HVAC Controls Operation & Maintenance

HVAC Controls Operation & Maintenance
اسم المؤلف
Guy W. Gupton, Jr.
8 يناير 2021
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HVAC Controls Operation & Maintenance
Third Edition
Guy W. Gupton, Jr.
Table of Contents
Chapter 1 Basic Functions of HVAC
Systems and Control Systems
Chapter 2 HVAC Equipment-to-Control Interactions
Chapter 3 Operating and Maintaining HVAC Control Systems
Chapter 4 The Mathematics of
Control Systems: Controller Equations
Chapter 5 Performance Prediction in HVAC Control Systems
Chapter 6 HVAC Control System Set-Up
Chapter 7 Maintaining Electric and Electronic Control Systems
Chapter 8 Maintaining Pneumatic Control Systems
Chapter 9 Maintaining Local Loop to BAS Interfaces
Chapter 10 HVAC Control System Checkout Procedures
Chapter 11 Fine Tuning Program for Pneumatic Control Systems
Chapter 12 Troubleshooting ATC Systems
Chapter 13 Tools & Fixtures for
ATC System Operation and Maintenance
Chapter 14 Training Control System
Operating and Maintenance Personnel
Chapter 15 Installing Hybrid Pneumatic and
Direct Digital Control Systems
Chapter 16 Operating Direct Digital Control Systems
Chapter 17 Testing Direct Digital Control Systems
Chapter 18 A Short Course in Psychrometrics
Glossary of Terms
Glossary of HVAC Terms
Used in Controls System
Operation and Maintenance
ACTUATOR—a controlled device such as a motor, relay, or solenoid in
which the central energy source is converted into a rotary, linear, or
switching action as required to position a final control element to cause
a change in the controlled variable. Valves and dampers are examples of
final control elements positioned by actuators.
ADAPTIVE DUTY CYCLING—a technique to automatically change the
duty cycling program based on actual environmental conditions, usually
temperature and humidity.
ADIABATIC PROCESS—a thermodynamic or conditioning process in
which no heat is extracted from or added to the system.
ADJUSTABLE DIFFERENTIAL—a means of changing the difference
between controller cut-in and cut-out points.
AIR CLEANER—a device used to remove airborne impurities from air,
such as dust, gas, vapor, fume, and smoke. Air cleaners include devices
such as air washers, air filters, electrostatic precipitators, and charcoal
AIR CONDITIONING, COMFORT—treating air to control its temperature, relative humidity, cleanliness, and distribution to meet the comfort
requirements of the occupied space.
an industrial process rather than for the comfort of occupants.
AIR CONDITIONER, ROOM—encased assembly designed as a unit for
mounting in a window, through a wall, or as a console.HVAC CONTROLS—OPERATION & MAINTENANCE
AIR CONDITIONER, SPLIT SYSTEM—a 2-piece system with an indoor
section with fan, evaporator coil, and filter and an outdoor section having compressor and air cooled condenser.
AIR CONDITIONER, UNITARY—a fan, evaporator coil, filter, compressor, and condenser combination designed in a single unit.
ALARM—a signal, audible or visible to warn of an abnormal and critical
operation condition.
ALERT—a form of alarm to warn of an abnormal but not critical operating condition.
AMBIENT AIR—the surrounding air, may be outdoor air or air in an
enclosure under study.
AMBIENT-COMPENSATED—a control design such that varying temperatures of air at the control do not affect the control setting.
AMBIENT TEMPERATURE—the temperature of the ambient air.
ANALOG DATA—data as an analog of the variable represented, presented in a continuous form, as compared to binary or digital data presented in discrete or discontinuous form (on, off), variable value to a
BAS or DDC system from a sensing device.
ANALOG INPUT—a control input in analog form, such as variable
pressure or voltage, transmitted to a controller from a sensor or other
control device.
ANALOG OUTPUT—a control output, such as a variable pressure or
voltage, transmitted from a controller to an actuator or another control
ANALOG POINT—a point that has a variable value, such as temperature, which will be measured by a sensing device to provide an analog
input to a control system.
ANEMOMETER—a device to sense and measure velocity of air flow at
ANTICIPATING CONTROL—a method of reducing the operating differential of the system by adding a small resistive heater inside the thermostat to raise the internal temperature of the thermostat faster than the
surrounding room temperature. This causes the thermostat to shut off
the heating equipment and start the cooling equipment sooner than it
would if affected only by the room temperature.
AQUASTAT—a thermostat used in water.
AUXILIARY CONTACTS—a secondary set of electrical contacts
mounted on a modulating motor or magnetic starter whose operation
coincides with the operation of the motor or starter. Usually low ampere
rating for pilot duty.
AUXILIARY POTENTIOMETER—a potentiometer, usually 135 ohm resistance, on an electric modulating motor, which is used to control other
modulating devices in response to the position of the original motor.
May be called a “follow-up pot.”
AUXILIARY SWITCH—an electric switch, usually spst, mounted on end
of an actuator motor. May also be called an “end switch.”
AUTOMATIC CONTROL—a system that reacts to a change or unbalance in the controlled condition by adjusting the variables, such as temperature and humidity, to restore the system to the desired balance.
AVERAGING ELEMENT—a temperature sensing element that responds
to the average temperature of the sensed medium, usually air.
BIMETAL ELEMENT—formed of two metals having different coefficients of thermal expansion; used in temperature control devices.
BLOWER—an air moving device of centrifugal type, may also be called
a “fan.”
BOILER—a closed vessel in which a liquid is heated with or without
vaporization; boiling need not occur.
BRITISH THERMAL UNIT (BTU)—a measure of heat approximating the
heat energy required to raise the temperature of one pound of water
BULB, CONTROL—portion of a temperature sensing system that is
placed in the controlled or measured variable.
BYPASS—a pipe or duct, usually controlled by valve or damper, for
conveying a fluid around an element of a system.
CAPACITY—maximum load for which a machine, apparatus, or system
is designed; the cooling or heating potential of a system is usually stated
in tons of refrigeration (TR) or Btu/hour for cooling systems and in Btu/
hour or MBH for heating systems.
CAPILLARY TUBE—a tube of small internal diameter. Used as a liquid
refrigerant flow control or expansion device between high and low sides
in refrigeration systems. Also used to transmit pressure from the sensitive bulb of temperature controls to the operating element.
CAPITAL INVESTMENT—an expenditure for an investment whose returns are expected to extend beyond one year.
ccf—hundreds of cubic feet, standard measurement for natural gas flow.
CENTRALIZED CONTROL—see Distributed Control.
CENTRAL FAN SYSTEM—an air conditioning system in which the air
is processed at a central location outside the conditioned space and distributed by means of a fan and duct system.
cfm—cubic feet per minute; a measurement of volume flow.
CHANGEOVER—the process of switching an air conditioning system
from heating cycle to cooling cycle, or vice versa.
CHANNEL—a separate, programmable control function in a BAS or
DDC system, usually controlling more than one point.
CHILLED WATER SYSTEM—a cooling system which conveys heat
through chilled water as the secondary refrigerant.
CHILLER—a refrigerating machine, usually comprised of a compressor,
condenser, refrigerant flow control device, and evaporator in a package,
that cools the liquid in a secondary refrigerant system.GLOSSARY OF TERMS
CIRCUIT—see Channel.
CLOSE-OFF PRESSURE—a value of the maximum allowable pressure
difference to which a valve may be subjected while fully closed without
overcoming actuator power to keep the valve closed.
COEFFICIENT OF PERFORMANCE—a term used to measure the efficiency of a heat transfer system. For a heat pump or an electrical element, it is defined as the heat output divided by the heating value of
power consumed in watts at standard test conditions. Abbreviated: COP.
COIL—a heat transfer element made of pipe or tubing. Air coils for
cooling or heating are generally extended-surface type with aluminum
fins on either copper or aluminum tubing.
COLD DECK—the cooling section of a multi-zone or dual-duct central
station apparatus.
COMBUSTION—the act or process of burning, as in fuels.
COMMUNICATION PORT—the input port on a DDC control system
component through which communication flows to the remainder of the
system; may be a local area network or telephone lines.
be programmed and controlled from a distant point, usually over ordinary telephone lines.
COMPRESSION—in mechanical refrigeration, the process by which the
pressure of the refrigerant is increased.
COMPRESSOR, HERMETICALLY SEALED REFRIGERANT—a motorcompressor unit consisting of a compressor and a motor enclosed in the
same housing, without external shaft seals, and with the motor operating in the refrigerant atmosphere. May be made as serviceable hermetic
COMPRESSOR, REFRIGERANT—the component of a mechanical refrigeration system which compresses the refrigerant vapor into a smallerHVAC CONTROLS—OPERATION & MAINTENANCE
volume, thereby raising the pressure of the refrigerant and consequently
its boiling temperature.
COMPUTER—a programmable electronic device that can store, retrieve,
and process information.
COMPUTER-BASED SYSTEM—a BAS or DDC system in which a computer is the central controlling device.
CONDENSATE—the liquid formed by condensation of a vapor. In
steam heating, water condensed from steam; in air conditioning, water
extracted from air, as by condensation on the cooling coil of a cooling
CONDENSATION—the process of changing a vapor into liquid by the
extraction of heat.
CONDENSER—arrangement of pipe or tubing inside a containing shell
in which a vapor is liquefied by removal of heat.
circulation of atmospheric air, usually fan-forced circulation.
CONDENSER COIL—in mechanical refrigeration, a section of coiled
tubing where gas refrigerant is cooled below its boiling point by circulating fluid, such as condenser water.
CONSUMPTION—the total amount of energy used, usually measured
in kilowatt hours (kWh). On the average, about 80% of a typical commercial or industrial utility bill is based on consumption and about 20%
on demand.
CONSUMPTION CHARGE—see Energy (Consumption) Charge.
CONTACTOR—an electromagnetic switching device.
CONTROL—any device for regulation of a system or component in
normal operation, manual or automatic. If automatic, it is responsive to
changes of pressure, temperature, or other property whose magnitude is
to be regulated.GLOSSARY OF TERMS
CONTROL PANEL—an electrical cabinet that contains control devices
and/or indicating devices.
CONTROL POINT—the value of the controlled variable maintained by
operation of the controller.
CONTROLLED DEVICE—the control component, such as damper,
valve, or relay, which is positioned to effect control of controlled medium.
CONTROLLED MEDIUM—the substance, such as air, water, or steam,
whose temperature, pressure, flow rate, volume, or concentration, is
being controlled.
CONTROLLED SPACE—the volume of the controlled medium: for example, a room in which the air temperature is being controlled.
CONTROLLED VARIABLE—that quantity or condition of a controlled
medium which is measured and controlled: for example, temperature,
pressure, flow rate, volume, or concentration.
CONTROLLER—a control component which translates signals from
sensors to output signals to actuators.
CONVECTION—transfer of heat by movement of fluid.
COOLING SYSTEM, CHILLED WATER—a closed, circulating system in
which a mechanical refrigeration system at a central location cools water
which is then piped to various parts of the building.
COOLING SYSTEM, DIRECT EXPANSION (DX)—a cooling and dehumidification process which cools air or other fluids by the evaporation
of mechanically compressed gas in an evaporator. A condenser then
removes this transferred heat to a different space. See Refrigeration
System, Mechanical.
COOLING SYSTEM, EVAPORATIVE—an adiabatic cooling system, generally housed in a cabinet containing a pump, distribution tubes, filter
pads, and a blower. The pump supplies water to the distribution tubesHVAC CONTROLS—OPERATION & MAINTENANCE
which carry the water to pads on sides of the cabinet. The blower draws
outdoor air through the moist filter pads. Some of the water in the pads
absorbs heat from the air and evaporates, cooling the air. The air is
nearly saturated and cannot be recirculated. This system works most
effectively in relatively dry climates.
COOLING SYSTEM, MULTI-STAGE—a cooling system that changes its
capacity by stages in response to changes in cooling demand.
CPU—the central processing unit or microprocessor in a computer that
controls a BAS or DDC system.
Cv—the flow coefficient for valves, representing the flow rate in gallons
of water per minute at 62°F that will cause a pressure drop across the
valve of 1 psig.
CYCLE—a complete course of operation of working fluid back to a starting point, measured in thermodynamic terms (functions). Also used in
general for any repeated process on any system.
CYCLE, COOLING—the functions in an HVAC system which provide
heat removal, or cooling, from a conditioned space.
CYCLE, HEATING—the functions in an HVAC system which provide
heat addition, or heating, to a conditioned space.
CYCLE, REFRIGERATION—complete course of operation of refrigerant
back to a starting point, evidenced by a repeated series of thermodynamic processes, or flow through a series of apparatus, or a repeated
series of mechanical operations.
CYCLING RATE—the number of complete cycles that the system goes
through in one hour. One complete cycle includes both on and off times.
DAMPER—an adjustable metal plate, louver, or set of louvers that controls airflow, especially through an air inlet, outlet, or duct.
DAMPER LINKAGE—linkage used to connect an actuator to a damper,
usually consisting of a pushrod, two crank arms, and two ball joints.GLOSSARY OF TERMS
DAMPER, OPPOSED BLADE—louver-type damper with alternating
blades rotating in opposite directions. Provides an equal percentage flow
characteristic in which successive equal increments of rotation produce
equal percentage increases in flow. Used for throttling and mixing applications where the sum of two airflows must remain nearly constant and
where accurate control of airflow is necessary.
DAMPER, PARALLEL BLADE—damper with all blades rotating in the
same direction. Provides a non-linear airflow characteristic in which
flow is not proportional to damper shaft rotation. Used in two-position
DATABASE—the data in memory in the CPU that controls a BAS or
DDC system.
DDC SYSTEM—a distributed control system made up of one or more
digital controllers and providing control and energy management functions for complete operation of HVAC and process systems in a system
linked in a communications network composed of one or more levels of
local area networks (LAN). No conventional control devices, pneumatic
or electronic, such as receiver/controllers, thermostats, or logic units are
present within or interface with a DDC control loop.
DEADBAND—in HVAC control terminology, a temperature range in
which neither heating nor cooling are turned on.
DEGREE DAY—a unit, based upon temperature difference and time,
used in estimating building environmental system energy usage. For
example, on any one day, when the mean temperature is less than the
base temperature, usually 65°F for heating degree days, there exist as
many heating degree days as there are degrees F difference in temperature between the mean temperature for the day and the base temperature.
DEHUMIDIFICATION—the condensation of water vapor from air by
cooling below the dew point or removal of water vapor from air by
chemical or physical methods.
DEHUMIDIFIER—an air cooler or washer used for lowering the moisHVAC CONTROLS—OPERATION & MAINTENANCE
ture content of the air passing through it. An absorption or adsorption
device for removing moisture from the air.
DEHYDRATION—the removal of water vapor from air by the use of
absorbing or adsorbing materials. The removal of water from stored
goods. In refrigeration terminology, the removal of water vapor from
refrigerant by use of molecular sieves.
DELTA SERVICE—a 3-wire or 4-wire 3-phase electrical wiring configuration commonly noted by a triangle.
DEMAND—the average rate of electrical usage, measured in kilowatts
demand (kWD), over a given period of time, called the demand interval.
An electric utility must determine its required generating and distribution capacity from the total demand of all its customers. The utility thus
bases its charges not only on total consumption but also on the measured peak demand.
DEMAND CHARGE—that part of an electric bill based on kW demand
and the demand interval. Expressed in dollars per kilowatt demand per
month. Demand charges offset costs for construction and maintenance
of a utility’s generating and distribution capacity.
DEMAND CONTROL—a device that controls the kW demand level by
shedding loads to prevent the kW demand from exceeding a predetermined set point, called the “target.”
DEMAND INTERVAL—the period of time on which kW demand is
monitored and billed by a utility, usually 15 or 30 minutes long.
DEMAND LIMITING—a technique to reduce demand by measuring
incoming electrical power and turning off specified loads so as to keep
the rate of electrical usage under a present level.
DEMAND READING—highest or maximum demand for electricity an
individual customer registers in a given interval, for example a 15-
minute interval. The metered demand reading sets the demand charge
for the month.
DEVIATION—the difference between the set point and the value of the
controlled variable at any instant.GLOSSARY OF TERMS
DEW-POINT TEMPERATURE—the temperature at which moisture
would begin to condense out of the air if the air should be cooled to that
temperature. The temperature corresponding to saturation (100 percent
relative humidity) for a given absolute humidity at constant pressure.
The moisture content of the air establishes the dew-point temperature.
See Moisture Content and Humidity Ratio.
DIFFERENTIAL—in a control, the difference between cut-out and cut-in
DIFFERENTIAL, INTERSTAGE—in a sequencing system, the amount of
change in the controlled medium required to sequence from “on” point
of one stage to “on” point of successive stage.
DIGITAL CONTROLLER—a microprocessor-based control module, programmable by user, with integral I/O, that performs stand-alone operations.
DIGITAL POINT—a point that has an “either/or” value, such as on/off,
which will be sensed to provide direct input to the BAS or DDC system.
DIRECT CURRENT—a source of electrical power that flows in one direction only. Abbreviated: dc.
DIRECT DIGITAL CONTROL (DDC)—a control system where digital
controllers directly sense building environment and make control decisions based on user defined, controller resident programs, and output
control signals that directly operate valves and damper actuators and
motor controllers, with controller output converted to the appropriate
type of signal for electric or pneumatic actuators.
DIRECT EXPANSION (DX) SYSTEM—see Cooling System, Direct Expansion.
DISCHARGE AIR—conditioned air that is distributed to the controlled
DISTRIBUTED CONTROL—a control system built up of stand-alone
controllers, with controllers installed near the controlled equipment to
distribute the processing to each stand-alone DDC panel, with a limitedHVAC CONTROLS—OPERATION & MAINTENANCE
number of utilized sensor inputs and control outputs to a controller,
often 48 or less, so failure of any single module will not cause the loss
of more than the number of points served by that module.
DISTRIBUTED PROCESSING SYSTEMS—a control system with a number of microprocessor-based modules, each performing its own specified
task, yet working together as an integrated system under the supervision of a central microprocessor or computer.
DRY AIR—air without water vapor; air only.
DRY BULB TEMPERATURE—the temperature of a gas or mixture of
gases indicated by a thermometer. Air temperature as read by any ordinary dry bulb thermometer.
DRYER—device containing a desiccant, placed in the refrigerant circuit
to collect and hold moisture in the system in excess of the amount that
can be tolerated by the system refrigerant.
DUCT—a passageway made of sheet metal or other suitable material,
not necessarily leaktight, used for conveying air or other gases at low
DUTY CYCLING—energizing a load for part of a specified time period.
Accomplished by a duty cycler.
DYNAMIC CONTROL—a process that optimizes operation of HVAC
system components, such as air handling units, converters, chillers, and
boilers, by increasing and decreasing setpoints or starting and stopping
equipment in response to heating and cooling needs of downstream
equipment. A requirement of dynamic control is input data as to heating/cooling demand status of downstream equipment, therefore dynamic control requires controllers connected in a communications network.
ECONOMIZER CYCLE CONTROL—a form of control that positions
dampers in an HVAC system to introduce up to 100% outside air for free
cooling whenever the outside temperature is below the required supply
air temperature, often about 55°F.GLOSSARY OF TERMS
ELECTRIC CONTROL SYSTEM—a system of controls utilizing electric
devices for sensing controlled parameters and for positioning actuators
on controlled devices.
ELECTRICAL BILLING CHARGE, UTILITY—a charge for the use of a
unit of electricity. See Demand Charge. See also Watt, Kilowatt, and
ELECTRICAL CIRCUIT—a power supply, a load, and a path for current
flow are the minimum requirements.
ELECTROMECHANICAL—a term used to describe devices which contain both electrical and mechanical components.
ELECTRONIC AIR CLEANER—a device that cleans circulating air by
producing an electric field to ionize particulate contaminants in the air
and then collect ionized particles on electrically charged plates.
ELECTRONIC CONTROL SYSTEM—a system of controls utilizing electronic devices for sensing controlled parameters and for providing input
to devices for positioning actuators on controlled devices.
ELEMENT, ELECTRIC DUCT HEATING—a unit assembly consisting of
electric resistance coils, insulated supports, and duct mounting frame
with control box mounting operating and safety controls with terminals
for connecting the assembly to electric power.
EMCS—acronym for energy management and control system.
ENERGY (CONSUMPTION) CHARGE—that part of an electric bill
based on kWh consumption. Expressed in cents per kWh. Energy charge
covers the cost of utility fuel, general operating costs, and part of the
amortization of the utility’s equipment.
ENERGY EFFICIENCY RATIO—a term used to measure the efficiency
of air conditioning equipment components, abbreviated EER. It is defined as the number of Btus removed, divided by the power consumed
in watts at standard test conditions. See also Seasonal Energy Efficiency
ENERGY MANAGEMENT—the process of managing energy consumption, usually in a building, to conserve energy.
ENERGY MANAGEMENT SYSTEM—a system based on a microprocessor, microcomputer, or minicomputer whose primary function is to control energy-using equipment so as to reduce the amount of energy used.
ENTHALPY—the total energy or heat content of the air. Includes sensible heat due to air temperature and latent heat due to moisture content. Expressed in Btu per pound of dry air.
economizer control in which 100% outdoor air and mechanical cooling
are used simultaneously for the most economical operation.
ENTHALPY CHANGEOVER CONTROL—a system of changeover between natural cooling with outdoor air and mechanical cooling with
refrigeration. Control devices compare enthalpy or total heat values of
return air and outdoor air, then position the economizer cycle to admit
air with lowest enthalpy. This may cause mechanical refrigeration of
100% outside air when outside air enthalpy is lower than return air
enthalpy, although dry bulb temperature is not low enough for supply
air to space.
ENVIRONMENTAL CONTROL SYSTEM—the process of controlling
the environment by heating, cooling, humidifying, dehumidifying, or
cleaning the air. This book uses the term HVAC control system for the
same process.
EVAPORATION—a change of state from liquid to vapor.
EVAPORATOR COIL, REFRIGERANT—in mechanical refrigeration, the
part of the refrigeration system where the refrigerant produces a cooling
effect by vaporizing or absorbing heat from air or water, a section of
coiled tubing where liquid refrigerant absorbs heat and evaporates.
EXHAUST AIR—that air which is removed from the conditioned space
by the ventilation system and discharged outdoors.GLOSSARY OF TERMS
EXPANSION DEVICE—in a mechanical refrigeration system, a restriction or orifice which regulates the flow of refrigerant into the evaporator
coil. May be in the form of a thermal expansion valve or a capillary tube.
FACE AND BYPASS DAMPER SYSTEM—an airflow system in which
the airflow is divided to flow through the cooling or heating coil and
face damper and around the coil through the bypass damper. Dampers
work in opposition, face damper closes while bypass damper opens, and
vice versa to regulate the amount of air that is conditioned. Used with
either a chilled or hot water coil. Proprietary systems include internal
face and bypass dampers in steam coil.
FACILITY AUTOMATION SYSTEM—see Building Automation System.
FAIL-SAFE—in HVAC controls terminology, placing damper or valve in
normal position which will minimize damage to building in case of
controls failure, such as placing heating valves open and humidifier
valves closed upon loss of control power; in BAS terminology, returning
all controlled devices to conventional control in case of load management panel failure.
FAN COIL UNIT—a complete unit located in the room being conditioned consisting of a coil through which hot or chilled water is circulated, a fan that circulates room air through the coil, a filter to remove
lint and dust, a cabinet, a grille, and a control system. The chilled water
or heated water is supplied from equipment located remotely from the
FILTER—a device that removes contaminants from liquids or gases such
as air.
FIRMWARE—software programmed into read only memory (ROM) and
erasable programmable read only memory (EPROM) chips. Software
may not be changed without physically altering the chip.
FLOW RATE—the rate at which fluids or gases will flow over a specified amount of time. Units are gallons per minute (gpm) for water,HVAC CONTROLS—OPERATION & MAINTENANCE
pounds per hour for steam, cubic feet per hour (cfh) for natural gas, and
cubic feet per minute (cfm) for air.
FLUID—gas, vapor, or liquid.
FULL LOAD CURRENT—see Running Current.
GRAPHIC SEQUENCE OF OPERATION—a drawing or graphical representation of the sequence of operation, showing all interlocks and
control loop sequences between input and output points, and inputs,
outputs, and logic blocks.
GRAPHICS—a pictorial representation on a computer screen of material, often control diagrams or building system schematics.
HAND-HELD TERMINAL—a portable device, control system manufacturer-specific, which can be connected directly to a communications port
on a digital controller and through which the digital controller can be
interrogated and/or programmed.
HARD WIRING—permanent wiring.
HEADER—a manifold or supply pipe to which a number of branch
pipes are connected.
HEAT EXCHANGER—a device specifically designed to transfer heat
between two physically separated fluids.
HEAT OF FUSION—latent heat involved in the change between solid
and liquid states. For ice melting to water—144 Btu/pound at 32°F.
HEAT OF VAPORIZATION—latent heat involved in the change between liquid and vapor states. For water—970 Btu/pound at 212°F.
HEAT PUMP—mechanical refrigeration system with the added capability of reversing the normal cooling cycle so the evaporator removes heat
from outdoor air and the condenser rejects that heat plus the heat of
compression to indoor air to heat the space. During cooling, the functions are normal.GLOSSARY OF TERMS
HEAT RECLAMATION—the process of reclaiming waste heat from
such sources as exhaust fans, condenser coils, and hot water drains to do
useful work, such as heating makeup air and heating domestic or process water.
HEATING SYSTEM, DIRECT FIRED—a heating system for outside air
in which combustion takes place in the airstream being introduced into
the building. The outdoor air temperature is increased by direct contact
with the flame of the fuel. Recirculation is restricted by code. Fuel efficiency is 100 percent.
HEATING SYSTEM, DUCT HEATER—a heating system in which the
heater is installed directly in the distribution duct of a central air conditioning or heating system. May be either electric, gas-fired, or oil-fired.
HEATING SYSTEM, ELECTRIC—a heating system that consists of one
or more stages of resistive heating elements installed in a duct, central
furnace, or boiler.
which heated water is supplied by a central hot water boiler.
HEATING SYSTEM, INDIRECT FIRED—a heating system in which
combustion takes place in a boiler or furnace. The fuel is burned in a
combustion chamber and the flue gases do not mix with the air delivered to the space.
HEATING SYSTEM, RADIANT—a heating system in which only the
heat radiated from the heat exchanger is effective in providing the heating requirements. The term radiant heating may include both radiant
panels and radiant strips.
HEATING SYSTEM, STEAM—a heating system in which heat is transferred from the heat source, such as a boiler, to the heating units by
means of steam. Steam pressure may be above atmospheric pressure or
at, or below atmospheric pressure.
HEATING SYSTEM, WARM AIR—a warm air heating plant consisting
of a heating unit (electric or fuel burning furnace) enclosed in a casing,HVAC CONTROLS—OPERATION & MAINTENANCE
from which the heated air is distributed to various rooms in a building
through ducts.
HIGH LIMIT CONTROL—a device that normally monitors the condition of the controlled medium and interrupts system operation if the
monitored condition becomes excessive.
HIGH SIDE—parts of the refrigerating system subjected to condenser
pressure or higher; the system from the compression side of the compressor through the condenser to the expansion point of the evaporator.
HORSEPOWER—unit of power in foot-pound-second system; work
done at the rate of 550 ft-lb per sec, or 33,000 ft-lb per min.
HOT DECK—the heating section of a multi-zone or dual-duct system.
HUMIDIFICATION—the process of increasing the water vapor content
of the conditioned air.
HUMIDIFIER—a device to add moisture to the air.
HUMIDISTAT—a regulatory device, actuated by changes in humidity,
used for the automatic control of relative humidity.
HUMIDITY—water vapor within a given space.
HUMIDITY RATIO—see Specific Humidity.
HUNTING—an undesirable condition where a controller is unable to
stabilize the state of the controlled medium causing rapid cycling.
HVAC—Heating, Ventilating, and Air Conditioning.
HYBRID CONTROL SYSTEM—a system of controls utilizing electric
devices or electronic devices for sensing controlled parameters and using pneumatic actuators for positioning controlled devices.
HYDROMETER—an instrument that, by the extent of its submergence,
indicates the specific gravity of the liquid in which it floats.GLOSSARY OF TERMS
HYDRONIC SYSTEM—a heating and/or cooling system that uses a liquid (usually hot or cold water) as the medium for heat transfer.
HYDRONICS—the science of heating and cooling with liquids.
HYGROMETER—an instrument responsive to humidity conditions,
usually calibrated in percent relative humidity of the atmosphere.
“IN” CONTACTS—those relay contacts which complete circuits when
the relay armature is energized. Also referred to as “Normally Open
INDOOR AIR QUALITY—the condition of air within the built environment, measured in multiple parameters including temperature, humidity, air motion, and presence of gaseous and particulate contaminants.
Abbreviated IAQ.
INDUCTIVE LOADS—loads whose voltage and current are out of
phase. True power consumption for inductive loads is calculated by
multiplying voltage, current, and the power factor of the load.
INFILTRATION—in air conditioning, the natural leakage of unconditioned outdoor air into a building.
INPUT/OUTPUT (I/O)—The acronym I/O refers to inputs and outputs
of a digital controller; analog inputs (AI), digital inputs (DI), analog
outputs (AO), and digital outputs (DO). Analog inputs (AI) are from
analog sensors of temperature, pressure, humidity, flow. Digital inputs
(DI) are from digital sensors such as motor status contacts, flow
switches, switch position indicators, and pulse output devices. Analog
outputs (AO) position modulating devices. Digital outputs (DO) operate
on/off or open/close controlled devices.
INRUSH CURRENT—the current that flows the instant after the switch
controlling current flow to a load is closed. Also called “Locked Rotor
INTERSTAGE DIFFERENTIAL—in a multistage HVAC system, the
change in temperature at the thermostat needed to turn additional heating or cooling equipment on.HVAC CONTROLS—OPERATION & MAINTENANCE
I/O UNIT—device installed on a digital controller to provide additional
point capacity and communicate with the stand-alone digital controller
on a LAN. An I/O unit itself is not stand-alone because the control
program does not reside in the microprocessor of the I/O unit.
INPUT DEVICE—device to provide input to control system, such as
temperature sensor or operating status device.
ISOTHERMAL PROCESS—a process in which there is no change in dry
bulb temperature.
KILOWATT—1,000 watts. Abbreviated kW.
KILOWATT-HOUR—a measure of electrical energy consumption: 1,000
watts being consumed per hour. Abbreviated kWh.
kW DEMAND—the maximum rate of electrical power usage for a 15- or
30-minute interval in a commercial building for each billing period. A
utility meter records this maximum rate, and customers are billed for
this peak rate usually once per month. Abbreviated kWD.
kWh CONSUMPTION—the amount of electrical energy used over a
period of time; the number of kWh used per month. Often called “Consumption.”
LADDER DIAGRAM—a wiring diagram showing the system connections between two conductors, represented with system conductors run
vertically and with connections run across, as rungs on a ladder.
LAG—a delay in the effect of a changed condition at one point in the
system on some other condition to which it is related. Also, the delay in
action of the sensing element of a control, due to the time required for
the sensing element to reach equilibrium with the property being controlled; i.e., temperature lag, flow lag, etc.
LATENT HEAT—the amount of heat necessary to change a quantity of
water to water vapor without changing either temperature or pressure,
when water is vaporized and passes into the air along with the vapor.
Likewise, latent heat is removed when water vapor is condensed.GLOSSARY OF TERMS
LIGHT EMITTING DIODE—a low current and voltage light used as an
indicator. Abbreviated LED.
LIMIT—control applied in the line or low voltage control circuit to break
the circuit if conditions move outside a preset range. In a motor, a switch
that cuts off power to the motor windings when the motor reaches its
full open position.
LIMIT CONTROL—a temperature, pressure, humidity, dew-point, or
other control that is used as an override to prevent undesirable or unsafe
conditions in a controlled system.
LIMIT SHUTDOWN—a condition in which the system has been
stopped because the value of the temperature or pressure has exceeded
a pre-established limit.
LINE VOLTAGE—in the control industry, the electric supply voltages as
opposed to low voltage.
LIQUID LINE—the tube or pipe carrying the refrigerant liquid from the
condenser or receiver of a refrigerating system to the evaporator or other
pressure-reducing device.
LOAD—that part of an electrical circuit in which useful work is performed. In a heating or cooling system, the heat transfer that the system
will be called upon to provide. Any equipment that can be connected to
a load management system.
LOAD FACTOR—a comparison of kilowatt-hours of electricity consumed to the peak rate at which power was consumed. Load factor is
always a number between zero and one and is expressed as the kilowatthours consumed over the specified period divided by the product of the
kilowatt peak demand registered times the number of hours in the period.
LOAD SHEDDING—the process of turning off electrical loads under
specified conditions, primarily to reduce demand.
LOCAL AREA NETWORK (LAN)—a communications bus that interconnects digital controllers for peer-to-peer communications with differHVAC CONTROLS—OPERATION & MAINTENANCE
ent levels of LANs within a single DDC system. In this case a digital
controller on a higher level LAN acts as a network controller to the
controllers on the lower level LAN. The network controller must have at
least two LAN communications ports with one port supporting peer-topeer communications with other digital controllers on the higher level
LAN and the other port supporting communications with the digital
controllers on the lower level LAN. LANs permit sharing global information, making it possible to apply building-wide control strategies,
such as peak demand limiting, permit dynamic control strategies, allow
coordinated response to alarm conditions, and allow remote monitoring
and programming of digital controllers.
LOCAL LOOP CONTROL—an existing control, such as a thermostat
system, that will continue to function after the installation of a BAS or
DDC system when the latter is not operating.
LOCKED ROTOR CURRENT—see Inrush Current.
LOUVER—an assembly of sloping vanes intended to permit air ventilation to pass through and to inhibit the transfer of water droplets.
LOW LIMIT CONTROL—a device that normally monitors the condition
of the controlled medium and interrupts system operation if the monitored condition drops below the desired minimum value.
LOW SIDE—the refrigerating system from the expansion point to the
point where the refrigerant vapor is compressed; where the system is at
or below evaporator pressure.
LOW VOLTAGE—in the control industry, a power supply of 25 volts or
MAIN—a pipe or duct for distributing to, or collecting from, various
MAKEUP AIR—outdoor air that is brought into a building to compensate for air removed by exhaust fans or other methods.
MAKEUP AIR SYSTEM—a ventilating system used to replace exhaustedGLOSSARY OF TERMS
air with outdoor air that is then heated or cooled. The system includes
a supply fan, a filter section, a heating section (either indirect or direct
heat), and automatic controls to regulate the air temperature.
MANIFOLD—portion of a main in which several branches are close
together. Also, a single piece in which there are several fluid paths. Also
called a “Header.”
MANOMETER—an instrument for measuring pressures. Essentially a
U-tube partially filled with a liquid, usually water, mercury, or a light
oil, so constructed that the amount of displacement of the liquid indicates the pressure being exerted on the instrument.
MECHANICAL REFRIGERATION SYSTEM—a cooling system consisting basically of a refrigerant compressor, condenser coil, expansion device, and evaporator coil. In a basic cycle, the refrigerant is compressed,
liquefied and cooled in an expansion device in the condenser, below its
boiling point. It then enters the evaporator coil where it boils, absorbing
heat from its surroundings. It is then compressed again and a new cycle
MEDIUM, HEATING—a solid or fluid, such as water, steam, air or flue
gas, used to convey heat from a boiler, furnace or other heat source, and
to deliver it, directly or through a suitable heating device, to a substance
or space being heated.
MICROPROCESSOR—the central processing unit (CPU) of a DDC
system that contains all the registers and logic circuitry that make it
possible for digital controllers to compute; in EMCS systems, a small
computer used in load management to analyze energy demand and
consumption so that loads are turned on and off according to a predetermined program.
MINIMUM ON-TIME—the shortest period of time that a load can be
energized when it is being duty cycled.
MINIMUM OUTSIDE AIR REQUIREMENTS—the code-mandated volume of outside air or equivalent outside air needed to replenish interior
space air to maintain acceptable indoor air quality.HVAC CONTROLS—OPERATION & MAINTENANCE
MIXED AIR—a mixture of air in an air handling system composed of
return air and outdoor air.
MIXING BOX—a container, located at the room being conditioned, in
which hot and cold air is mixed as required to maintain the desired
room temperature.
MODULATING CONTROL—a mode of automatic control in which the
action of the final control element is proportional to the deviation, from
set point, of the controlled medium.
MODULATING MOTOR—an electric motor, used to drive a damper or
valve, which can position the damper or valve anywhere between fully
open or fully closed in proportion to deviation of the controlled medium.
MODULATING—tending to adjust by increments and decrements.
MODULATING RANGE—see Proportional Band.
MODULE—a microprocessor-controlled device in a distributed processing system that performs a specific task under the supervision of the
central computer.
MODUTROL MOTOR—Honeywell’s trade name for a line of electric
and electronic two-position and modulating motor actuators used to
position dampers or valves.
MOISTURE CONTENT—amount of water vapor in a given amount of
air, usually expressed in grains of moisture per pound of dry air. (7,000
grains are equal to 1 lb.) Also called Specific Humidity.
MORNING PICKUP OR WARMUP—a control system that keeps outside air dampers closed after night setback until the desired space temperature is achieved.
MULTISTAGE THERMOSTAT—a temperature control that sequences
two or more switches in response to the amount of heating or cooling
MULTIZONE SYSTEM—centralized HVAC system that serves several
zones from a multi-zone unit with each zone having a thermostat.
NC—normally closed; in relays, normally closed contacts are closed
when the relay is de-energized; in dampers and valves, the position
assumed when the actuator is de-energized.
NIGHT SETBACK—the ability to reduce heating expense during unoccupied hours by lowering temperature, closing outside air dampers, and
intermittently operating blowers.
NO—normally open; in relays, normally open contacts are open when
the relay is de-energized; in dampers and valves, the position assumed
when the actuator is de-energized.
NOTEBOOK COMPUTER—a small personal computer.
OFFSET—a sustained deviation between the actual control point and the
set point under stable operating conditions.
ON-OFF CONTROL—a simple control system, consisting basically of a
switch, in which the device being controlled is either fully on or fully off
and no intermediate positions are available.
OPTIMUM START/STOP—a refined form of HVAC control that automatically adjusts the programmed start/stop schedule depending on
inside and outside air temperature and humidity, resulting in the latest
possible start and earliest possible stop of the HVAC equipment.
ORIFICE—an opening or construction in a passage to regulate the flow
of a fluid.
“OUT” CONTACTS—those relay contacts which complete circuits when
the relay coil is de-energized. Also referred to as “Normally Closed
OUTPUT SIGNAL CONVERSION—the changing of one kind of control
output into a proportionally related signal appropriate for direct actuation of the controlled device, such as 4 to 20 mA analog output signalsHVAC CONTROLS—OPERATION & MAINTENANCE
converted by a transducer to 3 to 15 psig pneumatic pressure or a contact closure originating in a digital controller converted into an on/off or
open/close signal to a 2-position device.
OUTSIDE AIR—air that is brought into the ventilation system from
outside the building and, therefore, has not been previously circulated
through the system. Also called “fresh air.”
OVERRIDE—a manual or automatic action taken to bypass the normal
operation of a device or system.
PACKAGED SYSTEM—a complete set of components and controls factory-assembled for ease of installation. A packaged system may perform
one or more of the air conditioning functions.
PACKAGED TERMINAL AIR CONDITIONER (PTAC)—a single-package air conditioning unit in a decorative cabinet for mounting in the
conditioned space, with electric-drive, air-cooled refrigerating system
and heating system as either electric resistance coil, reverse-cycle heat
pump, or hydronic coil.
PEAK DEMAND—the greatest amount of kilowatts needed during a
demand interval.
PEAK DEMAND LIMITING—see Demand Limiting.
PEAK LOAD CONTROL—see Demand Limiting.
PEAK LOAD LIMITING—see Demand Limiting.
PEER-TO-PEER COMMUNICATION—the relationship of controllers
connected on a communications LAN that act independently and communicate with each other as equals to pass information which facilitates
PHASE—an electrical term used to describe the number of distinct harmonic waves in alternating current electrical services. Residential service is single-phase; commercial facilities are usually three-phase.GLOSSARY OF TERMS
PID—an acronym referring to the three types of control action, proportional, integral, and derivative, that are used in controlling modulating
PILOT-DUTY RELAY—a relay used for switching loads such as another
relay or solenoid valve coils. The pilot duty relay contacts are located in
a second control circuit. Pilot duty relays are rated in volt-amperes (VA).
PLENUM CHAMBER—an air compartment connected to one or more
distributing ducts.
PNEUMATIC—operated by air pressure.
PNEUMATIC CONTROLS—a system of controls utilizing air pressure
for sensing controlled parameters and for positioning actuators on controlled devices.
POINT—an individual monitor, control, or sensing device connected to
a BAS or DDC system, such as a temperature sensor or a relay.
POTENTIAL TRANSFORMER—a voltage transformer. The voltage supplied to a primary coil induces a voltage in a secondary coil according
to the ratio of the wire windings in each of the coils.
POTENTIOMETER—an electromechanical device having a terminal
connected to each end of the resistive element, and third terminal connected to the wiper contact. The electrical input is divided as the contact
moves over the element, thus making it possible to mechanically change
the resistance.
POWER—in electricity, the watt. A time rate measurement for the use of
electrical energy. Joules per second.
POWER FACTOR—a ratio, sometimes expressed as a percent of actual
power (watts) in an ac circuit to apparent power (volt-amperes). A
measure of power loss in an inductive circuit. When the power factor is
less than 0.8, the utility may impose a penalty, as prescribed in the utility
POWER FACTOR CHARGE—a utility charge for “poor” power factor. It
is more expensive to provide power to a facility with a poor power
factor (usually less than 0.8).
POWER FACTOR CORRECTION—improvement of power factor on a
building’s electrical service by installation of capacitors on the utility’s
supply line.
POWER LINE SUBCARRIER—a device to allow the use of a building’s
existing electrical power system to carry the signals of the BAS or DDC
POWER SUPPLY—the voltage and current source for an electrical circuit. A battery, a utility service, and a transformer are power supplies.
PREHEAT—a process of raising the temperature of outdoor air before
incorporating it into the rest of the ventilating system. Used when large
amounts of very cold outdoor air must be used.
PRESSURE—the normal force exerted by a homogeneous liquid or gas,
per unit of area, on the wall of its container.
PRESSURE, ABSOLUTE—the sum of gauge pressure and atmospheric
pressure. Absolute pressure can be zero only in a perfect vacuum.
PRESSURE, ATMOSPHERIC—the pressure exerted in every direction at
any given point by the weight of the atmosphere. It is the pressure indicated by a barometer. Standard Atmospheric Pressure or Standard
Atmosphere is the pressure of 76 cm of mercury having a density of
13.5951 grams per cubic centimeter, under standard gravity of 980,665
cm per sec. It is equivalent to 14.696 psi or 29.921 inches of mercury at
PRESSURE, SUCTION—the refrigerant pressure as measured at the inlet
of a compressor in a direct expansion refrigeration system. Also known
as “backpressure.”
PRESSURE CONTROLS—used as limit protectors in the cooling system.
They establish pressure control limits to protect the system from exGLOSSARY OF TERMS
tremes in refrigeration suction and discharge line pressures. If the pressure deviates from normal, the pressure control breaks the circuit to the
compressor until the pressure returns to normal. Pressure controls have
automatic or manual reset, depending upon the construction of the
equipment and preference of the manufacturer.
PRESSURE DROP—the difference between the upstream pressure and
the downstream pressure of a fluid or gas passing through a pressure
loss causing device, such as a damper or valve.
PRESSURE GAUGE—pressure measured above atmospheric pressure;
indicated by a pressure gauge. Units are pounds per square inch gauge
PRESSURE HEAD—operating pressure measured in the discharge line
at a compressor outlet.
PRESSURE REGULATOR—automatic valve between the evaporator
outlet and compressor inlet that is responsive to pressure or temperature. It functions to throttle the vapor flow when necessary to prevent
the evaporator pressure from falling below a preset level.
PRIMARY CONTROL—a device that directly or indirectly controls the
control agent in response to needs indicated by the controller. Typically,
a motor, valve, relay, or similar device.
PROPORTIONAL BAND—the range of values of a proportional positioning controller through which the controlled variable must pass to
move the final control element through its full operating range. Commonly used equivalents are “throttling range” and “modulating range.”
PROPORTIONAL CONTROL—see Modulating Control.
PSYCHROMETER—an instrument with wet and dry bulb thermometers, for measuring the amount of moisture in the air. See Wet Bulb
RADIATION, THERMAL (HEAT)—the transmission of energy by
means of electromagnetic waves of very long wavelength. Radiant enHVAC CONTROLS—OPERATION & MAINTENANCE
ergy of any wavelength may, when absorbed, become thermal energy
and result in an increase in the temperature of the absorbing body.
RADIATOR—a heating unit exposed to view within the room or space
to be heated. A radiator transfers heat by radiation to objects within
visible range, and by conduction to the surrounding air which in turn is
circulated by natural convection; a so-called radiator is also a convector,
but the term radiator has been established by long usage.
RAPID CYCLING—see Hunting and Short Cycling.
RECEIVER—storage chamber for liquid refrigerant in a mechanical refrigeration system; often the bottom part of the condenser.
RECIRCULATED AIR—return air passed through the conditioner before
being again supplied to the conditioned space.
RECLAMATION—the process of reclaiming and reusing discharged
heat from such sources as exhaust fans, condenser coils, and hot water
drains to do useful work.
REFRIGERANT—a substance with a large latent heat of vaporization
and low boiling point that produces a refrigerating effect by absorbing
heat while expanding or vaporizing (boiling).
REFRIGERATING SYSTEM, ABSORPTION—a refrigerating system in
which the refrigerated gas evolved in the evaporator is taken up in an
absorber and released in a generator upon the application of heat.
more low sides connected to a single, central high side; a multiple system.
REFRIGERATING SYSTEM, CHILLED WATER—an indirect refrigerating system employing water as the circulating liquid.
which the pressure-imposing element is mechanically operated.GLOSSARY OF TERMS
REFRIGERATING SYSTEM, DIRECT-EXPANSION—a refrigerating system in which the evaporator is in direct contact with the refrigerated
material or space or is located in air circulating passages communicating
with such spaces.
evaporator coil produces cooling by absorbing heat from the surrounding air, raising the refrigerant to its boiling point and causing it to vaporize. The superheated vapor flows through the condenser, which condenses it into a liquid and gives off heat picked up in the evaporator
coil. Then the liquid flows to the expansion point, where it expands
(lowering its temperature and pressure) to start the cooling cycle again.
REFRIGERATING SYSTEM, SINGLE-PACKAGE—a complete factorymade and factory-tested refrigerating system in a suitable frame or enclosure, which is fabricated and shipped in one or more sections and in
which no refrigerant-containing parts are connected in the field.
REFRIGERATION SYSTEM—combination of interconnected refrigerantcontaining devices in which the refrigerant is circulated for the purpose
of extracting heat to produce cooling.
REHEAT—the process of adding heat to air to maintain the correct temperature after it has previously been cooled to some specified dew point
to control humidity.
RELATIVE HUMIDITY—the ratio of the existing vapor pressure of the
water in the air to the vapor pressure of water in saturated air at the
same dry bulb temperature.
RELAY—an electromechanical switch that opens or closes contacts in
response to some controlled action. Relay contacts are normally open
(NO) and normally closed (NC).
RELAY, MAGNETIC—solenoid-operated relay or contactor; a switching
relay that utilizes an electromagnet (solenoid) and an armature to provide the switching force.
control point for a space from outside the space. Often used in public
RESET—a process of automatically adjusting the control point of a given
controller to compensate for changes in the outdoor temperature. The
hot deck control point is normally reset upward as the outdoor temperature drops. The cold deck control point is normally reset downward as
the outdoor temperature increases.
RESET RATIO—the ratio of change in outdoor temperature to the
change in control point temperature. For example, a 2:1 reset ratio
means that for every 2 degrees change in outdoor temperature the control point will increase 1 degree.
RESISTANCE—the opposition that limits the amount of current that can
be produced by an applied voltage in an electrical circuit. Measured in
ohms, abbreviated with the Greek letter omega.
RESISTANCE TEMPERATURE DEVICE (RTD)—a wire-wound electronic temperature sensor device with stable temperature coefficient,
such as a 1,000 ohm Balco sensor with a resistance constant of 2.2 ohms
per °F.
RESISTIVE LOADS—electrical loads whose power factor is one. Usually
contain heating elements.
RESOLUTION—the number of possible states an input value or output
value can take; a function of the digital controller I/O circuitry, an analog-to-digital (A/D) converter for input and a digital-to-analog (D/A)
converter for output. Ten-bit resolution has 1,024 possible states; eightbit resolution has 256 possible states.
RESTORE—to energize a load that has been shed.
RETURN AIR—air that is drawn back into the ventilation system from
the controlled space.
ROOFTOP UNIT—HVAC system placed on a roof and connected to
ducts that supply conditioned air to the area below it.GLOSSARY OF TERMS
RUNNING CURRENT—the current that flows through a load after inrush current. Usually called “Full Load Current.”
SEASONAL PEAK—the maximum demand placed on the utility’s capacity resulting from seasonal factors. Some utilities have summer
peaks, some have winter peaks, some have both.
SELF DIAGNOSTICS—the function of a control system, usually DDC, to
detect errors, diagnose the cause of the errors, and notify the operator of
the problem.
SENSIBLE HEAT—that heat which changes the temperature of the air
without a change in moisture content. Changes in dry bulb thermometer
readings are indicative of changes in sensible heat.
SENSING ELEMENT—the first system element or group of elements.
The sensing element performs the initial measurement operation.
SENSOR—a control system sensing element.
SEQUENCER—an electronic device that may be programmed or set to
initiate a series of events and to make the events follow in sequence.
SEQUENCING CONTROL—a control that energizes successive stages
of heating or cooling equipment as its sensor detects the need for increased heating or cooling capacity. May be electronic or electromechanical. See Sequencer and Step Controller.
SETBACK—reduction of heating or cooling at night or during hours
when a building is unoccupied.
SET POINT—the value on the controller scale at which the controller
indicator is set.
SHIELDED CABLE—special cable used with equipment that generates
a low voltage output. Used to minimize the effects of frequency “noise”
on the output signal.
SHORT CYCLING—unit runs and then stops at short intervals; generally, this excessive cycling rate is hard on the system equipment.HVAC CONTROLS—OPERATION & MAINTENANCE
SINGLE-ZONE SYSTEM—HVAC system controlled by one thermostat.
SPACE THERMOSTAT—a thermostat whose sensor is located in the
STAND-ALONE CONTROL—the ability of a digital controller to perform required control and energy management functions without connection to another digital controller or central site computer. Stand-alone
ability requires a time clock, a microprocessor, microchip resident control programs, PID control, a communications port for interfacing with
and programming the controller, firmware for interrogation and programming, and I/O for sensing and effecting control of its control environment.
STAND-ALONE SYSTEM—a BAS or DDC system that performs all
control functions without direction from a central unit.
STANDARD AIR—a measure of air conditions used in psychrometric
calculations. In the inch-pounds (I-P) system, dry air at 70°F and 14.696
psi. At these conditions, dry air has a mass density of 0.075 pounds per
cubic foot.
START/STOP SCHEDULING—see Time Based Scheduling.
STARTER, MAGNETIC MOTOR—an adaptation of the basic contactor
with overload relays and selector switches or pushbuttons for start-stop.
Combination starters are a combination of starters and disconnect devices, either switches or circuit breakers, in a common enclosure. Starters
for large motors may be reduced voltage type with reactors, step resistors, and other features required for the specific application.
STEP CONTROLLER—an electromechanical device used with electric or
pneumatic systems, which may be set to initiate a series of events and
to make the events follow in sequence.
SUBCOOLING—liquid temperature and pressures are directly related;
subcooling is cooling the liquid line below the saturation temperature
corresponding to pressure. Subcooling is measured by comparing the
temperature of the liquid line to what the liquid temperature would
ordinarily be at the measured discharge pressure.GLOSSARY OF TERMS
SUBLIMATION—change of state directly from solid to gas without
appearance of liquid.
SUCTION LINE—the tube or pipe that carries the refrigerated vapor
from the evaporator to the compressor inlet.
SUPERHEAT—refrigerant vapor temperature and pressure are directly
related; superheating is raising the temperature of the vapor above the
saturation temperature corresponding to the pressure. Superheat is
measured by comparing the temperature measured with that which
would correspond to the measured pressure.
SURGE—an increase in electric energy on a conductor, such as would be
caused by lightning or by starting a large motor.
SWITCHING RELAYS—relays are devices that operate by a variation in
the conditions of one electrical circuit, to affect the operation of devices
in the same or another circuit. General purpose switching relays are
used to increase switching capability and isolate electrical circuits, such
as in systems where the heating and cooling equipment have separate
power supplies, and provide electrical interlocks with the system.
SYSTEM, FORCED-CIRCULATION—a heating, air conditioning, or refrigerating system in which the heating or cooling fluid circulation is
effected by a fan or pump.
SYSTEM, GRAVITY CIRCULATION—a heating or refrigerating system
in which the heating or cooling fluid circulation is effected by the motive
head due to difference in densities of cooler and warmer fluids in the
two sides of the system.
TEMPERATURE—the thermal state of matter with reference to its tendency to communicate heat to matter in contact with it. If no heat flows
upon contact, there is no difference in temperature.
TERMINAL CONTROL UNIT (TCU)—an off-the-shelf, stand-alone digital controller equipped for communication with digital controllers on a
lower level LAN and that may deviate from the stand-alone controller
only in receiving energy management and time information from aHVAC CONTROLS—OPERATION & MAINTENANCE
stand-alone controller. A TCU is commonly application specific and is
used for distributed control of specific HVAC subsystems, such as for
control of small air handling units (AHUs), variable air volume (VAV)
boxes, fan coil units, and heat pumps.
TERMINAL REHEAT SYSTEM—centralized blower and cooling system
that supplies cool air to multiple zones. Each zone contains a hot water
coil or electric heater that reheats the cooled supply air as determined by
the zone thermostat.
THERM—measurement used by gas utilities for billing purposes. A
therm is equal to 100,000 Btus or about 100 cubic feet of natural gas.
THERMISTOR—semiconductor material that responds to temperature
changes by changing its resistance.
THERMOCOUPLE—a junction of two wires of dissimilar materials, not
necessarily metal, but with the property of generating an electromotive
force related to the temperature of their junction.
THERMOSTAT—an automatic control device responsive to temperature;
a temperature actuated switch; an instrument that responds to changes
in temperature and directly or indirectly controls temperature. The thermostat serves as the basic controller for the HVAC system.
THROTTLING RANGE—the range of values of a proportional controller through which the controlled variable must pass to drive the final
control element through its full operating range. Also called “Proportional Band.” In a thermostat, the temperature change required to drive
the potentiometer wiper from one end to the other, typically 3 to 5°F.
TIGHT SHUTOFF—virtually no flow or leakage through a valve in its
closed position.
TIME BASED SCHEDULING—the process of scheduling electrical loads
on and off based on the time of the day, the day of the week, the month,
and the date.GLOSSARY OF TERMS
TIME CLOCK—a mechanical, electrical, or electronic timekeeping device connected to electrical equipment for the purpose of turning the
equipment on and off at selected times.
TIME SCHEDULING—see “Time-based Scheduling.”
TON OF REFRIGERATION—refrigerating effect equal to 12,000 Btus per
hour, the amount of heat required to melt one ton of ice in 24 hours.
TOTAL HEAT—the sum of the sensible and latent heats. Changes in wet
bulb thermometer readings are indicative of changes in total heat. For
convenience total heat is measured from 0°F.
TRANSFORMER—the system power supply. A transformer is an inductive stationary device that transfers electrical energy from one circuit to
another. The transformer has two windings, primary and secondary. A
changing voltage applied to one of these, usually the primary, induces
a current to flow in the other winding. A coupling transformer transfers
energy at the same voltage; a step-down transformer transfers energy at
a lower voltage, and a step-up transformer transfers energy at a higher
TRANSIENT—a current or voltage change that is not related to the
normal system characteristics.
TRANSMITTER—a control input device, such as a pneumatic temperature transmitter that sends a pneumatic pressure signal proportional to
the sensed temperature.
TREND LOG—a graphic representation of trends, either on screen or in
printout form.
TRENDING—an accumulation of observed data points from DDC or
BAS that is plotted to show the trend changes in observed values.
TURNKEY INSTALLATION—an installation of a BAS or DDC system in
which the equipment manufacturer takes responsibility for all elements
including design engineering, application engineering, provision of the
BAS or DDC system and related equipment, general contracting, installation acceptance training, documentation, and system maintenance.HVAC CONTROLS—OPERATION & MAINTENANCE
UNIT COOLER—a direct-cooling, factory-made, encased assembly including a cooling element, fan and motor (usually), and directional
UNIT HEATER—a direct-heating, factory-made, encased assembly including a heating element, fan and motor, and directional outlet.
UNITARY SYSTEM—an HVAC system that uses packaged or unitary
equipment. May use a central fan system for ventilating air.
UNLOADER—device on or in a compressor for equalizing the high and
low side pressures for a brief period during starting, in order to decrease
the starting load on the motor; also a device for controlling compressor
capacity by rendering one or more cylinders ineffective.
UTILITY SERVICE—a description of the characteristics of electrical service supplied by a utility company. Service characteristics frequently
found include 120/208 volt, 3-phase, 4-wire wye, 230/460 volt, 3-phase,
delta, and 277/480 volt, 3-phase, 4-wire wye.
UTILITY TRANSFORMER—a transformer, owned by the utility company, which is used to step-down the distribution voltage to the utilization voltage for a facility.
VALVE—a device in a pipe or tube that diverts, mixes, or stops fluid
flow by means of a flap, lid, plug, etc., acting to open and/or block
VALVE, AUTOMATIC CONTROL—a combined valve body and actuator or motor, actuated by an automatic control signal from a remote
point to position the valve either to open or close, or to proportion the
medium flow rate through the valve.
VALVE BODY—the portion of the valve through which the medium
VALVE, CHECK—a valve that allows fluid to flow in one direction only.GLOSSARY OF TERMS
VALVE, EXPANSION—a valve that controls the refrigerant flow to an
VALVE, MODULATING—a valve that can be positioned anywhere between fully on and fully off to proportion the rate of flow in response
to a modulating controller. See Modulating Control.
VALVE, ON-OFF—see Valve, Two-Position.
VALVE, SOLENOID—an electrically operated, two-position valve, either
normally open or normally closed, positioned by action of an electromagnet.
VALVE, THERMOSTATIC RADIATOR—a self-contained device that
combines a temperature-sensitive power unit with a valve. Mounts in
the hot water or steam supply pipe adjacent to a convector. May also be
applied to baseboard and finned-tube radiation. Requires no electricity.
VALVE, TWO-POSITION—a valve that is either fully open or fully
closed, with no positions between. Also called an “On-Off Valve.”
that supplies conditioned air at a fixed temperature to zones where a
temperature controlled airflow regulator varies the volume of air delivered to the space.
VELOCITY—a vector quantity that denotes at once the time rate and the
direction of a linear motion.
VELOCITY, OUTLET—the average discharge velocity of primary air
being discharged from the outlet, normally measured in the plane of the
VELOCITY, ROOM—the average sustained residual air velocity level in
the occupied zone of the conditioned space, measured in feet per
VELOCITY, TERMINAL—the highest sustained airstream velocity existing in the mixed air path at the end of the throw.
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