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HVAC NEC 2023

HVAC Certification Practice Test 9

This test covers thermostat wire colors and terminal functions (R, C, Y, G, W, O/B), variable frequency drives (VFDs) and inverter-driven equipment, and how troubleshooting an inverter board differs from troubleshooting a simple relay-based control circuit.

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HVAC electrical exams cover electrical theory (Ohm's Law, circuits), transformers, single-phase and three-phase motors, control wiring, safety, and troubleshooting.

Requirements vary by state. HVAC technicians typically need EPA 608 certification and may need a separate electrical license for line voltage work.

Exam Tip: Thermostat wire colors are a widely followed convention, not a strict rule — always confirm terminal function against the equipment diagram, especially for O/B on heat pumps.

25

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25 min

Time

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HVAC Certification Practice Test 9 — Question List

Question 1: On a standard thermostat, what does the R terminal provide?

24V hot power from the control transformer
The cooling call signal
The fan call signal
A ground reference for the thermostat

Question 2: Which thermostat terminal energizes the compressor contactor for a cooling call?

W
G
Y
C

Question 3: What does the G terminal control on a thermostat?

The reversing valve
The indoor blower (fan) relay
The outdoor condenser fan only
The compressor contactor

Question 4: The W terminal on a thermostat is used for which function?

Cooling call
Fan call
Heat call
24V common

Question 5: Why has the C (common) terminal become more important with modern smart thermostats?

It is required by NEC Article 440
Smart thermostats need continuous 24V power to run their display and electronics
It replaced the R terminal on new systems
It controls the reversing valve instead of O/B

Question 6: Why is the O/B terminal on a heat pump thermostat a common source of confusion?

O and B always mean the same thing on every heat pump
Manufacturers differ on whether O or B represents the energized state for cooling vs. heating
O/B is only used on gas furnaces, never heat pumps
O/B replaces the Y terminal on newer systems

Question 7: On a two-stage cooling system, what does the Y2 terminal typically call for?

A backup transformer
Second-stage compressor capacity
A second thermostat wired in parallel
An alarm output only

Question 8: A technician finds a yellow wire connected to the W terminal instead of Y at an older thermostat base. What should the technician do?

Assume the wire is miswired because yellow always means Y
Cut the wire since it must be a mistake
Trust the terminal label, not the wire color, and verify function with the equipment diagram
Ignore it — wire color has no bearing on thermostat wiring

Question 9: What does a variable frequency drive (VFD) do to control motor speed?

It converts incoming AC to DC, then re-synthesizes AC output at a controllable frequency
It simply adds a resistor in series with the motor windings
It switches the motor fully on and off faster than a contactor can
It increases line voltage to speed the motor up

Question 10: Why do modern high-efficiency HVAC systems increasingly use variable-speed compressors and fans instead of simple on/off staging?

Variable speed can match output to the actual load, reducing cycling losses and improving comfort
Variable-speed motors never need a disconnect
Variable speed eliminates the need for a thermostat
Variable-speed compressors do not require refrigerant

Question 11: How does troubleshooting an inverter-driven compressor differ from troubleshooting a conventional contactor-driven compressor?

Inverter systems can only be diagnosed by replacing the compressor
The fault may be in the board's logic, a communication wire, or a bad sensor input, not just the motor circuit
There is no difference — the same voltage checks always find the problem
Inverter boards cannot be diagnosed with a meter at all

Question 12: What is usually the fastest first step when troubleshooting an inverter-driven HVAC system?

Replace the compressor first, since it is always the cause
Check the fault codes and diagnostic LEDs on the control board
Disconnect the thermostat and rewire it from scratch
Assume the refrigerant charge is wrong and add more

Question 13: Why should a technician follow manufacturer-specified wait times before servicing an inverter board after removing power?

Internal DC bus capacitors can retain hazardous voltage after power is removed
The board needs time to reboot its Wi-Fi connection
It is only a manufacturer suggestion with no real safety basis
Waiting allows the refrigerant charge to settle

Question 14: Why can't you always calculate a VFD-driven motor's speed the same way you would for a motor running directly on line power?

A VFD deliberately outputs frequencies other than the standard line frequency
VFD motors do not have a synchronous speed at all
VFDs only work on single-phase power
Line frequency calculations always apply exactly the same way

Question 15: What do the R and C terminals together provide to a thermostat?

A complete 24V AC power circuit — R is hot, C is the return/common leg
A 120V circuit for the blower motor
A signal path used only for the reversing valve
A ground fault path back to the panel

Question 16: A thermostat calls for cooling and the compressor runs, but the indoor blower never starts. Which wiring connection is the most likely place to check first?

The R wire, since it powers the whole thermostat
The G wire and its connection at both the thermostat and the air handler
The O/B wire, since it controls the reversing valve
The outdoor disconnect, since the compressor is running fine

Question 17: A single-speed system cycles on and off frequently on a mild day. What downside does this create compared to a variable-speed system?

More start-up energy waste and less stable temperature/humidity control
The compressor will never wear out
Single-speed systems always use less total electricity
There is no meaningful difference in comfort or efficiency

Question 18: On a heat pump thermostat, what does the W terminal typically call for compared to a straight air-conditioning system?

Auxiliary or emergency electric heat, rather than a gas furnace
The condenser fan only
The reversing valve directly
Nothing — heat pumps do not use a W terminal

Question 19: In a VFD, what is the purpose of converting the incoming AC power to DC before re-synthesizing it as AC again?

It lets the drive electronically build a new AC waveform at whatever frequency is needed
It reduces the motor's horsepower rating
It is only done to charge a backup battery
DC conversion has no functional purpose in a VFD

Question 20: A technician is replacing a thermostat on an unfamiliar heat pump and is unsure whether O or B energizes the reversing valve in cooling mode. What should the technician do?

Guess O, since it is energized in cooling on most residential systems seen so far
Confirm against that specific unit's wiring diagram before finishing the installation
Wire both O and B together to be safe
Skip the reversing valve wire entirely

Question 21: True or False: On an inverter-driven system, a basic set of voltage and continuity checks that would fully diagnose a conventional system is always enough to find the fault.

True
False
Only true on single-phase inverter systems
Only true if the compressor is under 3 tons

Question 22: A thermostat's fan switch is set to "on" with no heating or cooling call active. What happens to the G terminal?

It stays energized, running the blower continuously
It de-energizes immediately since there is no call
It only works during a cooling call
It controls the reversing valve instead in this mode

Question 23: Why does a variable-speed cooling system generally remove more humidity than a single-speed system running the same total run time?

Running longer at lower speed keeps air in contact with the cold coil longer, condensing more moisture
Variable-speed systems use a completely different refrigerant
Humidity removal has nothing to do with airflow speed
Single-speed systems always remove more humidity

Question 24: Match the correct pairing: which terminal is most closely associated with the compressor contactor coil on a straight cooling call?

C
Y
G
W

Question 25: A technician is about to open an inverter board after switching off the disconnect. What is the correct safety step before touching any internal components?

Open the cover immediately since the disconnect is off
Follow the manufacturer's specified wait time and verify safe voltage with a meter first
Short the terminals with a screwdriver to discharge them quickly
No precaution is needed once the breaker is off

Key NEC References for This Test

Article	What It Covers
Art. 440	Air Conditioning Equipment — use nameplate MCA and MOCP, not FLA alone
Art. 430	Motors — general motor rules that Art. 440 modifies

Study Tip: Practice reading ladder diagrams. Trace each rung from L1 to the load: contacts open → circuit dead. Contacts closed → current flows. This is how HVAC electrical troubleshooting works.

EXAM CARD HVAC

Questions25

Time limit25 min

Pass score70%

Code editionNEC 2023

DifficultyIntermediate

Last reviewedJun 2026

Timer starts immediately. Progress is auto-saved.

Source: NEC 2026 Art. 440 & electrical theory