Manual Automatic Control SI

AUTOMATICCONTROL forENGINEERING MANUAL of

COMMERCIAL BUILDINGSSI Edition

ENGINEERING MANUAL OF AUTOMATIC CONTROLii

Copyright 1989, 1995, and 1997 by Honeywell Inc.

All rights reserved. This manual or portions thereof may not be reporducedin any form without permission of Honeywell Inc.

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Home and Building ControlHoneywell Inc.Honeywell PlazaP.O. Box 524Minneapolis MN 55408-0524

Printed in USA

ENGINEERING MANUAL OF AUTOMATIC CONTROL iii

FOREWORD

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

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

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

All of this manuals original sections have been updated and enhanced to include the latest developments incontrol technology and use the International System of Units (SI). A new section has been added on indoor airquality and information on district heating has been added to the Chiller, Boiler, and Distribution SystemControl Applications Section.

This third SI edition of the Engineering Manual of Automatic Control is our contribution to ensure that wecontinue to satisfy our customers requirements. The contributions and encouragement received from previoususers are gratefully acknowledged. Further suggestions will be most welcome.

Minneapolis, MinnesotaDecember, 1997

KEVIN GILLIGANPresident, H&BC Solutions and Services

ENGINEERING MANUAL OF AUTOMATIC CONTROLiv

ENGINEERING MANUAL OF AUTOMATIC CONTROL v

PREFACE

The purpose of this manual is to provide the reader with a fundamental understanding of controls and howthey 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, buildingairflow, 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 airconditioning systems. Discussions of pneumatic, electric, electronic, and digital controls illustrate that applicationsmay use one or more of several different control methods. Engineering data such as equipment sizing, use ofpsychrometric charts, and conversion formulas supplement and support the control information. To enhanceunderstanding, definitions of terms are provided within individual sections.

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

The control recommendations in this manual are general in nature and are not the basis for any specific job orinstallation. Control systems are furnished according to the plans and specifications prepared by the controlengineer. In many instances there is more than one control solution. Professional expertise and judgment arerequired 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 theirknowledge base and help develop sound approaches to automatic control.

ENGINEERING MANUAL OF AUTOMATIC CONTROLvi

ENGINEERING MANUAL OF AUTOMATIC CONTROL vii

CONTENTSForeward ............................................................................................................ iii

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

Control System Fundamentals .......................................................................................... 1Control Fundamentals ............................................................................................................ 3

Introduction.......................................................................................... 5Definitions ............................................................................................ 5HVAC System Characteristics ............................................................. 8Control System Characteristics ........................................................... 15Control System Components .............................................................. 30Characteristics and Attributes of Control Methods .............................. 35

Psychrometric Chart Fundamentals ............................................................................................................ 37Introduction.......................................................................................... 38Definitions ............................................................................................ 38Description of the Psychrometric Chart ............................................... 39The Abridged Psychrometric Chart ..................................................... 40Examples of Air Mixing Process .......................................................... 42Air Conditioning Processes ................................................................. 43Humidifying Process ............................................................................ 44Process Summary ............................................................................... 53ASHRAE Psychrometric Charts .......................................................... 53

Pneumatic Control Fundamentals ............................................................................................................ 57Introduction.......................................................................................... 59Definitions ............................................................................................ 59Abbreviations ....................................................................................... 60Symbols ............................................................................................... 61Basic Pneumatic Control System ........................................................ 61Air Supply Equipment .......................................................................... 65Thermostats ........................................................................................ 69Controllers ........................................................................................... 70Sensor-Controller Systems ................................................................. 72Actuators and Final Control Elements ................................................. 74Relays and Switches ........................................................................... 77Pneumatic Control Combinations ........................................................ 84Pneumatic Centralization .................................................................... 89Pneumatic Control System Example ................................................... 90

Electric Control Fundamentals ............................................................................................................ 95Introduction.......................................................................................... 97Definitions ............................................................................................ 97How Electric Control Circuits are Classified ........................................ 99Series 40 Control Circuits .................................................................... 100Series 80 Control Circuits .................................................................... 102Series 60 Two-Position Control Circuits ............................................... 103Series 60 Floating Control Circuits ...................................................... 106Series 90 Control Circuits .................................................................... 107Motor Control Circuits .......................................................................... 114

ENGINEERING MANUAL ofAUTOMATICCONTROL

ENGINEERING MANUAL OF AUTOMATIC CONTROLviii

Electronic Control Fundamentals ............................................................................................................ 119Introduction.......................................................................................... 120Definitions ............................................................................................ 120Typical System .................................................................................... 122Components ........................................................................................ 122Electronic Controller Fundamentals .................................................... 129Typical System Application .................................................................. 130

Microprocessor-Based/DDC Fundamentals .................................................................................................... 131Introduction.......................................................................................... 133Definitions ............................................................................................ 133Background ......................................................................................... 134Advantages ......................................................................................... 134Controller Configuration ...................................................................... 135Types of Controllers ............................................................................. 136Controller Software .............................................................................. 137Controller Programming ...................................................................... 142Typical Applications ............................................................................. 145

Indoor Air Quality Fundamentals ............................................................................................................ 149Introduction.......................................................................................... 151Definitions ............................................................................................ 151Abbreviations ....................................................................................... 153Indoor Air Quality Concerns ................................................................ 154Indoor Air Quality Control Applications................................................ 164Bibliography ......................................................................................... 170

Smoke Management Fundamentals ............................................................................................................ 171Introduction.......................................................................................... 172Definitions ............................................................................................ 172Objectives ............................................................................................ 173Design Considerations ........................................................................ 173Design Priniples .................................................................................. 175Control Applications ............................................................................ 178Acceptance Testing ............................................................................. 181Leakage Rated Dampers .................................................................... 181Bibliography ......................................................................................... 182

Building Management System Fundamentals ................................................................................................. 183Introduction.......................................................................................... 184Definitions ............................................................................................ 184Background ......................................................................................... 185System Configurations ........................................................................ 186System Functions ................................................................................ 189Integration of Other Systems............................................................... 196

ENGINEERING MANUAL OF AUTOMATIC CONTROL ix

Control System Applications .......................................................................................... 199Air Handling System Control Applications ...................................................................................................... 201

Introduction.......................................................................................... 203Abbreviations ....................................................................................... 203Requirements for Effective Control ...................................................... 204Applications-General ........................................................................... 206Valve and Damper Selection ............................................................... 207Symbols ............................................................................................... 208Ventilation Control Processes ............................................................. 209Fixed Quantity of Outdoor Air Control ................................................. 211Heating Control Processes .................................................................. 223Preheat Control Processes ................................................................. 228Humidification Control Process ........................................................... 235Cooling Control Processes .................................................................. 236Dehumidification Control Processes ................................................... 243Heating System Control Process ........................................................ 246Year-Round System Control Processes .............................................. 248ASHRAE Psychrometric Charts .......................................................... 261

Building Airflow System Control Applications ............................................................................................... 263Introduction.......................................................................................... 265Definitions ............................................................................................ 265Airflow Control Fundamentals ............................................................. 266Airflow Control Applications ................................................................. 280References .......................................................................................... 290

Chiller, Boiler, and Distribution System Control Applications ....................................................................... 291Introduction.......................................................................................... 295Abbreviations ....................................................................................... 295Definitions ............................................................................................ 295Symbols ............................................................................................... 296Chiller System Control ......................................................................... 297Boiler System Control .......................................................................... 327Hot and Chilled Water Distribution Systems Control ........................... 335High Temperature Water Heating System Control ............................... 374District Heating Applications................................................................ 380

Individual Room Control Applications ............................................................................................................ 395Introduction.......................................................................................... 397Unitary Equipment Control .................................................................. 408Hot Water Plant Considerations .......................................................... 424

ENGINEERING MANUAL OF AUTOMATIC CONTROLx

Engineering Information .......................................................................................... 425Valve Selection and Sizing ............................................................................................................ 427

Introduction.......................................................................................... 428Definitions............................................................................................ 428Valve Selection .................................................................................... 432Valve Sizing ......................................................................................... 437

Damper Selection and Sizing ............................................................................................................ 445Introduction.......................................................................................... 447Definitions............................................................................................ 447Damper Selection ................................................................................ 448Damper Sizing ..................................................................................... 457Damper Pressure Drop ....................................................................... 462Damper Applications ........................................................................... 463

General Engineering Data ............................................................................................................ 465Introduction.......................................................................................... 466Conversion Formulas and Tables ........................................................ 466Electrical Data ..................................................................................... 473Properties of Saturated Steam Data ................................................... 476Airflow Data ......................................................................................... 477Moisture Content of Air Data ............................................................... 479

Index .......................................................................................... 483

SMOKE MANAGEMENT FUNDAMENTALS

ENGINEERING MANUAL OF AUTOMATIC CONTROL 1

CONTROLSYSTEM

FUNDAMENTALS

ENGINEERING MANUAL OF AUTOMATIC CONTROL


2



CONTROL FUNDAMENTALS

3

CONTENTS

Control FundamentalsENGINEERING MANUAL OF AUTOMATIC CONTROL

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

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

HVAC System Characteristics ............................................................................................................ 8General ................................................................................................ 8Heating ................................................................................................ 9

General ........................................................................................... 9Heating Equipment ......................................................................... 10

Cooling ................................................................................................ 11General ........................................................................................... 11Cooling Equipment .......................................................................... 12

Dehumidification .................................................................................. 12Humidification ...................................................................................... 13Ventilation ............................................................................................ 13Filtration ............................................................................................... 14

Control System Characteristics ............................................................................................................ 15Controlled Variables ............................................................................ 15Control Loop ........................................................................................ 15Control Methods .................................................................................. 16

General ........................................................................................... 16Analog and Digital Control .............................................................. 16

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

General ....................................................................................... 17Basic Two-Position Control ......................................................... 17Timed Two-Position Control ........................................................ 18

Step Control .................................................................................... 19Floating Control ............................................................................... 20Proportional Control ........................................................................ 21

General ....................................................................................... 21Compensation Control ................................................................ 22

Proportional-Integral (PI) Control .................................................... 23Proportional-Integral-Derivative (PID) Control ................................ 25Enhanced Proportional-Integral-Derivative (EPID) Control ............. 25Adaptive Control .............................................................................. 26

Process Characteristics....................................................................... 26Load ................................................................................................ 26Lag .................................................................................................. 27

General ....................................................................................... 27Measurement Lag ....................................................................... 27Capacitance ................................................................................ 28Resistance .................................................................................. 29Dead Time .................................................................................. 29

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



4

Control System Components ............................................................................................................ 30Sensing Elements ............................................................................... 30

Temperature Sensing Elements ...................................................... 30Pressure Sensing Elements ............................................................ 31Moisture Sensing Elements ............................................................ 32Flow Sensors .................................................................................. 32Proof-of-Operation Sensors ............................................................ 33

Transducers ......................................................................................... 33Controllers ........................................................................................... 33Actuators ............................................................................................. 33Auxiliary Equipment ............................................................................. 34

Characteristics and Attributes of Control Methods .............................................................................. 35



5

INTRODUCTIONThis section describes heating, ventilating, and air

conditioning (HVAC) systems and discusses characteristics andcomponents of automatic control systems. Cross-references aremade to sections that provide more detailed information.

A correctly designed HVAC control system can provide acomfortable environment for occupants, optimize energy costand consumption, improve employee productivity, facilitateefficient manufacturing, control smoke in the event of a fire,and support the operation of computer and telecommunicationsequipment. Controls are essential to the proper operation ofthe system and should be considered as early in the designprocess as possible.

Properly applied automatic controls ensure that a correctlydesigned HVAC system will maintain a comfortableenvironment and perform economically under a wide range ofoperating conditions. Automatic controls regulate HVAC systemoutput in response to varying indoor and outdoor conditions tomaintain general comfort conditions in office areas and providenarrow temperature and humidity limits where required inproduction areas for product quality.

Automatic controls can optimize HVAC system operation.They can adjust temperatures and pressures automatically toreduce demand when spaces are unoccupied and regulateheating and cooling to provide comfort conditions while limitingenergy usage. Limit controls ensure safe operation of HVACsystem equipment and prevent injury to personnel and damageto the system. Examples of limit controls are low-limittemperature controllers which help prevent water coils or heatexchangers from freezing and flow sensors for safe operationof some equipment (e.g., chillers). In the event of a fire,controlled air distribution can provide smoke-free evacuationpassages, and smoke detection in ducts can close dampers toprevent the spread of smoke and toxic gases.

HVAC control systems can also be integrated with securityaccess control systems, fire alarm systems, lighting controlsystems, and building and facility management systems tofurther optimize building comfort, safety, and efficiency.

DEFINITIONSThe following terms are used in this manual. Figure 1 at the

end of this list illustrates a typical control loop with thecomponents identified using terms from this list.

Analog: Continuously variable (e.g., a faucet controlling waterfrom off to full flow).

Automatic control system: A system that reacts to a change orimbalance in the variable it controls by adjusting othervariables to restore the system to the desired balance.

Algorithm: A calculation method that produces a control outputby operating on an error signal or a time series of errorsignals.

Compensation control: A process of automatically adjustingthe setpoint of a given controller to compensate forchanges in a second measured variable (e.g., outdoorair temperature). For example, the hot deck setpointis normally reset upward as the outdoor air temperaturedecreases. Also called reset control.

Control agent: The medium in which the manipulated variableexists. In a steam heating system, the control agent isthe steam and the manipulated variable is the flow ofthe steam.

Control point: The actual value of the controlled variable(setpoint plus or minus offset).

Controlled medium: The medium in which the controlledvariable exists. In a space temperature control system,the controlled variable is the space temperature andthe controlled medium is the air within the space.

Controlled Variable: The quantity or condition that is measuredand controlled.

Controller: A device that senses changes in the controlledvariable (or receives input from a remote sensor) andderives the proper correction output.

Corrective action: Control action that results in a change ofthe manipulated variable. Initiated when the controlledvariable deviates from setpoint.

Cycle: One complete execution of a repeatable process. In basicheating operation, a cycle comprises one on periodand one off period in a two-position control system.

Cycling: A periodic change in the controlled variable from onevalue to another. Out-of-control analog cycling iscalled hunting. Too frequent on-off cycling is calledshort cycling. Short cycling can harm electricmotors, 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.



6

Deadband: A range of the controlled variable in which nocorrective action is taken by the controlled system andno energy is used. See also zero energy band.

Deviation: The difference between the setpoint and the valueof the controlled variable at any moment. Also calledoffset.

DDC: Direct Digital Control. See also Digital and Digitalcontrol.

Digital: A series of on and off pulses arranged to conveyinformation. 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 onsensor inputs and setpoint parameters. Theprogrammed control sequence determines the outputto the equipment.

Droop: A sustained deviation between the control point andthe setpoint in a two-position control system causedby a change in the heating or cooling load.

Enhanced proportional-integral-derivative (EPID) control:A control algorithm that enhances the standard PIDalgorithm by allowing the designer to enter a startupoutput value and error ramp duration in addition tothe gains and setpoints. These additional parametersare configured so that at startup the PID output variessmoothly to the control point with negligible overshootor undershoot.

Electric control: A control circuit that operates on line or lowvoltage and uses a mechanical means, such as atemperature-sensitive bimetal or bellows, to performcontrol functions, such as actuating a switch orpositioning a potentiometer. The controller signal usuallyoperates or positions an electric actuator or may switchan electrical load directly or through a relay.

Electronic control: A control circuit that operates on lowvoltage and uses solid-state components to amplifyinput signals and perform control functions, such asoperating a relay or providing an output signal toposition an actuator. The controller usually furnishesfixed control routines based on the logic of the solid-state components.

Final control element: A device such as a valve or damperthat acts to change the value of the manipulatedvariable. Positioned by an actuator.

Hunting: See Cycling.

Lag: A delay in the effect of a changed condition at one point inthe system, or some other condition to which it is related.Also, the delay in response of the sensing element of acontrol due to the time required for the sensing elementto sense a change in the sensed variable.

Load: In a heating or cooling system, the heat transfer that thesystem will be called upon to provide. Also, the workthat the system must perform.

Manipulated variable: The quantity or condition regulatedby the automatic control system to cause the desiredchange in the controlled variable.

Measured variable: A variable that is measured and may becontrolled (e.g., discharge air is measured andcontrolled, outdoor air is only measured).

Microprocessor-based control: A control circuit that operateson low voltage and uses a microprocessor to performlogic and control functions, such as operating a relayor providing an output signal to position an actuator.Electronic devices are primarily used as sensors. Thecontroller often furnishes flexible DDC and energymanagement control routines.

Modulating: An action that adjusts by minute increments anddecrements.

Offset: A sustained deviation between the control point andthe setpoint of a proportional control system understable operating conditions.

On/off control: A simple two-position control system in whichthe device being controlled is either full on or full offwith no intermediate operating positions available.Also called two-position control.

Pneumatic control: A control circuit that operates on airpressure and uses a mechanical means, such as atemperature-sensitive bimetal or bellows, to performcontrol functions, such as actuating a nozzle andflapper or a switching relay. The controller outputusually operates or positions a pneumatic actuator,although relays and switches are often in the circuit.

Process: A general term that describes a change in a measurablevariable (e.g., the mixing of return and outdoor airstreams in a mixed-air control loop and heat transferbetween cold water and hot air in a cooling coil).Usually considered separately from the sensingelement, control element, and controller.

Proportional band: In a proportional controller, the controlpoint range through which the controlled variable mustpass to move the final control element through its fulloperationg range. Expressed in percent of primarysensor span. Commonly used equivalents arethrottling range and modulating range, usuallyexpressed in a quantity of Engineering units (degreesof temperature).

Proportional control: A control algorithm or method in whichthe final control element moves to a positionproportional to the deviation of the value of thecontrolled variable from the setpoint.



7

Proportional-Integral (PI) control: A control algorithm thatcombines the proportional (proportional response) andintegral (reset response) control algorithms. Resetresponse tends to correct the offset resulting fromproportional control. Also called proportional-plus-reset or two-mode control.

Proportional-Integral-Derivative (PID) control: A controlalgorithm that enhances the PI control algorithm byadding a component that is proportional to the rate ofchange (derivative) of the deviation of the controlledvariable. Compensates for system dynamics andallows faster control response. Also called three-mode or rate-reset control.

Reset Control: See Compensation Control.

Sensing element: A device or component that measures thevalue of a variable.

Setpoint: The value at which the controller is set (e.g., thedesired room temperature set on a thermostat). Thedesired control point.

Short cycling: See Cycling.

Step control: Control method in which a multiple-switchassembly sequentially switches equipment (e.g.,electric heat, multiple chillers) as the controller inputvaries through the proportional band. Step controllers

may be actuator driven, electronic, or directly activatedby the sensed medium (e.g., pressure, temperature).

Throttling range: In a proportional controller, the control pointrange through which the controlled variable must passto move the final control element through its fulloperating range. Expressed in values of the controlledvariable (e.g., Kelvins or degrees Celsius, percentrelative humidity, kilopascals). Also calledproportional band. In a proportional roomthermostat, the temperature change required to drivethe 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 63.2percent of the total response to an instantaneous (orstep) change to its input. Typically used to judgethe responsiveness of the component or system.

Two-position control: See on/off control.

Zero energy band: An energy conservation technique thatallows temperatures to float between selected settings,thereby preventing the consumption of heating orcooling energy while the temperature is in this range.

Zoning: The practice of dividing a building into sections forheating and cooling control so that one controller issufficient to determine the heating and coolingrequirements for the section.

Fig. 1. Typical Control Loop.

SETPOINT

15

-15

55

90

RESET SCHEDULE

HWSETPOINT

OA TEMPERATURE

70

71

64

AUTO

41

INPUTOUTPUT

-2

PERCENTOPEN

VALVE

STEAM

FLOW

OUTDOORAIR

OUTDOORAIR

CONTROLPOINT

HOT WATERRETURN

HOT WATERSUPPLY

HOT WATERSUPPLY

TEMPERATURECONTROLLED

MEDIUM

CONTROLLEDVARIABLE

MEASUREDVARIABLE

MEASUREDVARIABLE

SETPOINT

ALGORITHM INCONTROLLER

FINAL CONTROLELEMENT

CONTROLAGENT

MANIPULATEDVARIABLE

M15127



8

HVAC SYSTEM CHARACTERISTICS

Figure 2 shows how an HVAC system may be distributed ina 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. Throughoutthe 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 floorsor areas of the building.

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

The control system for a commercial building comprisesmany control loops and can be divided into central system andlocal- or zone-control loops. For maximum comfort andefficiency, all control loops should be tied together to shareinformation and system commands using a buildingmanagement system. Refer to the Building Management SystemFundamentals section of this manual.

The basic control loops in a central air handling system canbe classified as shown in Table 1.

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

DAMPERAIR FILTER

COOLING COIL

FAN

CHILLER

PUMP

COOLINGTOWER HEATING

UNIT

DUCTWORK

VAV BOXDIFFUSER

BOILER CONTROLPANELM10506

GENERAL

An HVAC system is designed according to capacityrequirements, an acceptable combination of first cost and operatingcosts, system reliability, and available equipment space.



9

Table 1. Functions of Central HVAC Control Loops.

HEATING

GENERAL

Building heat loss occurs mainly through transmission,infiltration/exfiltration, and ventilation (Fig. 3).

Fig. 3. Heat Loss from a Building.

The heating capacity required for a building depends on thedesign temperature, the quantity of outdoor air used, and thephysical activity of the occupants. Prevailing winds affect therate of heat loss and the degree of infiltration. The heatingsystem must be sized to heat the building at the coldest outdoortemperature the building is likely to experience (outdoor designtemperature).

Transmission is the process by which energy enters or leavesa space through exterior surfaces. The rate of energytransmission is calculated by subtracting the outdoortemperature from the indoor temperature and multiplying theresult by the heat transfer coefficient of the surface materials.The rate of transmission varies with the thickness andconstruction of the exterior surfaces but is calculated the sameway for all exterior surfaces:

Energy Transmission perUnit Area and Unit Time = (TIN - TOUT) x HTC

Where:TIN = indoor temperature

TOUT = outdoor temperatureHTC = heat transfer coefficient

HTC = jouleUnit Time x Unit Area x Unit Temperature

ROOF -7CTRANSMISSION

VENTILATION DUCT

EXFILTRATION

DOOR WINDOW

PREVAILINGWINDS

INFILTRATION

20C

C3971

ControlLoop Classification Description

Ventilation Basic Coordinates operation of the outdoor, return, and exhaust air dampers to maintainthe 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 ofoutdoor and return air under varying indoor conditions (essential in variable airvolume systems). Low-temperature protection may be required.

Cooling Chiller control Maintains chiller discharge water at preset temperature or resets temperatureaccording to demand.

Cooling towercontrol

Controls cooling tower fans to provide the coolest water practical under existingwet 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. Ifcompressor is unloading type, cylinders are unloaded as required to maintaintemperature.

Fan Basic Turns on supply and return fans during occupied periods and cycles them asrequired during unoccupied periods.

Better Adjusts fan volumes to maintain proper duct and space pressures. Reduces systemoperating 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 asa function of demand or outdoor temperature.



10

Infiltration is the process by which outdoor air enters abuilding through walls, cracks around doors and windows, andopen doors due to the difference between indoor and outdoorair pressures. The pressure differential is the result oftemperature difference and air intake or exhaust caused by fanoperation. Heat loss due to infiltration is a function oftemperature difference and volume of air moved. Exfiltrationis the process by which air leaves a building (e.g., through wallsand 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 requireheating. As with heat loss from infiltration and exfiltration, heatloss from ventilation is a function of the temperature differenceand the volume of air brought into the building or exhausted.

HEATING EQUIPMENT

Selecting the proper heating equipment depends on manyfactors, including cost and availability of fuels, building sizeand 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 mosteconomical. Boilers are typically fueled by gas and may havethe option of switching to oil during periods of high demand.Solar heat can be used as an alternate or supplementary sourcewith any type of fuel.

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

THERMOSTAT

HOT WATERSUPPLY

VALVE

DISCHARGEAIR

FAN

HOT WATERRETURN C2702

Fig. 4. System Using Heating Coil.

An air handling system provides heat by moving an air streamacross a coil containing a heating medium, across an electricheating coil, or through a furnace. Unit heaters (Fig. 5) aretypically used in shops, storage areas, stairwells, and docks.Panel heaters (Fig. 6) are typically used for heating floors andare usually installed in a slab or floor structure, but may beinstalled in a wall or ceiling.

C2703

UNIT HEATER

COIL

FAN

STEAM ORHOT WATERSUPPLY

CONDENSATEOR HOT WATERRETURN

STEAM TRAP(IF STEAM SUPPLY)

Fig. 5. Typical Unit Heater.

C3035

DISCHARGE AIR

WALL

OUTDOORAIR

MIXINGDAMPERS

RETURNAIR

COOLING COIL

DRAIN PAN

HEATING COIL

FAN

Fig. 6. Panel Heaters.

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

HOT WATERSUPPLY

HOT WATERRETURN

GRID PANEL

HOT WATERSUPPLY

HOT WATERRETURN

SERPENTINE PANELC2704

Fig. 7. Unit Ventilator.



11

Fig. 8. Convection Heater.

WARM AIR

FINNED TUBE

RETURN AIR

FLOORSUPPLY

RETURN

TO OTHERHEATING UNITS

FROM OTHERHEATING UNITS

C2705

REFLECTOR

INFRAREDSOURCE

C2706

RADIANT HEAT

Fig. 9. Infrared Heater.

In mild climates, heat can be provided by a coil in the centralair handling system or by a heat pump. Heat pumps have theadvantage of switching between heating and cooling modes asrequired. Rooftop units provide packaged heating and cooling.Heating in a rooftop unit is usually by a gas- or oil-fired furnaceor an electric heat coil. Steam and hot water coils are availableas well. Perimeter heat is often required in colder climates,particularly under large windows.

A heat pump uses standard refrigeration components and areversing valve to provide both heating and cooling within thesame unit. In the heating mode, the flow of refrigerant throughthe coils is reversed to deliver heat from a heat source to theconditioned space. When a heat pump is used to exchange heatfrom the interior of a building to the perimeter, no additionalheat source is needed.

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

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

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

A heat pipe is a long, sealed, finned tube charged with arefrigerant. The tube is tilted slightly with one end in the outdoorair intake and the other end in the exhaust air. In a heating

application, the refrigerant vaporizes at the lower end in thewarm exhaust air, and the vapor rises toward the higher end inthe cool outdoor air, where it gives up the heat of vaporizationand condenses. A wick carries the liquid refrigerant back to thewarm end, where the cycle repeats. A heat pipe requires noenergy input. For cooling, the process is reversed by tilting thepipe the other way.

Controls may be pneumatic, electric, electronic, digital, or acombination. Satisfactory control can be achieved usingindependent control loops on each system. Maximum operatingefficiency and comfort levels can be achieved with a controlsystem which adjusts the central system operation to thedemands of the zones. Such a system can save enough inoperating costs to pay for itself in a short time.

Controls for the air handling system and zones are specificallydesigned for a building by the architect, engineer, or team whodesigns the building. The controls are usually installed at the jobsite. 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.

COOLINGGENERAL

Both sensible and latent heat contribute to the cooling loadof a building. Heat gain is sensible when heat is added to theconditioned space. Heat gain is latent when moisture is addedto the space (e.g., by vapor emitted by occupants and othersources). To maintain a constant humidity ratio in the space,water vapor must be removed at a rate equal to its rate of additioninto the space.

Conduction is the process by which heat moves betweenadjoining spaces with unequal space temperatures. Heat maymove through exterior walls and the roof, or through floors,walls, or ceilings. Solar radiation heats surfaces which thentransfer the heat to the surrounding air. Internal heat gain isgenerated by occupants, lighting, and equipment. Warm airentering a building by infiltration and through ventilation alsocontributes to heat gain.

Building orientation, interior and exterior shading, the angleof the sun, and prevailing winds affect the amount of solar heatgain, which can be a major source of heat. Solar heat receivedthrough windows causes immediate heat gain. Areas with largewindows may experience more solar gain in winter than insummer. Building surfaces absorb solar energy, become heated,and transfer the heat to interior air. The amount of change intemperature through each layer of a composite surface dependson the resistance to heat flow and thickness of each material.

Occupants, lighting, equipment, and outdoor air ventilationand infiltration requirements contribute to internal heat gain.For example, an adult sitting at a desk produces about 117 watts.Incandescent lighting produces more heat than fluorescentlighting. Copiers, computers, and other office machines alsocontribute significantly to internal heat gain.



12

COOLING EQUIPMENT

An air handling system cools by moving air across a coilcontaining a cooling medium (e.g., chilled water or arefrigerant). Figures 10 and 11 show air handling systems thatuse a chilled water coil and a refrigeration evaporator (directexpansion) coil, respectively. Chilled water control is usuallyproportional, whereas control of an evaporator coil is two-position. In direct expansion systems having more than onecoil, a thermostat controls a solenoid valve for each coil andthe compressor is cycled by a refrigerant pressure control. Thistype of system is called a pump down system. Pump downmay be used for systems having only one coil, but more oftenthe compressor is controlled directly by the thermostat.

Fig. 10. System Using Cooling Coil.

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, therefrigeration system cools water which is then pumped to coilsin the central air handling system or to the coils of fan coilunits, a zone system, or other type of cooling system. In a DXsystem, the DX coil of the refrigeration system is located inthe duct of the air handling system. Condenser cooling forchillers may be air or water (using a cooling tower), while DXsystems are typically air cooled. Because water cooling is moreefficient than air cooling, large chillers are always water cooled.

Compressors for chilled water systems are usually centrifugal,reciprocating, or screw type. The capacities of centrifugal andscrew-type compressors can be controlled by varying thevolume of refrigerant or controlling the compressor speed. DXsystem compressors are usually reciprocating and, in somesystems, capacity can be controlled by unloading cylinders.Absorption refrigeration systems, which use heat energy directlyto produce chilled water, are sometimes used for large chilledwater systems.

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

Initial and operating costs are prime factors in selectingcooling equipment. DX systems can be less expensive thanchillers. However, because a DX system is inherently two-position (on/off), it cannot control temperature with the accuracyof a chilled water system. Low-temperature control is essentialin 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 SystemControl Applications.

Air Handling System Control Applications. Individual Room Control Applications.

DEHUMIDIFICATION

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

For dehumidification, a cooling coil must have a capacityand surface temperature sufficient to cool the air below its dewpoint. Cooling the air condenses water, which is then collectedand drained away. When humidity is critical and the coolingsystem is used for dehumidification, the dehumidified air maybe reheated to maintain the desired space temperature.

When cooling coils cannot reduce moisture contentsufficiently, sorption units are installed. A sorption unit useseither a rotating granular bed of silica gel, activated alumina orhygroscopic salts (Fig. 12), or a spray of lithium chloride brineor glycol solution. In both types, the sorbent material absorbsmoisture from the air and then the saturated sorbent materialpasses through a separate section of the unit that applies heatto remove moisture. The sorbent material gives up moisture toa stream of scavenger air, which is then exhausted. Scavengerair is often exhaust air or could be outdoor air.

TEMPERATURECONTROLLER

SENSOR

CONTROLVALVE

CHILLED WATERSUPPLY

CHILLEDWATERCOIL COOL AIR

CHILLED WATER RETURN

C2707-2

D

X

TEMPERATURECONTROLLER SENSOR

COOL AIR

C2708-1

EVAPORATORCOIL

SOLENOIDVALVE

REFRIGERANTLIQUID

REFRIGERANTGAS



13

Fig. 12. Granular Bed Sorption Unit.

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

DRY AIR

HUMIDAIR

ROTATINGGRANULARBED

SORPTIONUNIT

SCAVENGERAIR

HEATINGCOIL

HUMID AIREXHAUST

C2709

MOISTUREELIMINATORS

SPRAYPUMP M10511

COOLINGCOIL

Fig. 13. Sprayed Coil Dehumidifier.

For more information on dehumidification, refer to thefollowing sections of this manual:

Psychrometric Chart Fundamentals. Air Handling System Control Applications.

HUMIDIFICATION

Low humidity can cause problems such as respiratorydiscomfort and static electricity. Humidifiers can humidify aspace either directly or through an air handling system. Forsatisfactory environmental conditions, the relative humidity ofthe air should be 30 to 60 percent. In critical areas whereexplosive gases are present, 50 percent minimum isrecommended. Humidification is usually required only duringthe heating season except in extremely dry climates.

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

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

Psychrometric Chart Fundamentals. Air Handling System Control Applications.

VENTILATION

Ventilation introduces outdoor air to replenish the oxygensupply and rid building spaces of odors and toxic gases.Ventilation can also be used to pressurize a building to reduceinfiltration. While ventilation is required in nearly all buildings,the design of a ventilation system must consider the cost ofheating and cooling the ventilation air. Ventilation air must bekept at the minimum required level except when used for freecooling (refer to ASHRAE Standard 62, Ventilation forAcceptable Indoor Air Quality).

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

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

Figure 14 shows a ventilation system that supplies 100 percentoutdoor air. This type of ventilation system is typically usedwhere 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 toprovide an acceptable environment.

EXHAUST

TOOUTDOORS

EXHAUSTFAN

RETURNAIR

SPACE

MAKE-UPAIR

SUPPLY FAN

COILFILTER

OUTDOORAIR

SUPPLY

C2711

Fig. 14. Ventilation System Using100 Percent Outdoor Air.

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

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



14

losses. The exhaust-air system may be incorporated into the airconditioning unit, or it may be a separate remote exhaust. Supplyair is heated or cooled, humidified or dehumidified, anddischarged into the space.

DAMPER RETURN FAN

RETURNAIR

EXHAUSTAIR

DAMPERS

OUTDOORAIR

MIXEDAIR

FILTER COIL SUPPLY FAN

SUPPLYAIR

C2712

Fig. 15. Ventilation System Using Return Air.

Ventilation systems as shown in Figures 14 and 15 shouldprovide an acceptable indoor air quality, utilize outdoor air forcooling (or to supplement cooling) when possible, and maintainproper building pressurization.

For more information on ventilation, refer to the followingsections 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 handlingsystem and is usually considered part of the ventilation system.Two basic types of filters are available: mechanical filters andelectrostatic precipitation filters (also called electronic aircleaners). Mechanical filters are subdivided into standard andhigh efficiency.

Filters are selected according to the degree of cleanlinessrequired, the amount and size of particles to be removed, andacceptable maintenance requirements. High-efficiencyparticulate air (HEPA) mechanical filters (Fig. 16) do not releasethe collected particles and therefore can be used for clean roomsand areas where toxic particles are released. HEPA filterssignificantly increase system pressure drop, which must beconsidered when selecting the fan. Figure 17 shows othermechanical filters.

C2713

CELL

PLEATED PAPER

AIR FLOW

Fig. 16. HEPA Filter.

PLEATED FILTER

BAG FILTER

Fig. 17. Mechanical Filters.

Other types of mechanical filters include strainers, viscouscoated filters, and diffusion filters. Straining removes particlesthat are larger than the spaces in the mesh of a metal filter andare often used as prefilters for electrostatic filters. In viscouscoated filters, the particles passing through the filter fiberscollide with the fibers and are held on the fiber surface. Diffusionremoves fine particles by using the turbulence present in theair stream to drive particles to the fibers of the filter surface.

An electrostatic filter (Fig. 18) provides a low pressure dropbut often requires a mechanical prefilter to collect large particlesand a mechanical after-filter to collect agglomerated particlesthat may be blown off the electrostatic filter. An electrostaticfilter electrically charges particles passing through an ionizingfield and collects the charged particles on plates with an oppositeelectrical charge. The plates may be coated with an adhesive.



15

Fig. 18. Electrostatic Filter.

CONTROL SYSTEM CHARACTERISTICSThe sensor can be separate from or part of the controller and

is located in the controlled medium. The sensor measures thevalue of the controlled variable and sends the resulting signalto the controller. The controller receives the sensor signal,compares it to the desired value, or setpoint, and generates acorrection signal to direct the operation of the controlled device.The controlled device varies the control agent to regulate theoutput of the control equipment that produces the desiredcondition.

HVAC applications use two types of control loops: open andclosed. An open-loop system assumes a fixed relationshipbetween a controlled condition and an external condition. Anexample of open-loop control would be the control of perimeterradiation heating based on an input from an outdoor airtemperature sensor. A circulating pump and boiler are energizedwhen an outdoor air temperature drops to a specified setting,and the water temperature or flow is proportionally controlledas a function of the outdoor temperature. An open-loop systemdoes not take into account changing space conditions frominternal heat gains, infiltration/exfiltration, solar gain, or otherchanging variables in the building. Open-loop control alonedoes not provide close control and may result in underheatingor overheating. For this reason, open-loop systems are notcommon in residential or commercial applications.

A closed-loop system relies on measurement of the controlledvariable to vary the controller output. Figure 19 shows a blockdiagram of a closed-loop system. An example of closed-loopcontrol would be the temperature of discharge air in a ductdetermining the flow of hot water to the heating coils to maintainthe discharge temperature at a controller setpoint.

AIRFLOW

AIRFLOW

ALTERNATEPLATESGROUNDED

INTERMEDIATEPLATESCHARGEDTO HIGHPOSITIVEPOTENTIAL

THEORETICALPATHS OFCHARGES DUSTPARTICLESPOSITIVELY CHARGED

PARTICLES

SOURCE: 1996 ASHRAE SYSTEMS AND EQUIPMENT HANDBOOK

PATH OFIONS

WIRES AT HIGHPOSITIVEPOTENTIAL

C2714

+

+

+

+

Automatic controls are used wherever a variable conditionmust be controlled. In HVAC systems, the most commonlycontrolled conditions are pressure, temperature, humidity, andrate of flow. Applications of automatic control systems rangefrom simple residential temperature regulation to precisioncontrol of industrial processes.

CONTROLLED VARIABLES

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

In a room heated by air moving through a hot water coil, forexample, the thermostat measures the temperature (controlledvariable) of the room air (controlled medium) at a specifiedlocation. As the room cools, the thermostat operates a valvethat regulates the flow (manipulated variable) of hot water(control agent) through the coil. In this way, the coil furnishesheat to warm the room air.

CONTROL LOOP

In an air conditioning system, the controlled variable ismaintained by varying the output of the mechanical equipmentby means of an automatic control loop. A control loop consistsof an input sensing element, such as a temperature sensor; acontroller that processes the input signal and produces an outputsignal; and a final control element, such as a valve, that operatesaccording to the output signal.



16

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

In this example, the sensing element measures the dischargeair 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 acorrective signal to a valve, which regulates the flow of hotwater to meet the process demand. Changes in the controlledvariable thus reflect the demand. The sensing element continuesto measure changes in the discharge air temperature and feedsthe new condition back into the controller for continuouscomparison and correction.

Automatic control systems use feedback to reduce themagnitude of the deviation and produce system stability asdescribed above. A secondary input, such as the input from anoutdoor air compensation sensor, can provide information aboutdisturbances that affect the controlled variable. Using an input inaddition to the controlled variable enables the controller toanticipate the effect of the disturbance and compensate for it, thusreducing the impact of disturbances on the controlled variable.

CONTROL METHODS

GENERAL

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

The most common forms of energy for automatic controlsystems are electricity and compressed air. Systems maycomprise one or both forms of energy.

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

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

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

Various control methods are described in the followingsections of this manual:

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

See CHARACTERISTICS AND ATTRIBUTES OFCONTROL METHODS.

ANALOG AND DIGITAL CONTROL

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

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

Figure 20 compares analog and digital control signals. Thedigital controller periodically updates the process as a functionof a set of measured control variables and a given set of controlalgorithms. 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 variablepressure output for pneumatic actuation of the final controlelement.

SETPOINT

FEEDBACKCONTROLLER SECONDARYINPUT

CORRECTIVESIGNAL

FINAL CONTROLELEMENT

PROCESS DISTURBANCES

CONTROLLEDVARIABLESENSING

ELEMENT

MANIPULATEDVARIABLE

C2072



17

Fig. 20. Comparison of Analogand Digital Control Signals.

CONTROL MODES

Control systems use different control modes to accomplishtheir purposes. Control modes in commercial applicationsinclude two-position, step, and floating control; proportional,proportional-integral, and proportional-integral-derivativecontrol; and adaptive control.

TWO-POSITION CONTROL

General

In two-position control, the final control element occupiesone of two possible positions except for the brief period whenit is passing from one position to the other. Two-position controlis used in simple HVAC systems to start and stop electric motorson unit heaters, fan coil units, and refrigeration machines, toopen water sprays for humidification, and to energize anddeenergize electric strip heaters.

In two-position control, two values of the controlled variable(usually equated with on and off) determine the position of thefinal control element. Between these values is a zone called thedifferential gap or differential in which the controller cannotinitiate an action of the final control element. As the controlledvariable reaches one of the two values, the final control elementassumes the position that corresponds to the demands of thecontroller, and remains there until the controlled variablechanges to the other value. The final control element moves tothe other position and remains there until the controlled variablereturns to the other limit.

An example of differential gap would be in a cooling systemin which the controller is set to open a cooling valve when thespace temperature reaches 26C, and to close the valve whenthe temperature drops to 25C. The difference between the twotemperatures (1C or 1 Kelvin) is the differential gap. Thecontrolled variable fluctuates between the two temperatures.

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

Basic Two-Position Control

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

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

The overshoot and undershoot conditions shown in Figure21 are caused by the lag in the system. When the heating systemis energized, it builds up heat which moves into the space towarm the air, the contents of the space, and the thermostat. Bythe time the thermostat temperature reaches the off point (e.g.,22C), the room air is already warmer than that temperature.When the thermostat shuts off the heat, the heating systemdissipates its stored heat to heat the space even more, causingovershoot. Undershoot is the same process in reverse.

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

ANALOG CONTROL SIGNAL

DIGITAL CONTROL SIGNAL

OPEN

FINALCONTROLELEMENTPOSITION

CLOSED

OPEN

FINALCONTROLELEMENTPOSITION

CLOSED

TIME

TIME C2080

TEMPERATURE(C)

OFF

ON

23.5

23

22.5

22

21.5

21

20.5

20

TIME

UNDERSHOOTCONDTION

DIFFERENTIAL

DIAL SETTING

OVERSHOOT CONDTION

C3972



18

Figure 22 shows a sample control loop for basic two-positioncontrol: a thermostat turning a furnace burner on or off inresponse to space temperature. Because the thermostat cannotcatch up with fluctuations in temperature, overshoot andundershoot enable the temperature to vary, sometimesconsiderably. Certain industrial processes and auxiliaryprocesses in air conditioning have small system lags and canuse two-position control satisfactorily.

Fig. 22. Basic Two-Position Control Loop.

Timed Two-Position Control

GENERAL

The ideal method of controlling the temperature in a space isto replace lost heat or displace gained heat in exactly the amountneeded. With basic two-position control, such exact operationis impossible because the heating or cooling system is eitherfull on or full off and the delivery at any specific instant iseither too much or too little. Timed two-position control,however, anticipates requirements and delivers measuredquantities of heating or cooling on a percentage on-time basisto reduce control point fluctuations. The timing is accomplishedby a heat anticipator in electric controls and by a timer inelectronic and digital controls.

In timed two-position control, the basic interaction betweenthe controller and the final control element is the same as forbasic two-position control. However, the controller respondsto gradual changes in the average value of the controlled variablerather than to cyclical fluctuations.

Overshoot and undershoot are reduced or eliminated becausethe heat anticipation or time proportioning feature results in afaster cycling rate of the mechanical equipment. The result iscloser control of the variable than is possible in basic two-position control (Fig. 23).

Fig. 23. Comparison of Basic Two-Positionand Timed Two-Position Control.

HEAT ANTICIPATION

In electromechanical control, timed two-position control canbe achieved by adding a heat anticipator to a bimetal sensingelement. In a heating system, the heat anticipator is connectedso that it energizes whenever the bimetal element calls for heat.On a drop in temperature, the sensing element acts to turn onboth the heating system and the heat anticipator. The heatanticipator heats the bimetal element to its off point early anddeenergizes the heating system and the heat anticipator. As theambient temperature falls, the time required for the bimetalelement to heat to the off point increases, and the cooling timedecreases. Thus, the heat anticipator automatically changes theratio of on time to off time as a function of ambient temperature.

THERMOSTATFURNACE

SOLENOIDGAS VALVE

C2715

22

21.5

22.5

21

20.5

23

23.5

20

CONTROLPOINT

TIME C3973

TIMED TWO-POSITION CONTROL

22

21.5

22.5

21

20.5

23

23.5

20

OFF

ON

TEMPERATURE(C)

DIFFERENTIAL

DIAL SETTING

UNDERSHOOTCONDITION

TIME

OVERSHOOTCONDITION

BASIC TWO-POSITION CONTROL

TEMPERATURE(C)

OFF

ON



19

Because the heat is supplied to the sensor only, the heatanticipation feature lowers the control point as the heat requirementincreases. The lowered control point, called droop, maintains alower temperature at design conditions and is discussed morethoroughly in the following paragraphs. Energizing the heaterduring thermostat off periods accomplishes anticipating action incooling thermostats. In either case, the percentage on-time variesin proportion to the system load.

TIME PROPORTIONING

Time proportioning control provides more effective two-position control than heat anticipation control and is availablewith some electromechanical thermostats and in electronic andmicroprocessor-based controllers. Heat is introduced into thespace using on/off cycles based on the actual heat load on thebuilding and programmable time cycle settings. This methodreduces large temperature swings caused by a large total lagand achieves a more even flow of heat.

In electromechanical thermostats, the cycle rate is adjustableby adjusting the heater. In electronic and digital systems, thetotal cycle time and the minimum on and off times of thecontroller are programmable. The total cycle time setting isdetermined primarily by the lag of the system under control. Ifthe total cycle time setting is changed (e.g., from 10 minutes to20 minutes), the resulting on/off times change accordingly (e.g.,from 7.5 minutes on/2.5 minutes off to 15 minutes on/5 minutesoff), but their ratio stays the same for a given load.

The cycle time in Figure 24 is set at ten minutes. At a 50percent load condition, the controller, operating at setpoint,produces a 5 minute on/5 minute off cycle. At a 75 percentload condition, the on time increases to 7.5 minutes, the offtime decreases to 2.5 minutes, and the opposite cycle ratiooccurs at 25 percent load. All load conditions maintain the preset10-minute total cycle.

10

7.5

5

2.5

0

SELECTEDCYCLE TIME(MINUTES)

100 75 50 25 0LOAD (%)

ON

OFF

C2090

Fig. 24. Time Proportioning Control.

Because the controller responds to average temperature orhumidity, it does not wait for a cyclic change in the controlledvariable before signaling corrective action. Thus control systemlags have no significant effect.

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

Fig. 25. Relationship between Control Point,Droop, and Load (Heating Control).

Time proportioning control of two-position loads isrecommended for applications such as single-zone systems thatrequire two-position control of heating and/or cooling (e.g., agas-fired rooftop unit with direct-expansion cooling). Timeproportioning control is also recommended for electric heatcontrol, particularly for baseboard electric heat. With timeproportioning control, care must be used to avoid cycling thecontrolled equipment more frequently than recommended bythe equipment manufacturer.

STEP CONTROL

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

Fig. 26. Electric Heat Stages.

22.5

22

21.5

0

0.1

1

DR

OO

P (C

)

CONT

ROL

POIN

T (C

)

DESIGNTEMPERATURE

OUTDOOR AIRTEMPERATURE

0% 100%LOADC3974

NO LOADTEMPERATURE

23

ONOFF

ONOFF

ONOFF

ONOFF

ONOFF

5

4

3

2

1

DIFFERENTIAL

THROTTLING RANGE

22SPACE TEMPERATURE (C)

100%0% LOADC3981

STAGES



20

Fig. 27. Staged Reciprocating Chiller Control.

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

ACTUATOR

AIRFLOW

DAMPER

REFERENCEPRESSURE

PICK-UP

STATICPRESSURE

PICK-UP

FLOATINGSTATIC

PRESSURECONTROLLER

C2717

full on, the modulating stage returns to zero, and the sequencerepeats 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 donewith multiple, digital, on-off outputs since software allowseasily adjustable on-to-off per stage and interstage differentialsas well as no-load and time delayed startup and minimum onand off adjustments.

FLOATING CONTROL

Floating control is a variation of two-position control and isoften called three-position control. Floating control is not acommon control mode, but is available in most microprocessor-based control systems.

Floating control requires a slow-moving actuator and a fast-responding sensor selected according to the rate of response inthe controlled system. If the actuator should move too slowly,the controlled system would not be able to keep pace withsudden changes; if the actuator should move too quickly, two-position control would result.

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

SPACE ORRETURN AIR

THERMOSTATACTUATOR

SOLENOIDVALVES

FAN

DISCHARGEAIR

DIRECT EXPANSIONCOILS

MULTISTAGEELECTRIC HEAT

STEPCONTROLLER

STAGE NUMBERS6

5

4

3

2

1

C2716

D

X

D

X

Fig. 28. Step Control with SequencedDX Coils and Electric Heat.

A variation of step control used to control electric heat isstep-plus-proportional control, which provides a smoothtransition between stages. This control mode requires one ofthe stages to be a proportional modulating output and the others,two-position. For most efficient operation, the proportionalmodulating stage should have at least the same capacity as onetwo-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 modulatingstage drops to zero output and begins to proportion its outputagain to match the increasing load. When the modulating stageagain reaches full output, the second two-position stage comes

Fig. 29. Floating Static Pressure Control.

In a typical application, the control point moves in and outof the deadband, crossing the switch differential (Fig. 30). Adrop in static pressure below the controller setpoint causes theactuator to drive the damper toward open. The narrowdifferential of the controller stops the actuator after it has moveda short distance. The damper remains in this position until thestatic pressure further decreases, causing the actuator to drivethe damper further open. On a rise in static pressure above thesetpoint, the reverse occurs. Thus, the control point can floatbetween open and closed limits and the actuator does not move.When the control point moves out of the deadband, thecontroller moves the actuator toward open or closed until thecontrol point moves into the deadband again.

ONOFF

ONOFF

ONOFF

ONOFF4

3

2

1

DIFFERENTIAL

THROTTLING RANGE

24

SPACE TEMPERATURE (C)

22

100%0% LOADC3982

STAGES

23SETPOINT



21

Fig. 30. Floating Control.

VALVE

CONTROLLER

SENSOR

CHILLEDWATER

RETURNAIR

DISCHARGEAIR

COIL C2718

PROPORTIONAL CONTROL

General

Proportional control proportions the output capacity of theequipment (e.

Documents

Manual Automatic Control SI