Honeywell AUTOMATIC CONTROL Engineering Manual

HONEYWELL

ENGINEERING MANUAL of

AUTOMATIC
CONTROL

for

COMMERCIAL BUILDINGS

ENGINEERING MANUAL OF AUTOMATIC CONTROL

i

Copyright 1934, 1940, 1953, 1988, 1991 and 1997 by Honeywell Inc.

All rights reserved. This manual or portions thereof may not be reporduced

in any form without permission of Honeywell Inc.

Library of Congress Catalog Card Number: 97-72971

Home and Building Control

Honeywell Inc.
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P.O. Box 524
Minneapolis MN 55408-0524

Honeywell Latin American Region

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Home and Building Control

Honeywell Limited-Honeywell Limitée
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North York, Ontario
M2H 3N7

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1140 Brussels
Belgium

Printed in USA

ii

Honeywell Asia Pacific Inc.

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ENGINEERING MANUAL OF AUTOMATIC CONTROL

FOREWORD

The Minneapolis Honeywell Regulator Company published the first edition of the Engineering Manual of
Automatic Control in l934. The manual quickly became the standard textbook for the commercial building
controls industry. Subsequent editions have enjoyed even greater success in colleges, universities, and contractor
and consulting engineering offices throughout the world.

Since the original 1934 edition, the building control industry has experienced dramatic change and made
tremendous advances in equipment, system design, and application. In this edition, microprocessor controls are
shown in most of the control applications rather than pneumatic, electric, or electronic to reflect the trends in
industry today. Consideration of configuration, functionality, and integration plays a significant role in the
design of building control systems.

Through the years Honeywell has been dedicated to assisting consulting engineers and architects in the
application of automatic controls to heating, ventilating, and air conditioning systems. This manual is an outgrowth
of that dedication. Our end user customers, the building owners and operators, will ultimately benefit from the
efficiently designed systems resulting from the contents of this manual.

All of this manual’s original sections have been updated and enhanced to include the latest developments in
control technology. A new section has been added on indoor air quality and information on district heating has
been added to the Chiller, Boiler, and Distribution System Control Applications Section.

This twenty-first edition of the Engineering Manual of Automatic Control is our contribution to ensure that
we continue to satisfy our customer’s requirements. The contributions and encouragement received from previous
users are gratefully acknowledged. Further suggestions will be most welcome.

Minneapolis, Minnesota
October, 1997

KEVIN GILLIGAN
President, H&BC Solutions and Services

ENGINEERING MANUAL OF AUTOMATIC CONTROL

iii

iv

ENGINEERING MANUAL OF AUTOMATIC CONTROL

PREFACE

The purpose of this manual is to provide the reader with a fundamental understanding of controls and how
they are applied to the many parts of heating, ventilating, and air conditioning systems in commercial buildings.

Many aspects of control are presented including air handling units, terminal units, chillers, boilers, building
airflow, water and steam distribution systems, smoke management, and indoor air quality. Control fundamentals,
theory, and types of controls provide background for application of controls to heating, ventilating, and air
conditioning systems. Discussions of pneumatic, electric, electronic, and digital controls illustrate that applications
may use one or more of several different control methods. Engineering data such as equipment sizing, use of
psychrometric charts, and conversion formulas supplement and support the control information. To enhance
understanding, definitions of terms are provided within individual sections. For maximum usability, each section
of this manual is available as a separate, self-contained document.

Building management systems have evolved into a major consideration for the control engineer when evaluating
a total heating, ventilating, and air conditioning system design. In response to this consideration, the basics of
building management systems configuration are presented.

The control recommendations in this manual are general in nature and are not the basis for any specific job or
installation. Control systems are furnished according to the plans and specifications prepared by the control
engineer. In many instances there is more than one control solution. Professional expertise and judgment are
required for the design of a control system. This manual is not a substitute for such expertise and judgment.
Always consult a licensed engineer for advice on designing control systems.

It is hoped that the scope of information in this manual will provide the readers with the tools to expand their
knowledge base and help develop sound approaches to automatic control.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

v

vi

ENGINEERING MANUAL OF AUTOMATIC CONTROL

ENGINEERING MANUAL of

AUTOMATIC
CONTROL

CONTENTS

Foreward ............................................................................................................................................................. iii

Preface ................................................................................................................................................................ v

Control System Fundamentals ............................................................................................ 1

Control Fundamentals ....................................................................................................................................... 3

Introduction.......................................................................................... 5

Definitions............................................................................................ 5

HVAC System Characteristics ............................................................. 8

Control System Characteristics ........................................................... 15

Control System Components .............................................................. 30

Characteristics And Attributes Of Control Methods............................. 35

Psychrometric Chart Fundamentals ................................................................................................................ 37

Introduction.......................................................................................... 38

Definitions............................................................................................ 38

Description of the Psychrometric Chart............................................... 39

The Abridged Psychrometric Chart ..................................................... 40

Examples of Air Mixing Process .......................................................... 42

Air Conditioning Processes ................................................................. 43

Humidifying Process............................................................................ 44

ASHRAE Psychrometric Chart ............................................................ 53

Pneumatic Control Fundamentals .................................................................................................................... 57

Introduction.......................................................................................... 59

Definitions............................................................................................ 59

Abbreviations ....................................................................................... 60

Symbols............................................................................................... 61

Basic Pneumatic Control System ........................................................ 61

Air Supply Equipment .......................................................................... 65

Thermostats ........................................................................................ 69

Controllers ........................................................................................... 70

Sensor-Controller Systems ................................................................. 72

Actuators and Final Control Elements ................................................. 74

Relays and Switches ........................................................................... 77

Pneumatic Control Combinations ........................................................ 84

Pneumatic Centeralization .................................................................. 89

Pneumatic Control System Example ................................................... 90

Electric Control Fundamentals ......................................................................................................................... 95

Introduction.......................................................................................... 97

Definitions............................................................................................ 97

How Electric Control Circuits Classified .............................................. 99

Series 40 Control Circuits.................................................................... 100

Series 80 Control Circuits.................................................................... 102

Series 60 Two-Position Control Circuits............................................... 103

Series 60 Floating Control Circuits ...................................................... 106

Series 90 Control Circuits.................................................................... 107

Motor Control Circuits.......................................................................... 114

ENGINEERING MANUAL OF AUTOMATIC CONTROL

vii

Electronic Control Fundamentals..................................................................................................................... 119

Introduction.......................................................................................... 120

Definitions............................................................................................ 120

Typical System .................................................................................... 122

Components ........................................................................................ 122

Electtonic Controller Fundamentals .................................................... 129

Typical System Application .................................................................. 130

Microprocessor-Based/DDC Fundamentals .................................................................................................... 131

Introduction.......................................................................................... 133

Definitions............................................................................................ 133

Background ......................................................................................... 134

Advantages ......................................................................................... 134

Controller Configuration ...................................................................... 135

Types of Controllers............................................................................. 136

Controller Software.............................................................................. 137

Controller Programming ...................................................................... 142

Typical Applications ............................................................................. 145

Indoor Air Quality Fundamentals ..................................................................................................................... 149

Introduction.......................................................................................... 151

Definitions............................................................................................ 151

Abbreviations ....................................................................................... 153

Indoor Air Quality Concerns ................................................................ 154

Indoor Air Quality Control Applications................................................ 164

Bibliography ......................................................................................... 170

Smoke Management Fundamentals ................................................................................................................. 171

Introduction.......................................................................................... 172

Definitions............................................................................................ 172

Objectives............................................................................................ 173

Design Considerations ........................................................................ 173

Design Principles ................................................................................ 175

Control Applications ............................................................................ 178

Acceptance Testing ............................................................................. 181

Leakage Rated Dampers .................................................................... 181

Bibliography ......................................................................................... 182

Building Management System Fundamentals................................................................................................. 183

Introduction.......................................................................................... 184

Definitions............................................................................................ 184

Background ......................................................................................... 185

System Configurations ........................................................................ 186

System Functions ................................................................................ 189

Integration of Other Systems............................................................... 197

viii

ENGINEERING MANUAL OF AUTOMATIC CONTROL

Control System Applications ............................................................................................... 199

Air Handling System Control Applications...................................................................................................... 201

Introduction.......................................................................................... 203

Abbreviations ....................................................................................... 203

Requirements For Effective Control .................................................... 204

Applications-General ........................................................................... 206

Valve and Damper Selection ............................................................... 207

Symbols............................................................................................... 208

Ventilation Control Processes ............................................................. 209

Fixed Quantity of Outdoor Air Control ................................................. 211

Heating Control Processes.................................................................. 223

Preheat Control Processes ................................................................. 228

Humidification Control Process ........................................................... 235

Cooling Control Processes .................................................................. 236

Dehumidification Control Processes ................................................... 243

Heating System Control process ......................................................... 246

Year-Round System Control processes .............................................. 248

ASHRAE Psychrometric Charts .......................................................... 261

Building Airflow System Control Applications ............................................................................................... 263

Introduction.......................................................................................... 265

Definitions............................................................................................ 265

Airflow Control Fundamentals ............................................................. 267

Airflow Control Applications................................................................. 281

References .......................................................................................... 292

Chiller, Boiler, and Distribution System Control Applications ....................................................................... 293

Introduction.......................................................................................... 297

Abbreviations....................................................................................... 297

Definitions............................................................................................ 297

Symbols............................................................................................... 298

Chiller System Control......................................................................... 299

Boiler System Control.......................................................................... 329

Hot And Chilled Water Distribution Systems Control........................... 337

High Temperature Water Heating System Control .............................. 376

District Heating Applications................................................................ 382

Individual Room Control Applications ............................................................................................................ 399

Introduction.......................................................................................... 401

Unitary Equipment Control .................................................................. 412

Hot Water Plant Considerations .......................................................... 428

ENGINEERING MANUAL OF AUTOMATIC CONTROL

ix

Engineering Information....................................................................................................... 429

Valve Selection and Sizing ................................................................................................................................ 431

Introduction.......................................................................................... 432

Definitions............................................................................................ 432

Valve Selection .................................................................................... 436

Valve Sizing ......................................................................................... 441

Damper Selection and Sizing ............................................................................................................................ 451

Introduction.......................................................................................... 453

Definitions............................................................................................ 453

Damper Selection ................................................................................ 454

Damper Sizing ..................................................................................... 463

Damper Pressure Drop ....................................................................... 468

Damper Applications ........................................................................... 469

General Engineering Data ................................................................................................................................. 471

Introduction.......................................................................................... 472

Weather Data ...................................................................................... 472

Conversion Formulas And Tables........................................................ 475

Electrical Data ..................................................................................... 482

Properties Of Saturated Steam Data................................................... 488

Airflow Data ......................................................................................... 489

Moisture Content Of Air Data .............................................................. 491

Index ....................................................................................................................................... 494

x

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL

CONTROL FUNDAMENTALS

SYSTEMS

FUNDMENTALS

1

ENGINEERING MANUAL OF AUTOMATIC CONTROL

Control

Fundamentals

Contents

Introduction ............................................................................................................ 5

Definitions ............................................................................................................ 5

HVAC System Characteristics ............................................................................................................ 8

General................................................................................................ 8

Heating ................................................................................................ 9

General................................................................................................ 9

Heating Equipment .............................................................................. 10

Cooling ................................................................................................ 11

General................................................................................................ 11

Cooling Equipment .............................................................................. 12

Dehumidification .................................................................................. 12

Humidification ...................................................................................... 13

Ventilation............................................................................................ 13

Filtration............................................................................................... 14

Control System Characteristics ............................................................................................................ 15

Controlled Variables ............................................................................ 15

Control Loop ........................................................................................ 15

Control Methods .................................................................................. 16

General ........................................................................................... 16

Analog And Digital Control .............................................................. 16

Control Modes ..................................................................................... 17

Two-Position Control....................................................................... 17

General ....................................................................................... 17

Basic Two-Position Control......................................................... 17

Timed Two-Position Control ........................................................ 18

Step Control .................................................................................... 19

Floating Control............................................................................... 20

Proportional Control ........................................................................ 21

General ....................................................................................... 21

Compensation Control ................................................................ 22

Proportional-Integral (Pi) Control .................................................... 23

Proportional-Integral-Derivative (Pid) Control ................................. 25

Enhanced Proportional-Integral-Derivative (epid) Control .............. 25

Adaptive Control ............................................................................. 26

Process Characteristics....................................................................... 26

CONTROL FUNDAMENTALS

Load ................................................................................................ 26

Lag .................................................................................................. 27

General ........................................................................................... 27

Measurement Lag ........................................................................... 27

Capacitance .................................................................................... 28

Resistance ...................................................................................... 29

Dead Time....................................................................................... 29

Control Application Guidelines ............................................................ 29

Control System Components ............................................................................................................ 30

Sensing Elements ............................................................................... 30

Te mperature Sensing Elements...................................................... 30

Pressure Sensing Elements............................................................ 31

Moisture Sensing Elements ............................................................ 32

Flow Sensors .................................................................................. 32

Proof-Of-Operation Sensors ........................................................... 33

Transducers ........................................................................................ 33

Controllers ........................................................................................... 33

Actuators ............................................................................................. 33

Auxiliary Equipment............................................................................. 34

Characteristics And Attributes Of Control Methods .............................................................................................. 35

ENGINEERING MANUAL OF AUTOMATIC CONTROL

4

INTRODUCTION

CONTROL FUNDAMENTALS

This section describes heating, ventilating, and air
conditioning (HVAC) systems and discusses characteristics and
components of automatic control systems. Cross-references
are made to sections that provide more detailed information.

A correctly designed HVAC control system can provide a
comfortable environment for occupants, optimize energy cost
and consumption, improve employee productivity, facilitate
efficient manufacturing, control smoke in the event of a fire,
and support the operation of computer and telecommunications
equipment. Controls are essential to the proper operation of
the system and should be considered as early in the design
process as possible.

Properly applied automatic controls ensure that a correctly
designed HVAC system will maintain a comfortable
environment and perform economically under a wide range of
operating conditions. Automatic controls regulate HVAC
system output in response to varying indoor and outdoor
conditions to maintain general comfort conditions in office
areas and provide narrow temperature and humidity limits
where required in production areas for product quality.

DEFINITIONS

The following terms are used in this manual. Figure 1 at the
end of this list illustrates a typical control loop with the
components identified using terms from this list.

Analog: Continuously variable (e.g., a faucet controlling water

from off to full flow).

Automatic control system: A system that reacts to a change

or imbalance in the variable it controls by adjusting
other variables to restore the system to the desired
balance.

Automatic controls can optimize HVAC system operation.
They can adjust temperatures and pressures automatically to
reduce demand when spaces are unoccupied and regulate
heating and cooling to provide comfort conditions while
limiting energy usage. Limit controls ensure safe operation of
HVAC system equipment and prevent injury to personnel and
damage to the system. Examples of limit controls are low­limit temperature controllers which help prevent water coils
or heat exchangers from freezing and flow sensors for safe
operation of some equipment (e.g., chillers). In the event of a
fire, controlled air distribution can provide smoke-free
evacuation passages, and smoke detection in ducts can close
dampers to prevent the spread of smoke and toxic gases.

HVAC control systems can also be integrated with security
access control systems, fire alarm systems, lighting control
systems, and building and facility management systems to
further optimize building comfort, safety, and efficiency.

Controlled medium: The medium in which the controlled

variable exists. In a space temperature control system,
the controlled variable is the space temperature and
the controlled medium is the air within the space.

Controlled Variable: The quantity or condition that is

measured and controlled.

Controller: A device that senses changes in the controlled

variable (or receives input from a remote sensor) and
derives the proper correction output.

Algorithm: A calculation method that produces a control

output by operating on an error signal or a time series
of error signals.

Compensation control: A process of automatically adjusting

the setpoint of a given controller to compensate for
changes in a second measured variable (e.g., outdoor
air temperature). For example, the hot deck setpoint
is normally reset upward as the outdoor air
temperature decreases. Also called “reset control”.

Control agent: The medium in which the manipulated variable

exists. In a steam heating system, the control agent is
the steam and the manipulated variable is the flow of
the steam.

Control point: The actual value of the controlled variable

(setpoint plus or minus offset).

Corrective action: Control action that results in a change of

the manipulated variable. Initiated when the
controlled variable deviates from setpoint.

Cycle: One complete execution of a repeatable process. In

basic heating operation, a cycle comprises one on
period and one off period in a two-position control
system.

Cycling: A periodic change in the controlled variable from

one value to another. Out-of-control analog cycling
is called “hunting”. Too frequent on-off cycling is
called “short cycling”. Short cycling can harm electric
motors, fans, and compressors.

Cycling rate: The number of cycles completed per time unit,

typically cycles per hour for a heating or cooling
system. The inverse of the length of the period of the
cycle.

5

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

Deadband: A range of the controlled variable in which no

corrective action is taken by the controlled system
and no energy is used. See also “zero energy band”.

related. Also, the delay in response of the sensing
element of a control due to the time required for the
sensing element to sense a change in the sensed
variable.

Deviation: The difference between the setpoint and the value

of the controlled variable at any moment. Also called
“offset”.

DDC: Direct Digital Control. See also Digital and Digital

control.

Digital: A series of on and off pulses arranged to convey

information. Morse code is an early example.
Processors (computers) operate using digital
language.

Digital control: A control loop in which a microprocessor-

based controller directly controls equipment based
on sensor inputs and setpoint parameters. The
programmed control sequence determines the output
to the equipment.

Droop: A sustained deviation between the control point and

the setpoint in a two-position control system caused
by a change in the heating or cooling load.

Enhanced proportional-integral-derivative (EPID) control:

A control algorithm that enhances the standard PID
algorithm by allowing the designer to enter a startup
output value and error ramp duration in addition to
the gains and setpoints. These additional parameters
are configured so that at startup the PID output varies
smoothly to the control point with negligible
overshoot or undershoot.

Electric control: A control circuit that operates on line or low

voltage and uses a mechanical means, such as a
temperature-sensitive bimetal or bellows, to perform
control functions, such as actuating a switch or
positioning a potentiometer. The controller signal
usually operates or positions an electric actuator or
may switch an electrical load directly or through a
relay.

Load: In a heating or cooling system, the heat transfer that

the system will be called upon to provide. Also, the
work that the system must perform.

Manipulated variable: The quantity or condition regulated

by the automatic control system to cause the desired
change in the controlled variable.

Measured variable: A variable that is measured and may be

controlled (e.g., discharge air is measured and
controlled, outdoor air is only measured).

Microprocessor-based control: A control circuit that operates

on low voltage and uses a microprocessor to perform
logic and control functions, such as operating a relay
or providing an output signal to position an actuator.
Electronic devices are primarily used as sensors. The
controller often furnishes flexible DDC and energy
management control routines.

Modulating: An action that adjusts by minute increments and

decrements.

Offset: A sustained deviation between the control point and

the setpoint of a proportional control system under
stable operating conditions.

On/off control: A simple two-position control system in which

the device being controlled is either full on or full off
with no intermediate operating positions available.
Also called “two-position control”.

Pneumatic control: A control circuit that operates on air

pressure and uses a mechanical means, such as a
temperature-sensitive bimetal or bellows, to perform
control functions, such as actuating a nozzle and
flapper or a switching relay. The controller output
usually operates or positions a pneumatic actuator,
although relays and switches are often in the circuit.

Electronic control: A control circuit that operates on low

voltage and uses solid-state components to amplify
input signals and perform control functions, such as
operating a relay or providing an output signal to
position an actuator. The controller usually furnishes
fixed control routines based on the logic of the solid­state components.

Final control element: A device such as a valve or damper

that acts to change the value of the manipulated
variable. Positioned by an actuator.

Hunting: See Cycling.

Lag: A delay in the effect of a changed condition at one point

in the system, or some other condition to which it is

ENGINEERING MANUAL OF AUTOMATIC CONTROL

Process: A general term that describes a change in a measurable

variable (e.g., the mixing of return and outdoor air
streams in a mixed-air control loop and heat transfer
between cold water and hot air in a cooling coil).
Usually considered separately from the sensing
element, control element, and controller.

Proportional band: In a proportional controller, the control

point range through which the controlled variable
must pass to move the final control element through
its full operating range. Expressed in percent of
primary sensor span. Commonly used equivalents are
“throttling range” and “modulating range”, usually
expressed in a quantity of engineering units (degrees
of temperature).

6

Proportional control: A control algorithm or method in which

the final control element moves to a position
proportional to the deviation of the value of the
controlled variable from the setpoint.

Proportional-Integral (PI) control: A control algorithm that

combines the proportional (proportional response)
and integral (reset response) control algorithms. Reset
response tends to correct the offset resulting from
proportional control. Also called “proportional-plus­reset” or “two-mode” control.

Proportional-Integral-Derivative (PID) control: A control

algorithm that enhances the PI control algorithm by
adding a component that is proportional to the rate of
change (derivative) of the deviation of the controlled
variable. Compensates for system dynamics and
allows faster control response. Also called “three­mode” or “rate-reset” control.

Reset Control: See Compensation control.

Sensing element: A device or component that measures the

value of a variable.

CONTROL FUNDAMENTALS

Step control: Control method in which a multiple-switch

assembly sequentially switches equipment (e.g.,
electric heat, multiple chillers) as the controller input
varies through the proportional band. Step controllers
may be actuator driven, electronic, or directly
activated by the sensed medium (e.g., pressure,
temperature).

Throttling range: In a proportional controller, the control point

range through which the controlled variable must pass
to move the final control element through its full
operating range. Expressed in values of the controlled
variable (e.g., degrees Fahrenheit, percent relative
humidity, pounds per square inch). Also called
“proportional band”. In a proportional room
thermostat, the temperature change required to drive
the manipulated variable from full off to full on.

Time constant: The time required for a dynamic component,

such as a sensor, or a control system to reach 63.2
percent of the total response to an instantaneous (or
“step”) change to its input. Typically used to judge
the responsiveness of the component or system.

Two-position control: See on/off control.

Setpoint: The value at which the controller is set (e.g., the

desired room temperature set on a thermostat). The
desired control point.

Short cycling: See Cycling.

MEASURED

VARIABLE

OUTDOOR

AIR

30

OUTDOOR

AIR

CONTROLLED

MEASURED

VARIABLE

MEDIUM

RESET SCHEDULE

60

0

OA

TEMPERATURE

CONTROLLED

VARIABLE

HOT WATER

SUPPLY

148

130

190

HW

SETPOINT

CONTROL

POINT

HOT WATER

SUPPLY

TEMPERATURE

Zero energy band: An energy conservation technique that

allows temperatures to float between selected settings,
thereby preventing the consumption of heating or
cooling energy while the temperature is in this range.

Zoning: The practice of dividing a building into sections for

heating and cooling control so that one controller is
sufficient to determine the heating and cooling

ALGORITHM IN

CONTROLLER

SETPOINT

160

SETPOINT

INPUT

OUTPUT

PERCENT

159

OPEN

VALVE

41

FLOW

FINAL CONTROL
ELEMENT

STEAM

MANIPULATED
VARIABLE

CONTROL
AGENT

HOT WATER

RETURN

requirements for the section.

AUTO

Fig. 1. Typical Control Loop.

7

M10510

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

HVAC SYSTEM CHARACTERISTICS

GENERAL

An HVAC system is designed according to capacity
requirements, an acceptable combination of first cost and
operating costs, system reliability, and available equipment
space.

DAMPER

AIR
FILTER

COOLING
COIL

FAN

COOLING

TOWER

Figure 2 shows how an HVAC system may be distributed in
a small commercial building. The system control panel, boilers,
motors, pumps, and chillers are often located on the lower level.
The cooling tower is typically located on the roof. Throughout
the building are ductwork, fans, dampers, coils, air filters,
heating units, and variable air volume (VAV) units and
diffusers. Larger buildings often have separate systems for
groups of floors or areas of the building.

DUCTWORK

HEATING

UNIT

VAV BOX

DIFFUSER

CHILLER

PUMP

Fig. 2. Typical HVAC System in a Small Building.

The control system for a commercial building comprises
many control loops and can be divided into central system and
local- or zone-control loops. For maximum comfort and
efficiency, all control loops should be tied together to share
information and system commands using a building
management system. Refer to the Building Management
System Fundamentals section of this manual.

BOILER

CONTROL
PANEL

M10506

The basic control loops in a central air handling system can

be classified as shown in Table 1.

Depending on the system, other controls may be required
for optimum performance. Local or zone controls depend on
the type of terminal units used.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

8

CONTROL FUNDAMENTALS

r

Ta ble 1. Functions of Central HVAC Control Loops.

Control

Loop Classification Description

Ventilation Basic Coordinates operation of the outdoor, return, and exhaust air dampers to maintain

the proper amount of ventilation air. Low-temperature protection is often required.

Better Measures and controls the volume of outdoor air to provide the proper mix of

outdoor and return air under varying indoor conditions (essential in variable air
volume systems). Low-temperature protection may be required.

Cooling Chiller control Maintains chiller discharge water at preset temperature or resets temperature

according to demand.

Cooling tower
control

Controls cooling tower fans to provide the coolest water practical under existing

wet bulb temperature conditions.
Water coil control Adjusts chilled water flow to maintain temperature.
Direct expansion

(DX) system
control

Cycles compressor or DX coil solenoid valves to maintain temperature. If

compressor is unloading type, cylinders are unloaded as required to maintain

temperature.

Fan Basic Turns on supply and return fans during occupied periods and cycles them as

required during unoccupied periods.
Better Adjusts fan volumes to maintain proper duct and space pressures. Reduces system

operating cost and improves performance (essential for variable air volume

systems).

Heating Coil control Adjusts water or steam flow or electric heat to maintain temperature.

Boiler control Operates burner to maintain proper discharge steam pressure or water temperature.

For maximum efficiency in a hot water system, water temperature should be reset

as a function of demand or outdoor temperature.

HEATING

GENERAL

Building heat loss occurs mainly through transmission,

infiltration/exfiltration, and ventilation (Fig. 3).

TRANSMISSION

VENTILATION DUCT

EXFILTRATION

DOOR

Fig. 3. Heat Loss from a Building.

The heating capacity required for a building depends on the
design temperature, the quantity of outdoor air used, and the
physical activity of the occupants. Prevailing winds affect the
rate of heat loss and the degree of infiltration. The heating
system must be sized to heat the building at the coldest outdoor
temperature the building is likely to experience (outdoor design
temperature).

ROOF

70°F

20°F

WINDOW

PREVAILING
WINDS

INFILTRATION

C2701

Transmission is the process by which energy enters or leaves
a space through exterior surfaces. The rate of energy
transmission is calculated by subtracting the outdoor
temperature from the indoor temperature and multiplying the
result by the heat transfer coefficient of the surface materials.
The rate of transmission varies with the thickness and
construction of the exterior surfaces but is calculated the same
way for all exterior surfaces:

Energy Transmission per
Unit Area and Unit Time = (TIN - T

) x HTC

OUT

Where:

TIN=indoor temperature

T

= outdoor temperature

OUT

HTC = heat transfer coefficient

HTC

=

Unit Time x Unit Area x Unit Temperatu

Btu

9

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

Infiltration is the process by which outdoor air enters a
building through walls, cracks around doors and windows, and
open doors due to the difference between indoor and outdoor
air pressures. The pressure differential is the result of
temperature difference and air intake or exhaust caused by fan
operation. Heat loss due to infiltration is a function of
temperature difference and volume of air moved. Exfiltration
is the process by which air leaves a building (e.g., through
walls and cracks around doors and windows) and carries heat
with it. Infiltration and exfiltration can occur at the same time.

Ventilation brings in fresh outdoor air that may require
heating. As with heat loss from infiltration and exfiltration,
heat loss from ventilation is a function of the temperature
difference and the volume of air brought into the building or
exhausted.

HEATING EQUIPMENT

Selecting the proper heating equipment depends on many
factors, including cost and availability of fuels, building size
and use, climate, and initial and operating cost trade-offs.
Primary sources of heat include gas, oil, wood, coal, electrical,
and solar energy. Sometimes a combination of sources is most
economical. Boilers are typically fueled by gas and may have
the option of switching to oil during periods of high demand.
Solar heat can be used as an alternate or supplementary source
with any type of fuel.

STEAM OR
HOT WATER
SUPPLY

FAN

COIL

UNIT HEATER

STEAM TRAP
(IF STEAM SUPPLY)

CONDENSATE
OR HOT WATER
RETURN

Fig. 5. Typical Unit Heater.

GRID PANEL

SERPENTINE PANEL

C2703

HOT WATER
SUPPLY

HOT WATER
RETURN

HOT WATER
SUPPLY

HOT WATER
RETURN

C2704

Figure 4 shows an air handling system with a hot water coil.
A similar control scheme would apply to a steam coil. If steam
or hot water is chosen to distribute the heat energy, high­efficiency boilers may be used to reduce life-cycle cost. Water
generally is used more often than steam to transmit heat energy
from the boiler to the coils or terminal units, because water
requires fewer safety measures and is typically more efficient,
especially in mild climates.

THERMOSTAT

HOT WATER
SUPPLY

FAN

VALVE

HOT WATER
RETURN

DISCHARGE
AIR

C2702

Fig. 4. System Using Heating Coil.

An air handling system provides heat by moving an air
stream across a coil containing a heating medium, across an
electric heating coil, or through a furnace. Unit heaters (Fig.

5) are typically used in shops, storage areas, stairwells, and
docks. Panel heaters (Fig. 6) are typically used for heating
floors and are usually installed in a slab or floor structure, but
may be installed in a wall or ceiling.

Fig. 6. Panel Heaters.

Unit ventilators (Fig. 7) are used in classrooms and may
include both a heating and a cooling coil. Convection heaters
(Fig. 8) are used for perimeter heating and in entries and
corridors. Infrared heaters (Fig. 9) are typically used for spot
heating in large areas (e.g., aircraft hangers, stadiums).

FAN

DISCHARGE
AIR

COOLING
COIL

RETURN
AIR

C3035

WALL

OUTDOOR
AIR

HEATING
COIL

DRAIN PAN

MIXING

DAMPERS

Fig. 7. Unit Ventilator.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

10

CONTROL FUNDAMENTALS

FINNED TUBE

WARM AIR

RETURN AIR

SUPPLY

RETURN

FLOOR

TO OTHER
HEATING UNITS

FROM OTHER
HEATING UNITS

C2705

Fig. 8. Convection Heater.

REFLECTOR

INFRARED
SOURCE

RADIANT HEAT

C2706

Fig. 9. Infrared Heater.

In mild climates, heat can be provided by a coil in the central
air handling system or by a heat pump. Heat pumps have the
advantage of switching between heating and cooling modes
as required. Rooftop units provide packaged heating and
cooling. Heating in a rooftop unit is usually by a gas- or oil­fired furnace or an electric heat coil. Steam and hot water coils
are available as well. Perimeter heat is often required in colder
climates, particularly under large windows.

A heat pump uses standard refrigeration components and a
reversing valve to provide both heating and cooling within the
same unit. In the heating mode, the flow of refrigerant through
the coils is reversed to deliver heat from a heat source to the
conditioned space. When a heat pump is used to exchange heat
from the interior of a building to the perimeter, no additional
heat source is needed.

A heat-recovery system is often used in buildings where a
significant quantity of outdoor air is used. Several types of
heat-recovery systems are available including heat pumps,
runaround systems, rotary heat exchangers, and heat pipes.

In a runaround system, coils are installed in the outdoor air
supply duct and the exhaust air duct. A pump circulates the
medium (water or glycol) between the coils so that medium
heated by the exhaust air preheats the outdoor air entering the
system.

A rotary heat exchanger is a large wheel filled with metal
mesh. One half of the wheel is in the outdoor air intake and
the other half, in the exhaust air duct. As the wheel rotates, the
metal mesh absorbs heat from the exhaust air and dissipates it
in the intake air.

A heat pipe is a long, sealed, finned tube charged with a
refrigerant. The tube is tilted slightly with one end in the
outdoor air intake and the other end in the exhaust air. In a
heating application, the refrigerant vaporizes at the lower end

in the warm exhaust air, and the vapor rises toward the higher
end in the cool outdoor air, where it gives up the heat of
vaporization and condenses. A wick carries the liquid
refrigerant back to the warm end, where the cycle repeats. A
heat pipe requires no energy input. For cooling, the process is
reversed by tilting the pipe the other way.

Controls may be pneumatic, electric, electronic, digital, or
a combination. Satisfactory control can be achieved using
independent control loops on each system. Maximum operating
efficiency and comfort levels can be achieved with a control
system which adjusts the central system operation to the
demands of the zones. Such a system can save enough in
operating costs to pay for itself in a short time.

Controls for the air handling system and zones are
specifically designed for a building by the architect, engineer,
or team who designs the building. The controls are usually
installed at the job site. Terminal unit controls are typically
factory installed. Boilers, heat pumps, and rooftop units are
usually sold with a factory-installed control package
specifically designed for that unit.

COOLING

GENERAL

Both sensible and latent heat contribute to the cooling load
of a building. Heat gain is sensible when heat is added to the
conditioned space. Heat gain is latent when moisture is added
to the space (e.g., by vapor emitted by occupants and other
sources). To maintain a constant humidity ratio in the space,
water vapor must be removed at a rate equal to its rate of
addition into the space.

Conduction is the process by which heat moves between
adjoining spaces with unequal space temperatures. Heat may
move through exterior walls and the roof, or through floors,
walls, or ceilings. Solar radiation heats surfaces which then
transfer the heat to the surrounding air. Internal heat gain is
generated by occupants, lighting, and equipment. Warm air
entering a building by infiltration and through ventilation also
contributes to heat gain.

Building orientation, interior and exterior shading, the angle
of the sun, and prevailing winds affect the amount of solar
heat gain, which can be a major source of heat. Solar heat
received through windows causes immediate heat gain. Areas
with large windows may experience more solar gain in winter
than in summer. Building surfaces absorb solar energy, become
heated, and transfer the heat to interior air. The amount of
change in temperature through each layer of a composite
surface depends on the resistance to heat flow and thickness
of each material.

Occupants, lighting, equipment, and outdoor air ventilation
and infiltration requirements contribute to internal heat gain.
For example, an adult sitting at a desk produces about 400 Btu
per hour. Incandescent lighting produces more heat than
fluorescent lighting. Copiers, computers, and other office
machines also contribute significantly to internal heat gain.

11

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

COOLING EQUIPMENT

An air handling system cools by moving air across a coil
containing a cooling medium (e.g., chilled water or a
refrigerant). Figures 10 and 11 show air handling systems that
use a chilled water coil and a refrigeration evaporator (direct
expansion) coil, respectively. Chilled water control is usually
proportional, whereas control of an evaporator coil is two­position. In direct expansion systems having more than one
coil, a thermostat controls a solenoid valve for each coil and
the compressor is cycled by a refrigerant pressure control. This
type of system is called a “pump down” system. Pump down
may be used for systems having only one coil, but more often
the compressor is controlled directly by the thermostat.

CHILLED
WATER
SUPPLY

TEMPERATURE
CONTROLLER

CONTROL
VALV E

CHILLED
WATER
COIL

SENSOR

CHILLED
WATER
RETURN

COOL AIR

C2707-2

Fig. 10. System Using Cooling Coil.

TEMPERATURE
CONTROLLER

EVAPORATOR
COIL

SOLENOID
VALV E

D

X

COOL AIR

SENSOR

REFRIGERANT
LIQUID

Compressors for chilled water systems are usually
centrifugal, reciprocating, or screw type. The capacities of
centrifugal and screw-type compressors can be controlled by
varying the volume of refrigerant or controlling the compressor
speed. DX system compressors are usually reciprocating and,
in some systems, capacity can be controlled by unloading
cylinders. Absorption refrigeration systems, which use heat
energy directly to produce chilled water, are sometimes used
for large chilled water systems.

While heat pumps are usually direct expansion, a large heat
pump may be in the form of a chiller. Air is typically the heat
source and heat sink unless a large water reservoir (e.g., ground
water) is available.

Initial and operating costs are prime factors in selecting
cooling equipment. DX systems can be less expensive than
chillers. However, because a DX system is inherently two­position (on/off), it cannot control temperature with the
accuracy of a chilled water system. Low-temperature control
is essential in a DX system used with a variable air volume
system.

For more information control of various system equipment,
refer to the following sections of this manual:

— Chiller, Boiler, and Distribution System

Control Application.
—Air Handling System Control Applications.
—Individual Room Control Applications.

DEHUMIDIFICATION

Air that is too humid can cause problems such as
condensation and physical discomfort. Dehumidification
methods circulate moist air through cooling coils or sorption
units. Dehumidification is required only during the cooling
season. In those applications, the cooling system can be
designed to provide dehumidification as well as cooling.

REFRIGERANT
GAS

C2708-1

Fig. 11. System Using Evaporator

(Direct Expansion) Coil.

Two basic types of cooling systems are available: chillers,
typically used in larger systems, and direct expansion (DX)
coils, typically used in smaller systems. In a chiller, the
refrigeration system cools water which is then pumped to coils
in the central air handling system or to the coils of fan coil
units, a zone system, or other type of cooling system. In a DX
system, the DX coil of the refrigeration system is located in
the duct of the air handling system. Condenser cooling for
chillers may be air or water (using a cooling tower), while DX
systems are typically air cooled. Because water cooling is more
efficient than air cooling, large chillers are always water cooled.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

For dehumidification, a cooling coil must have a capacity
and surface temperature sufficient to cool the air below its dew
point. Cooling the air condenses water, which is then collected
and drained away. When humidity is critical and the cooling
system is used for dehumidification, the dehumidified air may
be reheated to maintain the desired space temperature.

When cooling coils cannot reduce moisture content
sufficiently, sorption units are installed. A sorption unit uses
either a rotating granular bed of silica gel, activated alumina
or hygroscopic salts (Fig. 12), or a spray of lithium chloride
brine or glycol solution. In both types, the sorbent material
absorbs moisture from the air and then the saturated sorbent
material passes through a separate section of the unit that
applies heat to remove moisture. The sorbent material gives
up moisture to a stream of “scavenger” air, which is then
exhausted. Scavenger air is often exhaust air or could be
outdoor air.

12

CONTROL FUNDAMENTALS

HUMID

ROTATING
GRANULAR
BED

SORPTION
UNIT

AIR

DRY AIR

HUMID AIR
EXHAUST

HEATING
COIL

SCAVENGER
AIR

C2709

Fig. 12. Granular Bed Sorption Unit.

Sprayed cooling coils (Fig. 13) are often used for space
humidity control to increase the dehumidifier efficiency and
to provide year-round humidity control (winter humidification
also).

MOISTURE

COOLING
COIL

ELIMINATORS

VENTILATION

Ventilation introduces outdoor air to replenish the oxygen
supply and rid building spaces of odors and toxic gases.
Ventilation can also be used to pressurize a building to reduce
infiltration. While ventilation is required in nearly all buildings,
the design of a ventilation system must consider the cost of
heating and cooling the ventilation air. Ventilation air must be
kept at the minimum required level except when used for free
cooling (refer to ASHRAE Standard 62, Ventilation for
Acceptable Indoor Air Quality).

To ensure high-quality ventilation air and minimize the
amount required, the outdoor air intakes must be located to
avoid building exhausts, vehicle emissions, and other sources
of pollutants. Indoor exhaust systems should collect odors or
contaminants at their source. The amount of ventilation a
building requires may be reduced with air washers, high
efficiency filters, absorption chemicals (e.g., activated
charcoal), or odor modification systems.

Ventilation requirements vary according to the number of
occupants and the intended use of the space. For a breakdown
of types of spaces, occupancy levels, and required ventilation,
refer to ASHRAE Standard 62.

SPRAY
PUMP

M10511

Fig. 13. Sprayed Coil Dehumidifier.

For more information on dehumidification, refer to the

following sections of this manual:

— Psychrometric Chart Fundamentals.
—Air Handling System Control Applications.

HUMIDIFICATION

Low humidity can cause problems such as respiratory
discomfort and static electricity. Humidifiers can humidify a
space either directly or through an air handling system. For
satisfactory environmental conditions, the relative humidity
of the air should be 30 to 60 percent. In critical areas where
explosive gases are present, 50 percent minimum is
recommended. Humidification is usually required only during
the heating season except in extremely dry climates.

Humidifiers in air handling systems typically inject steam
directly into the air stream (steam injection), spray atomized
water into the air stream (atomizing), or evaporate heated water
from a pan in the duct into the air stream passing through the
duct (pan humidification). Other types of humidifiers are a
water spray and sprayed coil. In spray systems, the water can
be heated for better vaporization or cooled for
dehumidification.

For more information on humidification, refer to the
following sections of this manual:

— Psychrometric Chart Fundamentals.
—Air Handling System Control Applications.

Figure 14 shows a ventilation system that supplies 100
percent outdoor air. This type of ventilation system is typically
used where odors or contaminants originate in the conditioned
space (e.g., a laboratory where exhaust hoods and fans remove
fumes). Such applications require make-up air that is
conditioned to provide an acceptable environment.

EXHAUST

SUPPLY
FAN

RETURN
AIR

MAKE-UP
AIR

SPACE

C2711

TO
OUTDOORS

OUTDOOR
AIR

SUPPLY

FILTER

EXHAUST
FAN

COIL

Fig. 14. Ventilation System Using 100 Percent

Outdoor Air.

In many applications, energy costs make 100 percent outdoor
air constant volume systems uneconomical. For that reason,
other means of controlling internal contaminants are available,
such as variable volume fume hood controls, space
pressurization controls, and air cleaning systems.

A ventilation system that uses return air (Fig. 15) is more
common than the 100 percent outdoor air system. The return­air ventilation system recirculates most of the return air from
the system and adds outdoor air for ventilation. The return-air
system may have a separate fan to overcome duct pressure

13

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

losses. The exhaust-air system may be incorporated into the
air conditioning unit, or it may be a separate remote exhaust.
Supply air is heated or cooled, humidified or dehumidified,
and discharged into the space.

DAMPER RETURN FAN

EXHAUST
AIR

RETURN
AIR

FILTER COIL SUPPLY FAN

MIXED
AIR

SUPPLY
AIR

C2712

OUTDOOR
AIR

DAMPERS

Fig. 15. Ventilation System Using Return Air.

Ventilation systems as shown in Figures 14 and 15 should
provide an acceptable indoor air quality, utilize outdoor air
for cooling (or to supplement cooling) when possible, and
maintain proper building pressurization.

For more information on ventilation, refer to the following
sections of this manual:

—Indoor Air Quality Fundamentals.
—Air Handling System Control Applications.
— Building Airflow System Control Applications.

FILTRATION

Air filtration is an important part of the central air handling
system and is usually considered part of the ventilation system.
Two basic types of filters are available: mechanical filters and
electrostatic precipitation filters (also called electronic air
cleaners). Mechanical filters are subdivided into standard and
high efficiency.

PLEATED FILTER

Filters are selected according to the degree of cleanliness
required, the amount and size of particles to be removed, and
acceptable maintenance requirements. High-efficiency
particulate air (HEPA) mechanical filters (Fig. 16) do not
release the collected particles and therefore can be used for
clean rooms and areas where toxic particles are released. HEPA
filters significantly increase system pressure drop, which must
be considered when selecting the fan. Figure 17 shows other
mechanical filters.

CELL

AIR FLOW

PLEATED PAPER

C2713

Fig. 16. HEPA Filter.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

BAG FILTER

Fig. 17. Mechanical Filters.

Other types of mechanical filters include strainers, viscous
coated filters, and diffusion filters. Straining removes particles
that are larger than the spaces in the mesh of a metal filter and
are often used as prefilters for electrostatic filters. In viscous
coated filters, the particles passing through the filter fibers
collide with the fibers and are held on the fiber surface.
Diffusion removes fine particles by using the turbulence present
in the air stream to drive particles to the fibers of the filter
surface.

An electrostatic filter (Fig. 18) provides a low pressure drop
but often requires a mechanical prefilter to collect large
particles and a mechanical after-filter to collect agglomerated
particles that may be blown off the electrostatic filter. An
electrostatic filter electrically charges particles passing through
an ionizing field and collects the charged particles on plates
with an opposite electrical charge. The plates may be coated
with an adhesive.

14

CONTROL FUNDAMENTALS

–

PATH
OF
IONS

WIRES
AT HIGH
POSITIVE
POTENTIAL

SOURCE: 1996 ASHRAE SYSTEMS AND EQUIPMENT HANDBOOK

AIRFLOW

AIRFLOW

POSITIVELY CHARGED
PA RTICLES

+

ALTERNATE
PLATES

–

GROUNDED

+

–

INTERMEDIATE
PLATES

+

CHARGED
TO HIGH
POSITIVE

–

POTENTIAL

+

THEORETICAL
PATHS OF

–

CHARGES DUST
PARTICLES

C2714

Fig. 18. Electrostatic Filter.

CONTROL SYSTEM CHARACTERISTICS

Automatic controls are used wherever a variable condition
must be controlled. In HVAC systems, the most commonly
controlled conditions are pressure, temperature, humidity, and
rate of flow. Applications of automatic control systems range
from simple residential temperature regulation to precision
control of industrial processes.

CONTROLLED VARIABLES

The sensor can be separate from or part of the controller
and is located in the controlled medium. The sensor measures
the value of the controlled variable and sends the resulting
signal to the controller. The controller receives the sensor
signal, compares it to the desired value, or setpoint, and
generates a correction signal to direct the operation of the
controlled device. The controlled device varies the control
agent to regulate the output of the control equipment that
produces the desired condition.

Automatic control requires a system in which a controllable
variable exists. An automatic control system controls the
variable by manipulating a second variable. The second
variable, called the manipulated variable, causes the necessary
changes in the controlled variable.

In a room heated by air moving through a hot water coil, for
example, the thermostat measures the temperature (controlled
variable) of the room air (controlled medium) at a specified
location. As the room cools, the thermostat operates a valve
that regulates the flow (manipulated variable) of hot water
(control agent) through the coil. In this way, the coil furnishes
heat to warm the room air.

CONTROL LOOP

In an air conditioning system, the controlled variable is
maintained by varying the output of the mechanical equipment
by means of an automatic control loop. A control loop consists
of an input sensing element, such as a temperature sensor; a
controller that processes the input signal and produces an output
signal; and a final control element, such as a valve, that operates
according to the output signal.

HVAC applications use two types of control loops: open
and closed. An open-loop system assumes a fixed relationship
between a controlled condition and an external condition. An
example of open-loop control would be the control of perimeter
radiation heating based on an input from an outdoor air
temperature sensor. A circulating pump and boiler are energized
when an outdoor air temperature drops to a specified setting,
and the water temperature or flow is proportionally controlled
as a function of the outdoor temperature. An open-loop system
does not take into account changing space conditions from
internal heat gains, infiltration/exfiltration, solar gain, or other
changing variables in the building. Open-loop control alone
does not provide close control and may result in underheating
or overheating. For this reason, open-loop systems are not
common in residential or commercial applications.

A closed-loop system relies on measurement of the
controlled variable to vary the controller output. Figure 19
shows a block diagram of a closed-loop system. An example
of closed-loop control would be the temperature of discharge
air in a duct determining the flow of hot water to the heating
coils to maintain the discharge temperature at a controller
setpoint.

15

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

SETPOINT

FEEDBACK

SENSING

ELEMENT

CONTROLLER

CORRECTIVE
SIGNAL

FINAL CONTROL

ELEMENT

MANIPULATED
VARIABLE

PROCESS

CONTROLLED
VARIABLE

SECONDARY

INPUT

DISTURBANCES

C2072

Fig. 19. Feedback in a Closed-Loop System.

In this example, the sensing element measures the discharge
air temperature and sends a feedback signal to the controller.
The controller compares the feedback signal to the setpoint.
Based on the difference, or deviation, the controller issues a
corrective signal to a valve, which regulates the flow of hot
water to meet the process demand. Changes in the controlled
variable thus reflect the demand. The sensing element continues
to measure changes in the discharge air temperature and feeds
the new condition back into the controller for continuous
comparison and correction.

Automatic control systems use feedback to reduce the
magnitude of the deviation and produce system stability as
described above. A secondary input, such as the input from an
outdoor air compensation sensor, can provide information
about disturbances that affect the controlled variable. Using
an input in addition to the controlled variable enables the
controller to anticipate the effect of the disturbance and
compensate for it, thus reducing the impact of disturbances on
the controlled variable.

CONTROL METHODS

GENERAL

An automatic control system is classified by the type of
energy transmission and the type of control signal (analog or
digital) it uses to perform its functions.

The most common forms of energy for automatic control
systems are electricity and compressed air. Systems may
comprise one or both forms of energy.

Systems that use electrical energy are electromechanical,
electronic, or microprocessor controlled. Pneumatic control
systems use varying air pressure from the sensor as input to a
controller, which in turn produces a pneumatic output signal
to a final control element. Pneumatic, electromechanical, and
electronic systems perform limited, predetermined control
functions and sequences. Microprocessor-based controllers use
digital control for a wide variety of control sequences.

Self-powered systems are a comparatively minor but still
important type of control. These systems use the power of the
measured variable to induce the necessary corrective action.
For example, temperature changes at a sensor cause pressure
or volume changes that are applied directly to the diaphragm
or bellows in the valve or damper actuator.

Many complete control systems use a combination of the
above categories. An example of a combined system is the
control system for an air handler that includes electric on/off
control of the fan and pneumatic control for the heating and
cooling coils.

Va rious control methods are described in the following
sections of this manual:

— Pneumatic Control Fundamentals.
—Electric Control Fundamentals.
—Electronic Control Fundamentals.
—Microprocessor-Based/DDC Fundamental.

See CHARACTERISTICS AND ATTRIBUTES OF
CONTROL METHODS.

ANALOG AND DIGITAL CONTROL

Traditionally, analog devices have performed HVAC control.
A typical analog HVAC controller is the pneumatic type which
receives and acts upon data continuously. In a pneumatic
controller, the sensor sends the controller a continuous
pneumatic signal, the pressure of which is proportional to the
value of the variable being measured. The controller compares
the air pressure sent by the sensor to the desired value of air
pressure as determined by the setpoint and sends out a control
signal based on the comparison.

The digital controller receives electronic signals from
sensors, converts the electronic signals to digital pulses
(values), and performs mathematical operations on these
values. The controller reconverts the output value to a signal
to operate an actuator. The controller samples digital data at
set time intervals, rather than reading it continually. The
sampling method is called discrete control signaling. If the
sampling interval for the digital controller is chosen properly,
discrete output changes provide even and uninterrupted control
performance.

Figure 20 compares analog and digital control signals. The
digital controller periodically updates the process as a function
of a set of measured control variables and a given set of control
algorithms. The controller works out the entire computation,
including the control algorithm, and sends a signal to an
actuator. In many of the larger commercial control systems,
an electronic-pneumatic transducer converts the electric output
to a variable pressure output for pneumatic actuation of the
final control element.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

16

CONTROL FUNDAMENTALS

ANALOG CONTROL SIGNAL

OPEN

FINAL

CONTROL

ELEMENT
POSITION

CLOSED

OPEN

FINAL

CONTROL

ELEMENT
POSITION

CLOSED

DIGITAL CONTROL SIGNAL

TIME

TIME

C2080

Fig. 20. Comparison of Analog and Digital Control Signals.

CONTROL MODES

Control systems use different control modes to accomplish
their purposes. Control modes in commercial applications
include two-position, step, and floating control; proportional,
proportional-integral, and proportional-integral-derivative
control; and adaptive control.

An example of differential gap would be in a cooling system
in which the controller is set to open a cooling valve when the
space temperature reaches 78F, and to close the valve when
the temperature drops to 76F. The difference between the two
temperatures (2 degrees F) is the differential gap. The
controlled variable fluctuates between the two temperatures.

Basic two-position control works well for many applications.
For close temperature control, however, the cycling must be
accelerated or timed.

BASIC TWO-POSITION CONTROL

In basic two-position control, the controller and the final
control element interact without modification from a
mechanical or thermal source. The result is cyclical operation
of the controlled equipment and a condition in which the
controlled variable cycles back and forth between two values
(the on and off points) and is influenced by the lag in the
system. The controller cannot change the position of the final
control element until the controlled variable reaches one or
the other of the two limits of the differential. For that reason,
the differential is the minimum possible swing of the controlled
variable. Figure 21 shows a typical heating system cycling
pattern.

TEMPERATURE
(°F)

75

OVERSHOOT
CONDTION

DIAL SETTING

DIFFERENTIAL

OFF

ON

74

73

72

71

70

TWO-POSITION CONTROL

GENERAL

In two-position control, the final control element occupies
one of two possible positions except for the brief period when
it is passing from one position to the other. Two-position control
is used in simple HVAC systems to start and stop electric
motors on unit heaters, fan coil units, and refrigeration
machines, to open water sprays for humidification, and to
energize and deenergize electric strip heaters.

In two-position control, two values of the controlled variable
(usually equated with on and off) determine the position of
the final control element. Between these values is a zone called
the “differential gap” or “differential” in which the controller
cannot initiate an action of the final control element. As the
controlled variable reaches one of the two values, the final
control element assumes the position that corresponds to the
demands of the controller, and remains there until the controlled
variable changes to the other value. The final control element
moves to the other position and remains there until the
controlled variable returns to the other limit.

69

68

TIME

UNDERSHOOT
CONDTION

C2088

Fig. 21. Typical Operation of Basic Two-Position Control.

The overshoot and undershoot conditions shown in Figure
21 are caused by the lag in the system. When the heating system
is energized, it builds up heat which moves into the space to
warm the air, the contents of the space, and the thermostat. By
the time the thermostat temperature reaches the off point (e.g.,
72F), the room air is already warmer than that temperature.
When the thermostat shuts off the heat, the heating system
dissipates its stored heat to heat the space even more, causing
overshoot. Undershoot is the same process in reverse.

In basic two-position control, the presence of lag causes the
controller to correct a condition that has already passed rather
than one that is taking place or is about to take place.
Consequently, basic two-position control is best used in
systems with minimal total system lag (including transfer,
measuring, and final control element lags) and where close
control is not required.

17

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

Figure 22 shows a sample control loop for basic two-position
control: a thermostat turning a furnace burner on or off in
response to space temperature. Because the thermostat cannot
catch up with fluctuations in temperature, overshoot and
undershoot enable the temperature to vary, sometimes
considerably. Certain industrial processes and auxiliary
processes in air conditioning have small system lags and can
use two-position control satisfactorily.

THERMOSTAT

FURNACE

SOLENOID
GAS VALVE

C2715

Fig. 22. Basic Two-Position Control Loop.

TIMED TWO-POSITION CONTROL

GENERAL

The ideal method of controlling the temperature in a space

is to replace lost heat or displace gained heat in exactly the
amount needed. With basic two-position control, such exact
operation is impossible because the heating or cooling system
is either full on or full off and the delivery at any specific instant
is either too much or too little. Timed two-position control,
however, anticipates requirements and delivers measured
quantities of heating or cooling on a percentage on-time basis
to reduce control point fluctuations. The timing is accomplished
by a heat anticipator in electric controls and by a timer in
electronic and digital controls.

BASIC TWO-POSITION CONTROL

TEMPERATURE
(°F)

OFF

ON

TEMPERATURE
(°F)

OFF

ON

75

74

73

72

71

70

69

68

TIME

TIMED TWO-POSITION CONTROL

75

74

73

72

71

70

69

68

TIME

OVERSHOOT
CONDITION

DIAL SETTING

UNDERSHOOT
CONDITION

DIFFERENTIAL

CONTROL
POINT

C2089

Fig. 23. Comparison of Basic Two-Position and Timed

Two-Position Control.

In timed two-position control, the basic interaction between

the controller and the final control element is the same as for
basic two-position control. However, the controller responds
to gradual changes in the average value of the controlled
variable rather than to cyclical fluctuations.

Overshoot and undershoot are reduced or eliminated because

the heat anticipation or time proportioning feature results in a
faster cycling rate of the mechanical equipment. The result is
closer control of the variable than is possible in basic two­position control (Fig. 23).

HEAT ANTICIPATION

In electromechanical control, timed two-position control can
be achieved by adding a heat anticipator to a bimetal sensing
element. In a heating system, the heat anticipator is connected
so that it energizes whenever the bimetal element calls for heat.
On a drop in temperature, the sensing element acts to turn on
both the heating system and the heat anticipator. The heat
anticipator heats the bimetal element to its off point early and
deenergizes the heating system and the heat anticipator. As
the ambient temperature falls, the time required for the bimetal
element to heat to the off point increases, and the cooling time
decreases. Thus, the heat anticipator automatically changes
the ratio of on time to off time as a function of ambient
temperature.

Because the heat is supplied to the sensor only, the heat
anticipation feature lowers the control point as the heat
requirement increases. The lowered control point, called
“droop”, maintains a lower temperature at design conditions
and is discussed more thoroughly in the following paragraphs.
Energizing the heater during thermostat off periods
accomplishes anticipating action in cooling thermostats. In
either case, the percentage on-time varies in proportion to the
system load.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

18

TIME PROPORTIONING

73-1/4

72

70-3/4

0

1-1/4

2-1/2

DROOP (°F)

CONTROL POINT (°F)

DESIGN

TEMPERATURE

OUTDOOR AIR

TEMPERATURE

0%

100%

LOAD

C2091-1

NO LOAD

TEMPERATURE

Time proportioning control provides more effective two­position control than heat anticipation control and is available
with some electromechanical thermostats and in electronic and
microprocessor-based controllers. Heat is introduced into the
space using on/off cycles based on the actual heat load on the
building and programmable time cycle settings. This method
reduces large temperature swings caused by a large total lag
and achieves a more even flow of heat.

CONTROL FUNDAMENTALS

In electromechanical thermostats, the cycle rate is adjustable
by adjusting the heater. In electronic and digital systems, the
total cycle time and the minimum on and off times of the
controller are programmable. The total cycle time setting is
determined primarily by the lag of the system under control.
If the total cycle time setting is changed (e.g., from 10 minutes
to 20 minutes), the resulting on/off times change accordingly
(e.g., from 7.5 minutes on/2.5 minutes off to 15 minutes on/5
minutes off), but their ratio stays the same for a given load.

The cycle time in Figure 24 is set at ten minutes. At a 50
percent load condition, the controller, operating at setpoint,
produces a 5 minute on/5 minute off cycle. At a 75 percent
load condition, the on time increases to 7.5 minutes, the off
time decreases to 2.5 minutes, and the opposite cycle ratio
occurs at 25 percent load. All load conditions maintain the
preset 10-minute total cycle.

SELECTED

CYCLE TIME

(MINUTES)

10

7.5

5

2.5

0

100 75 50 25 0

LOAD (%)

ON

OFF

C2090

Fig. 25. Relationship between Control Point, Droop,

and Load (Heating Control).

Time proportioning control of two-position loads is
recommended for applications such as single-zone systems that
require two-position control of heating and/or cooling (e.g., a
gas-fired rooftop unit with direct-expansion cooling). Time
proportioning control is also recommended for electric heat
control, particularly for baseboard electric heat. With time
proportioning control, care must be used to avoid cycling the
controlled equipment more frequently than recommended by
the equipment manufacturer.

STEP CONTROL

Step controllers operate switches or relays in sequence to
enable or disable multiple outputs, or stages, of two-position
devices such as electric heaters or reciprocating refrigeration
compressors. Step control uses an analog signal to attempt to
obtain an analog output from equipment that is typically either
on or off. Figures 26 and 27 show that the stages may be
arranged to operate with or without overlap of the operating
(on/off) differentials. In either case, the typical two-position
differentials still exist but the total output is proportioned.

Fig. 24. Time Proportioning Control.

Because the controller responds to average temperature or
humidity, it does not wait for a cyclic change in the controlled
variable before signaling corrective action. Thus control system
lags have no significant effect.

Droop in heating control is a lowering of the control point
as the load on the system increases. In cooling control, droop
is a raising of the control point. In digital control systems,
droop is adjustable and can be set as low as one degree or even
less. Figure 25 shows the relationship of droop to load.

19

STAGES

THROTTLING RANGE

DIFFERENTIAL

5

4

3

2

1

ONOFF

74

ONOFF

SPACE TEMPERATURE (°F)

ONOFF

LOAD

ONOFF

Fig. 26. Electric Heat Stages.

ENGINEERING MANUAL OF AUTOMATIC CONTROL

ONOFF

72

100%0%

C2092-1

CONTROL FUNDAMENTALS

THROTTLING RANGE

STAGES

DIFFERENTIAL

4

3

2

OFF

1

72

SPACE TEMPERATURE (°F)

0%

74

SETPOINT

LOAD

ON

ONOFF

ONOFF

ONOFF

76

100%

C2093

Fig. 27. Staged Reciprocating Chiller Control.

Figure 28 shows step control of sequenced DX coils and
electric heat. On a rise in temperature through the throttling
range at the thermostat, the heating stages sequence off. On a
further rise after a deadband, the cooling stages turn on in
sequence.

SPACE OR

ACTUATOR

6

5

4

3

2

1

STEP

CONTROLLER

FAN

STAGE NUMBERS

SOLENOID

VALVES

D

X

DIRECT EXPANSION

COILS

RETURN AIR

THERMOSTAT

D

X

MULTISTAGE

ELECTRIC HEAT

DISCHARGE

AIR

C2716

zero, and the sequence repeats until all stages required to meet
the load condition are on. On a decrease in load, the process
reverses.

With microprocessor controls, step control is usually done
with multiple, digital, on-off outputs since software allows
easily adjustable on-to-off per stage and interstage differentials
as well as no-load and time delayed startup and minimum on
and off adjustments.

FLOATING CONTROL

Floating control is a variation of two-position control and is
often called “three-position control”. Floating control is not a
common control mode, but is available in most microprocessor­based control systems.

Floating control requires a slow-moving actuator and a fast­responding sensor selected according to the rate of response
in the controlled system. If the actuator should move too slowly,
the controlled system would not be able to keep pace with
sudden changes; if the actuator should move too quickly, two­position control would result.

Floating control keeps the control point near the setpoint at
any load level, and can only be used on systems with minimal
lag between the controlled medium and the control sensor.
Floating control is used primarily for discharge control systems
where the sensor is immediately downstream from the coil,
damper, or device that it controls. An example of floating
control is the regulation of static pressure in a duct (Fig. 29).

FLOATING

STATIC

PRESSURE

CONTROLLER

ACTUATOR

REFERENCE

PRESSURE

PICK-UP

AIRFLOW

STATIC

PRESSURE

PICK-UP

Fig. 28. Step Control with Sequenced DX Coils and

Electric Heat.

A variation of step control used to control electric heat is
step-plus-proportional control, which provides a smooth
transition between stages. This control mode requires one of
the stages to be a proportional modulating output and the others,
two-position. For most efficient operation, the proportional
modulating stage should have at least the same capacity as
one two-position stage.

Starting from no load, as the load on the equipment increases,
the modulating stage proportions its load until it reaches full
output. Then, the first two-position stage comes full on and
the modulating stage drops to zero output and begins to
proportion its output again to match the increasing load. When
the modulating stage again reaches full output, the second two­position stage comes full on, the modulating stage returns to

ENGINEERING MANUAL OF AUTOMATIC CONTROL

DAMPER

C2717

Fig. 29. Floating Static Pressure Control.

In a typical application, the control point moves in and out
of the deadband, crossing the switch differential (Fig. 30). A
drop in static pressure below the controller setpoint causes the
actuator to drive the damper toward open. The narrow
differential of the controller stops the actuator after it has moved
a short distance. The damper remains in this position until the
static pressure further decreases, causing the actuator to drive
the damper further open. On a rise in static pressure above the
setpoint, the reverse occurs. Thus, the control point can float
between open and closed limits and the actuator does not move.
When the control point moves out of the deadband, the
controller moves the actuator toward open or closed until the
control point moves into the deadband again.

20