Control drawings and specifications use a standardized vocabulary that must be understood
before any wiring diagram, schematic, or sequence-of-operation document can be correctly
interpreted. This lesson introduces the core control system terms, the difference between
open and closed loop operation, the key parameters found on HVAC/R control drawings,
and how to read symbols and legends on ladder diagrams.
Control SystemsOpen & Closed LoopLadder Diagrams313A / 313D
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5.1.1 — Control System Terminology
HVAC/R control drawings and specifications use a consistent vocabulary across wiring
diagrams, control schematics, and sequence-of-operation documents. A firm grasp of
these terms allows a technician to follow signal paths from sensor to actuator, interpret
safety interlocks, and participate effectively in commissioning and troubleshooting.
System Components & Control Concepts
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Control System
An arrangement of components that regulates HVAC/R equipment to maintain desired
conditions. Every control system has at least one input (a sensor), a decision-making
device (a controller), and an output (an actuator or switching device).
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Setpoint
The target value the control system is designed to maintain — for example,
21 °C (70 °F) room temperature or 690 kPa (100 psi)
suction pressure. The controller continuously works to keep the controlled variable
at or near the setpoint.
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Controlled Variable
The physical parameter being monitored and regulated — such as temperature,
pressure, humidity, or airflow. The sensor measures this value and reports it to the
controller so that output can be adjusted to minimize deviation from setpoint.
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Manipulated Variable
The quantity the controller adjusts to influence the controlled variable —
for example, refrigerant flow through an expansion valve, fan speed via a VFD signal,
or the opening position of a chilled water valve. This is what the system
“turns up or down” to correct an error.
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Sensor / Transducer
Detects a physical condition and converts it to an electrical or pneumatic signal.
A thermostat sensing element, a pressure transducer producing a 4–20 mA
signal, and a humidity sensor producing a 0–10 VDC signal are all
examples of sensors used in HVAC/R controls.
💻
Controller
Compares the measured value from the sensor to the setpoint and sends a corrective
output signal to the actuator or switching device. Controllers range from simple
bimetallic thermostats to microprocessor-based DDC systems with full
programmable logic.
🔨
Actuator
Physically moves or adjusts a component in response to a controller signal.
Examples: a motorized valve actuator that modulates water flow, a damper actuator
that positions an outdoor air damper, or a relay coil that closes contacts to
start a compressor.
∥
Dead Band (Differential)
A range around the setpoint within which the controller does not change its output.
For example, a thermostat set to 22 °C with a 1 K dead band will not
change output between 21.5 °C and 22.5 °C. The dead band prevents
short-cycling of compressors, fans, and contactors.
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Interlock
Prevents a device from operating unless a prerequisite condition is met. A common
example: a condenser fan interlock that prevents the compressor from starting unless
the condenser fan contactor is confirmed closed. Interlocks protect equipment and
enforce correct operating sequences.
🚨
Safety Circuit
Monitors unsafe conditions and shuts down equipment to prevent damage or injury.
High-pressure cutouts, low-pressure cutouts, motor overloads, and freeze-stats are
all safety circuit devices. Safety circuits are typically wired in series in the
control circuit so any single device can interrupt equipment operation.
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DDC (Direct Digital Control)
Microprocessor-based control with programmable logic, used in building automation
systems (BAS). DDC controllers accept analogue and digital inputs, execute control
algorithms in software, and produce analogue and digital outputs — replacing
hardwired relay logic with flexible, adjustable programs.
Control Mode Types
The table below summarizes the seven control modes commonly referenced in HVAC/R
specifications, from the simplest on/off switching through progressive levels of
accuracy and complexity.
Control Mode
How It Works
Typical HVAC/R Use
Two-Position (On/Off)
Output is either fully ON or fully OFF based on whether the variable is above or below setpoint
Adds Integral action to Proportional — eliminates steady-state offset (droop) by accumulating error over time
Chilled water supply temperature reset, discharge air control
PID Control
Adds Derivative action to PI — anticipates change by reacting to the rate of error change, reducing overshoot
Electronic expansion valve superheat control, precision process cooling
PWM (Pulse Width Modulation)
Varies the ON-time (duty cycle) of a repeated switching signal to regulate average power to a component
ECM motor speed signal, solenoid valve duty-cycle control
5.1.2 — Open and Closed Loop Control
All control systems fall into one of two fundamental categories based on whether the
output is corrected using feedback from the process. Understanding which type is in
use is the starting point for any commissioning or troubleshooting task.
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Open Loop Control
The controller sends a fixed command based on a schedule or preset input
without receiving any feedback from the process. It cannot detect
whether the desired result was achieved or self-correct for disturbances.
Example
A timer-based defrost that runs heaters for 20 minutes every 6 hours,
regardless of actual frost accumulation on the coil.
✓ Simple and low cost to design and maintain
✓ No sensor feedback required
✗ Cannot respond to changing load conditions
✗ Must be manually adjusted when conditions change
↻
Closed Loop Control
A sensor continuously measures the controlled variable and feeds the measured value
back to the controller. The controller compares it to the setpoint and adjusts the
output to minimize the error. The system is self-correcting.
Example
A thermostat cycling a compressor on and off to maintain 22 °C room
temperature — the sensor reports actual temperature and the controller
responds to any deviation from setpoint.
✓ Self-correcting under varying loads and disturbances
✓ Standard in modern DDC and EEV applications
✗ More complex — requires accurate, reliable sensors
✗ Requires proper tuning to avoid cycling or instability
💡
Most Modern HVAC/R Systems Use Closed Loop Control
Direct digital control (DDC) systems, electronic expansion valves, inverter-driven
compressors, and modulating chilled-water valves are all closed loop systems.
Open loop control is still found in simple timed devices — defrost timers,
morning warm-up schedules — but the trend in commercial HVAC/R is toward
closed loop feedback for every regulated variable.
5.1.3 — Key Terms on Control Drawings
Control drawings reference specific terms to define how each loop is configured and
how safety devices behave. These six terms appear on nearly every HVAC/R control
drawing and specification sheet encountered in the field.
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Setpoint
The desired value the control system maintains — e.g., 22 °C
(72 °F) room temperature or 690 kPa (100 psi) suction pressure.
Shown on specifications with both SI and imperial units; always check which unit
system applies to the equipment being serviced.
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Process Variable (PV)
The actual measured value at the sensor at any given moment — the current
air temperature the thermostat is reading, or the present discharge pressure the
transducer is reporting. The difference between PV and setpoint is the
error that the controller acts to eliminate.
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Output
The control action or signal sent to a device in response to the error —
for example, a 24 V signal to a contactor coil, a 0–10 VDC signal
to a modulating valve actuator, or a 4–20 mA signal to a VFD speed
reference input.
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Deadband
A temperature or pressure range within which the controller holds its output
steady and does not respond. A typical HVAC deadband is 1–3 K
(approximately 2–5 °F). Without a deadband, the slightest
fluctuation would cause continuous on/off switching that damages contactors and
compressors.
↔
Differential
The numerical difference between the cut-in and cut-out values of a switch or
pressure control. A low-pressure switch that cuts in at 207 kPa (30 psi)
and cuts out at 276 kPa (40 psi) has a differential of 69 kPa
(10 psi). Differential is set on the device; deadband is programmed
in the controller.
▶◀
Normally Open (NO) / Normally Closed (NC)
The contact positions of switches and relay contacts in their de-energized
state as shown on a ladder diagram. A normally open contact is an
open circuit at rest and closes when actuated. A normally closed contact
passes current at rest and opens when actuated. This convention is universal
across all ladder diagram standards.
5.1.4 — Symbols and Legends
Every control drawing includes a legend (symbol list) that defines each graphical
element used on that drawing. Apprentices must be able to read this legend before
tracing any circuit — symbols are not universally standardized across all
manufacturers, so a legend on one drawing may differ from another.
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Ladder Diagram Conventions
Ladder diagrams are the standard format for HVAC/R control wiring schematics.
The name comes from the appearance: two vertical power rails (the “legs”)
connected by horizontal rungs — each rung representing one control circuit.
Left rail — line voltage (L1), typically 120 V or 240 V AC; all control power originates here
Right rail — neutral (N) or L2; circuits complete through this rail to complete the path
Each rung — reads left to right; contacts (switches, relay contacts) are in series on the left; the load (coil, motor, indicator) is at the right end
Line types — thick solid lines typically indicate power (line voltage) wiring; thin or dashed lines indicate low-voltage control or signal wiring; always check the legend to confirm
Rung numbering — rungs are numbered sequentially (1, 2, 3…) to allow cross-referencing between diagrams and sequence-of-operation documents
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Common Symbol Abbreviations
The following abbreviations appear on most HVAC/R control drawings. When a legend
is present, always verify the drawing’s own definitions — some
manufacturers use non-standard abbreviations.
TSTemperature Sensor / Thermostat
PSPressure Switch / Pressure Sensor
HPSHigh Pressure Switch (safety)
LPSLow Pressure Switch (safety)
RRelay (control relay coil or contacts)
CContactor coil
OLOverload Relay contacts
FRFan Relay
CRControl Relay
TTransformer
CBCircuit Breaker
NONormally Open contact
NCNormally Closed contact
DDCDirect Digital Controller output point
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Sensing, Controlling, and Controlled Devices
Control drawings classify every component into one of three functional roles.
Keeping these categories clear when reading a drawing makes signal tracing faster
and reduces errors during troubleshooting:
Sensing devices — measure the controlled variable and generate an input signal; examples: temperature sensor (NTC thermistor, RTD), pressure transducer (4–20 mA output), humidity sensor (0–10 VDC output). Shown near the process point on the drawing.
Controlling devices — compare the sensed value to setpoint and generate an output; examples: electronic thermostat, DDC controller module, pressure switch. Shown as a block or symbol at the middle of the signal chain.
Controlled devices — execute the output command to change the manipulated variable; examples: contactor, motorized valve actuator, VFD (receiving a speed reference), solenoid valve. Shown at the load end of each ladder rung.
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313A and 313D Compliance
The symbol conventions and terminology used in this lesson align with the drawing
and specification standards referenced in Ontario’s 313A (Refrigeration and
Air Conditioning) and 313D (Domestic and Commercial Refrigeration) apprenticeship
standards. When reading any control drawing on site, always locate the legend first,
confirm which standard the drawing follows, and verify setting ranges against the
manufacturer’s specification sheet before making any adjustments.