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HVAC NEC 2023

HVAC Certification Practice Test 6

This test covers the electrical components of a refrigerant circuit: compressor contactors, start relays, low and high-pressure switches, defrost cycles, and crankcase heaters. You will practice tracing safety interlocks and diagnosing common failures.

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HVAC Practice tests

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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: Remember that low and high-pressure switches are both wired normally closed (NC) in series with the contactor coil. Either one tripping alone is enough to stop the compressor.

25

Questions

25 min

Time

70%

Pass

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HVAC Certification Practice Test 6 — Question List

Question 1: What does the compressor contactor do?

It measures refrigerant pressure
It switches line-voltage power to the compressor motor on and off
It converts AC power to DC power
It controls only the indoor blower motor

Question 2: What is the purpose of a start relay (current-sensing or potential relay) in a compressor circuit?

It keeps the start capacitor connected continuously
It disconnects the start capacitor after the compressor has started
It controls the reversing valve
It measures head pressure

Question 3: What happens if a start capacitor is left connected continuously instead of being disconnected by the start relay?

It improves compressor efficiency
It has no real effect on the compressor
It reduces the compressor's amp draw
It overheats the start winding

Question 4: In many systems, how is the condenser fan motor commonly wired relative to the compressor?

In parallel with the compressor, off the same contactor (or independently cycled by a fan-cycling pressure switch in low-ambient setups)
In series with the compressor motor windings
Only through the defrost board
Only through the crankcase heater circuit

Question 5: A low-pressure switch is wired normally closed (NC) in series with the contactor coil circuit. What condition causes it to open?

Low suction pressure, from a leak, restriction, or frozen coil
High head pressure
A dirty condenser coil
A failed run capacitor

Question 6: A high-pressure switch is wired normally closed (NC) in series with the contactor coil circuit. What condition causes it to open?

Low suction pressure
High head pressure, from a dirty condenser, failed fan, or non-condensables
A weak start capacitor
Low ambient temperature

Question 7: Why are the low-pressure switch and high-pressure switch usually wired in series with each other and the contactor coil?

So both must trip together to stop the compressor
To double the available voltage to the coil
To reduce wiring cost only
So that either switch tripping alone stops the compressor

Question 8: What triggers a heat pump's defrost cycle?

The defrost board, monitoring outdoor coil temperature and time/run-time
The room thermostat alone
The crankcase heater
The low-pressure switch alone

Question 9: During a heat pump's defrost cycle, what does the reversing valve do?

It switches the system temporarily into cooling mode, sending hot gas to the outdoor coil to melt frost
It shuts off the compressor completely
It increases the refrigerant charge
It activates the crankcase heater only

Question 10: During defrost, what typically happens to the outdoor fan?

It speeds up
It shuts off
It reverses direction
It has no change

Question 11: During defrost, why do many heat pump systems energize indoor electric heat strips?

To melt ice on the indoor coil
To power the outdoor fan
To test the compressor contactor
So occupants aren't blown with cold air while the system is temporarily in cooling mode

Question 12: What is the main purpose of a crankcase heater?

It heats the refrigerant in the evaporator
It keeps compressor oil warmer than the surrounding refrigerant while the compressor is off
It prevents the condenser coil from freezing
It powers the defrost cycle

Question 13: What problem does a crankcase heater help prevent?

High head pressure during normal operation
Refrigerant migrating into and condensing in the compressor oil, which can cause foaming and slugging on startup
Phase rotation reversal
Run capacitor failure

Question 14: When is a crankcase heater typically energized?

Only while the compressor is running
Whenever the compressor is off
Only during the defrost cycle
Only in summer

Question 15: Power to a rooftop unit was out for several days, and the crankcase heater was not energized during that time. What should a technician do before starting the compressor?

Start the compressor immediately, since crankcase heaters have no real effect
Bypass the crankcase heater permanently
Increase the refrigerant charge before starting
Follow the manufacturer's recommended wait period, with power restored and the crankcase heater energized, before starting the compressor

Question 16: What is "slugging" in a compressor?

Liquid refrigerant, often mixed with foaming oil, entering the compression chamber, which can cause mechanical damage
The sound a contactor makes when it energizes
A type of electrical short in the start winding
A normal part of every defrost cycle

Question 17: How does a contactor generally differ from a relay in an HVAC system?

A contactor only works on 24V circuits
A contactor has larger contacts built for higher current loads, like compressors, while a relay handles smaller control currents
A relay is used only for line-voltage compressor loads
There is no real difference between them

Question 18: A technician finds the compressor not running, and the low-pressure switch reads open. Which of these is a plausible cause?

A dirty condenser coil restricting airflow
A refrigerant leak causing low suction pressure
A failed run capacitor
Reversed phase rotation on a single-phase system

Question 19: A technician finds the compressor not running, and the high-pressure switch reads open. Which of these is a plausible cause?

A refrigerant leak causing low charge
A failed run capacitor
Low airflow across the evaporator coil
A dirty condenser coil restricting airflow

Question 20: Both the low-pressure switch and high-pressure switch are functioning correctly and wired in series with the contactor coil. What happens if the high-pressure switch trips?

The contactor coil de-energizes and the compressor stops, the same as if the low-pressure switch had tripped
Only the condenser fan stops
The compressor speeds up to compensate
Nothing happens until both switches trip together

Question 21: Why might non-condensables, like air, trapped inside a refrigerant system cause the high-pressure switch to trip?

Non-condensables raise head pressure beyond what refrigerant alone would produce, tripping the high-pressure safety
Non-condensables lower suction pressure only
Non-condensables have no effect on pressure
Non-condensables only affect the crankcase heater

Question 22: A condenser fan is wired independently through a fan-cycling pressure switch instead of directly off the compressor contactor. What is the purpose of this setup?

To save on capacitor cost
To allow the fan to run only during defrost
To cycle the fan on and off to maintain proper head pressure in low-ambient conditions
To allow the fan to run only when the crankcase heater is on

Question 23: What component switches power to the compressor motor on and off in response to a thermostat call?

The low-pressure switch
The crankcase heater
The defrost board alone
The compressor contactor

Question 24: Where in the electrical circuit are the low-pressure switch and high-pressure switch typically wired?

In series with the contactor coil, as safety interlocks
In parallel with the run capacitor
In series with the crankcase heater only
In parallel with the thermostat display

Question 25: A rooftop unit's compressor won't start. The technician checks and finds normal 24V at the thermostat call, but 0V at the contactor coil terminals. Both the low and high-pressure switches test closed with a meter when isolated. What is the next logical step?

Replace the compressor immediately
Check for another open device in series in that rung, such as a defrost interlock or a blown low-voltage fuse, since the pressure switches have been ruled out
Assume the thermostat is fully at fault with no further checks
Increase the refrigerant charge

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

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Source: NEC 2026 Art. 440 & electrical theory