When troubleshooting a malfunctioning HVAC system, it’s important to understand HVAC symbols. These symbols, found in electrical and electronic schematics, guide the design and repair of the system’s circuit boards and wiring. You can become familiar with these symbols by reading your HVAC system manual. Interpreting HVAC symbols allows you to accurately interpret circuit diagrams, identify issues, and make decisions regarding the repair of your HVAC system
HVAC schematics, including circuit diagrams or schematic diagrams feature mostly universal symbols. There are some variations between regions like Europe and North America. Mastering these HVAC symbols and their functions will help you with troubleshooting your HVAC system. This article provides you with common HVAC symbols and their meanings to support you in diagnosing your HVAC system issues.
What are HVAC Symbols?
Symbols are used in the construction trade for all different types of things like plumbing systems and HVAC systems. HVAC symbols can quickly show what something means without using lots of words or complicated terms. Abbreviations of complicated terms are often used with symbols.
Some common symbols used in all HVAC system plans include HVAC ductwork, compressor and supply air fans.
Understanding the function of each HVAC system component helps you accurately diagnose the problem you’re facing. Here are some of the commonly used HVAC symbols related to the schematics and electronic components of your HVAC system.
Typical HVAC Symbols in Circuit Diagram
Resistor is a common part that is used in any printed circuit board. This passive part resists the flow of current. Change to similar value and rating if burnt.
A variable resistor or potentiometer is a resistor whose resistance can be adjusted. Usually used when an adjustment is needed to the voltage.
AC Capacitors are another type of common part. Change to similar value and rating if damaged.
Electrolytic Capacitor is usually used as smoothing capacitor in power supply regulation. Change to similar rating and value when damaged.
Variable Capacitor is used in tuning the circuit to a required frequency.
Inductor resists the flow of current. Change to similar value and rating if open circuit.
Variable Inductor allows the value of the inductance to be changed. Usually used for tuning to a required frequency.
Transformer in HVAC circuit is usually used to step down the input AC voltage to a lower AC voltage that is further rectified to get the required DC voltage. Check the original input, output voltages and the VA if you need to change it.
NPN Transistor is used to switch a load by inputting a positive voltage of more than 0.7V between Base and Emitter terminals (Bipolar type of transistor). If damaged, get an equivalent part by checking its original part.
PNP Transistor is used to switch a load by inputting a positive voltage of more than 0.7V between Emitter and Base terminals (Bipolar type of transistor). If damaged, get an equivalent part by checking its original part.
Batteries are used in a design where there is no power supply available or as a backup DC power in case there is no supply. Change batteries to similar or equivalent type when the backup power is not working.
Fuse is used as a protection in case of short circuit or overcurrent. If burnt, correct the problem and replace fuse with the same type and rating.
Varistor is used to protect the circuit from surge in voltage or lightning. Replace with similar rating when burnt or damaged.
Triac is usually used in motor control application where the speed of the motor is controlled by controlling the amount of power to it. Phase angle control is the typical one used in controlling fan motors.
Zener Diode can be used to clamp the voltage to a fix level depending on its value. Change to similar rating when damaged.
Diode allows the current to pass in one direction from anode to cathode. Change to same rating if burnt.
A LEDÂ or light emitting diode is used to display the status of the system. Seldom damaged, but if burnt, change to an equivalent type.
The SCR or Thyristor can switch and carry high current by applying a small pulse to the gate of the device. Only half of the power cycle is used as this is a uni-directional device.  Change to an equivalent device if damaged.
A speaker or horn is used to emit sound to inform the user of events that happen.
Power ground of the circuit.
Opto-coupler is usually used to isolate a high-power circuit from another circuit for safety reasons via the optical circuit. When the diode is on, it will cause the load that is connected to the transistor to turn on. The primary circuit and the secondary circuit are electrically isolated.
Op-Amp is another useful device that has many applications. There are usually a few of this devices in a semiconductor package.
Crystal is needed to provide clock that runs the microprocessor or microcontroller.
Diode bridge is a combination of four diodes that are connected to convert AC voltage to DC voltage.
IGBT or insulated gate bipolar transistor is used to switch high current load. Usually used in inverter or power supply circuit.
The Vcc is the DC power supply that has been rectified and is usually used to power up semiconductor devices and other DC circuitry.
Lamp is used as a visual indicator of the status of a system.
Understanding and using the HVAC symbols and terms in this chart will help to diagnose and address many common issues with your HVAC system. Accurately interpreting these HVAC control symbols will pinpoint problems and necessary repairs. If unable to understand the abbreviations, terms and HVAC symbols I recommend you contact a professional technician. They are experts at diagnosing issues with airflow, refrigerant, HVAC ductwork, actuators and other HVAC system components. A technician familiar with HVAC control symbols has the necessary tools to handle complex issues, so that your system operates efficiently and safely.
Related: Types of HVAC Measuring Instruments
