for the building control
Control
systems.
Control: ‘purposeful action on or in a
Whilst there are many experts who can advise on suitable control systems,
process to meet specified objectives’
some of the jargon and terminology used in controls engineering can seem BS IEC 60050-351: 1998: International
impenetrable and incomprehensible to the non-expert, which can make it electrotechnical vocabulary. Automatic
control
difficult to have a useful and constructive discussion of user needs and
required functions. Experience suggests that many buildings do not work as
well as intended or meet occupant needs satisfactorily with, in many cases,
the cause being control problems.
Indeed many building services engineers also find the topic complex. In the
preface to CIBSE Guide H: Building control systems, the chair of the drafting
committee stated: ‘Controls are poorly understood in our industry. The
subject is also disliked in university courses as it can become highly
mathematical. It is probably due to this lack of appreciation of controls and
their commissioning that I have yet to find a building that works properly and
satisfies the occupants …”
1.1 Use of this guidance
This guidance is intended to enable and assist the non-expert client, facilities
manager and building user to:
— understand the functions and limitations of controls
— communicate their needs and requirements to their engineers/advisors.
CIBSEKnowledge Series — Understanding controls 1
It is also likely to be of interest to building services engineers involved in
design, installation and commissioning as a tool that they may find helpful to
use in discussion with clients, and provides students with an accessible
introduction to the subject of controls. Detailed technical information on
control systems can be found in CIBSE Guide H.
This publication provides an introduction to controls and is divided into two
main parts:
— sections 2–4 explain the basic principles and components of controls
systems, together with some of the jargon commonly used
— sections 5–7 provide an overview of what control systems can (and
cannot) do, together with key choice factors and guidance on the
information that may be needed when deciding on control
requirements and strategy.
Finally a selected bibliography is provided for those who require further
reading on this subject.
The publication answers the following questions, which can be used to help you
find the most relevant sections to you
Query Answer in
section
(cid:2) What do control systems do? 2.1
(cid:2) What are controls and control systems? 2.2
(cid:2) What are all the various bits in a control system
and what do they do? 2.3
(cid:2) What are all these abbreviations: BMS, DDC etc? 2.4
(cid:2) How do the various parts link together to make
the controls work? 3.1
(cid:2) Are they reliable? 3.2
(cid:2) How long will they last? 3.3
(cid:2) What does all this jargon mean? 4
(cid:2) What can I control? 5
(cid:2) What do I want to control? 5
(cid:2) What are the choices? 6
(cid:2) How do I develop a control strategy? 6.1
(cid:2) What can’t control systems do? 6.2
(cid:2) What affects system performance? 6.2
(cid:2) How do I decide what is needed? 6.3
(cid:2) What questions do I need to ask? 7
(cid:2) What information do I need to provide? 7
(cid:2) What do I need to know about commissioning and
maintenance? 7
2 CIBSEKnowledge Series — Understanding controls
2 Controls and control systems
Case study
(with acknowledgement to AEC and
Good controls are essential for the safe and efficient operation of modern Johnson Controls)
buildings. Well designed and operated control systems can:
— create and maintain a comfortable indoor environment
Barclays has achieved energy savings of over
4.2 megawatts per year, reduced costs by
— prevent systems from being on when not needed, and prevent heating
over £50000 annually and cut CO
2
and cooling operating simultaneously for the same space emissions by over 1000 tonnes, by
identifying and remedying inefficiencies in
— keep HVAC plant operating safely and efficiently the current control strategy for its
corporate offices in Poole and its
— reduce energy consumption and running costs Gloucester-based data centre.
Barclays House, Poole is a typical eight
— prevent energy wastage
storey 1970s building, providing both
heating and cooling with14 modular
— help meet Building Regulation requirements for energy efficiency and
boilers and two banks of chillers.
conservation
The main plant control strategy was
flawed, meaning that heating and cooling
— provide feedback data that can, for example, be used for monitoring
were sometimes occurring
system performance and planning maintenance requirements. simultaneously for the same zone.
The new building management system
has enabled substantial savings by
The reduction in energy usage delivered by well designed and operated
correctly demand-sequencing the main
controls systems will also cut the greenhouse gas emissions that are driving plant and ensuring that heating does not
fight chilling, as well as by ensuring that
climate change. Studies have shown that even well-managed buildings waste
the compensation strategy for provision
up to 15% of the energy they purchase and, typically, businesses could cut of heating and cooling, both on the main
headers and on relevant zones, was
their energy consumption by a quarter.
correct.
This has delivered a 20% saving on the
Controls are needed for all the various services and systems within the boiler gas consumption and a 10%
reduction of the total site electrical
building:
consumption.
— heating
— cooling
— ventilation
— lighting
— building electrical services
— fire fighting and alarm systems
— security systems
— transport systems
— water supply systems.
Any system that uses energy, and/or provides functional requirements such as
security, will need control. However there is often an order of priority for
controls, with safety-critical systems taking precedence, followed by those
systems that consume the most energy — usually space heating/cooling and
lighting. Whilst energy for heating is often provided by gas, cooling and
lighting require electricity and are often the largest consumers of electricity in
a building (see Figure 1).
CIBSEKnowledge Series — Understanding controls 3
Figure 1: Relative annual delivered energy Relative annual energy costs for
consumption for typical standard typical standard air conditioned office
Energy use in offices
air conditioned office
Computer room
Other electricity Computer room
Other electricity
Office
equipment Heating
Office
equipment
Heating and Cooling
Lighting hot water
(gas/oil)
Fans, pumps,
Lighting
controls
Fans,
pumps,
controls
Building Regulations 2000
Humidification
Approved Document L 2002
Cooling Humidification
The building should be provided with
zone, timing and temperature controls
Based on data from Energy Consumption Guide ECG19: Energy use in offices
such that each functional area is
maintained at the required temperature
only during the period when it is
occupied.
Where it is practical, the aim of lighting
Building Regulations 2000 Approved Document L (2002) specifically
controls should be to encourage the
maximum use of daylight and to avoid the addresses the issue of energy efficiency and requires heating, hot water,
unnecessary use of lighting when spaces
mechanical ventilation, air conditioning and lighting systems to ‘use no more
are unoccupied.
energy than is reasonable in the circumstances’. This requires the use of
(Forthcoming revision to the Regulations
are unlikely to change these general controls so that energy can be used efficiently. Forthcoming revisions to the
requirements.)
Building Regulations to incorporate some requirements of the European
Energy Performance of Buildings Directive (EPBD) will place even more
emphasis on demonstrating energy efficiency.
2.1 What do control systems do?
Controls are needed to ensure safe operation and to achieve the required
output from the building services systems, and as such they act to achieve
Figure 2:
and then maintain a specific condition, for example a temperature or lighting
The principle of controls
level. They work by measuring and adjusting specific variables within systems
such as temperature, velocity (speed), flow rate, pressure and electrical
System input
resistance. These will then alter the system output to enable the required
occupant conditions to be met (Figure 2).
Controls
Controls can be simple, for example a domestic plug-in timer, with metal pins
Feedback
Output that can be moved to set the on and off times for a security light, or a room
thermostat for central heating, or a light dimmer; or they can be complex, for
Maintained example an integrated control system that combines the control of HVAC
condition
systems with lighting, fire and security systems via a common interface
(management and display panel).
4 CIBSEKnowledge Series — Understanding controls
However, in all cases the two fundamental main functions of building control
systems are simply:
— to switch equipment on and off
— to adjust the output of equipment to maintain the required operating
conditions.
A simple example is the domestic heating system (Figure 3) which requires a
time switch (time clock) to turn the boiler and pump on and off, and a
thermostat to sense the space temperature and call for heat when the space
temperature is lower than the required temperature. The water temperature
from the boiler is normally preset and the space thermostat operates to turn
the boiler on and off to maintain the required temperature, with the pump
running continuously during the ‘on’ periods of the time clock. The boiler
should not start unless the pump is running. In some systems the pump runs
intermittently, with the space thermostat switching both the boiler and the
pump. The pump may also need to run for a period after the time switch (or
space thermostat) calls for shutdown in order to dissipate residual heat from
the boiler.
Figure 3:
Time Turns boiler and pump
clock on/off at preset times Thermostat Simple controls for
Radiators Senses room temperature domestic heating system
and turns boiler on and
off to maintain required
temperature
Boiler
(with inbuilt
controls to
regulate water Pump
temperature)
Even for domestic systems, more complex controls are available — from
programmable time controllers and programmable electronic thermostats to
individual thermostatic radiator valves, but all fundamentally fulfil the same
functions of on/off or modulating (varying) the output. The same applies with
complex controls for large buildings — there are just more systems and
system components to control, some of which need to be interlinked — for
example heating and cooling, so that they do not operate simultaneously for
the same space.
CIBSEKnowledge Series — Understanding controls 5
2.2 What are controls and control systems?
The two terms ‘controls’ and ‘control systems’ are often used
interchangeably, but, although for most practical purposes they mean the
same thing, strictly speaking there are some minor differences.
Figure 4: Controls/building controls
A control system
‘Controls’ is a more generic term, used for a collection of individual control
Sensor elements, from sensors, valves and timers to control panels. It is often also
used to mean a control system, although this does have a more specific
definition (see below).
Controller
Control system/building control system
A control system is the system that performs the control function. In its
Controlled device
simplest format, it comprises a sensor, a controller and the controlled device,
(Figure 4) which all work to provide control for the controlled load or
process — such as heating, cooling, lighting etc. A common example of this is
a temperature sensor, which operates a controller to control the heat input
to a heater (controlled load or process) by altering the flow through a control
valve (controlled device) (Figure 5).
Figure 5:
Temperature Controller Control valve Heating coil
Common control system sensor
Control system
A very simple example is a domestic thermostatic radiator valve (TRV)
(Figure 6), which contains the sensor (to sense room temperature), the
controller, and the controlled device (the valve that opens or shuts to allow
more or less hot water through the radiator). The controlled load or process
is the heat delivered by the hot water in the radiator, i.e. the radiator output.
Figure 6:
TRV Radiator
Simple radiator control system
Control system
(Sensor/controller/control valve)
Another example is lighting control linked to occupancy presence, often used
for an occasionally occupied space. A passive infra-red (PIR) sensor detects
6 CIBSEKnowledge Series — Understanding controls