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NATE_HEATPUMP_SERVICE NEC 2023

NATE Heat Pump Service Practice Test 3

This practice test covers how and why a heat pump defrost cycle happens: why frost forms on the outdoor coil, what triggers and ends a defrost cycle, and why auxiliary heat often runs at the same time. Several scenario questions cover diagnosing defrost control problems.

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NATE Heat Pump Service

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Frequently Asked Questions

Yes — completely free with no signup required. Every question includes a full explanation of the correct answer.

Two: the NATE Core exam plus at least one specialty exam (for example AC Service, Heat Pump Service, or Air Distribution Service). You can take them on the same day or separately.

The Core exam has 50 questions and each installation or service specialty exam has 100 questions. You need 70% on each to pass — 35 of 50 on Core and 70 of 100 on a specialty. Check natex.org for the current format before you book.

NATE certification lasts two years. To renew, you submit 16 hours of continuing education or retake the specialty exam before your certification expires.

No. These are independent practice tests written to match the published NATE exam topics. They are not official NATE exam questions.

Exam Tip: Remember that during defrost, the system is briefly running in cooling mode on purpose — that is why the outdoor coil gets hot gas and the indoor coil can feel cold without auxiliary heat.

30

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30 min

Time

70%

Pass

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NATE Heat Pump Service Practice Test 3 — Question List

Question 1: Why does frost build up on the outdoor coil during heating mode in cold, humid weather?

Because the outdoor coil is acting as the condenser and stays too hot
Because the compressor is oversized
Because the outdoor coil is acting as the evaporator and runs cold, so moisture in the air freezes on it
Because the indoor blower is running too fast

Question 2: What commonly triggers a defrost cycle to begin?

The thermostat display dimming
The homeowner pressing a reset button
The outdoor unit's electrical disconnect switch
A defrost control monitoring coil temperature and/or accumulated compressor run time

Question 3: What does the reversing valve do when defrost begins?

It energizes to temporarily run the system in cooling mode
It shuts off completely
It moves to a special third defrost-only position
Nothing; the valve does not move during defrost

Question 4: What happens to the outdoor fan during a defrost cycle?

It speeds up to blow warm air across the coil
It shuts off
It reverses direction
It runs at the same speed as normal heating mode

Question 5: Why does the outdoor fan shut off during defrost instead of continuing to run?

To save the fan motor from wear
Because the fan motor cannot run when the valve shifts
So the coil heats up and melts frost faster instead of being cooled by outdoor air
Because outdoor fans are not needed in heating mode at all

Question 6: Why do many heat pump systems energize auxiliary/emergency heat strips during a defrost cycle?

To help melt frost on the outdoor coil faster
To reduce electrical demand during defrost
Because the compressor is turned off during defrost
So occupants are not blown with cold air, since the indoor coil is temporarily acting as the evaporator

Question 7: What can end a defrost cycle?

Either a coil sensor detecting the coil is clear, or a maximum defrost time being reached
Only a manual reset by the homeowner
Only the outdoor temperature rising above freezing
Only the thermostat being turned off and back on

Question 8: True or False: Frost buildup on the outdoor coil is mainly a heating-mode concern, not a cooling-mode concern.

False, frost builds up equally in both modes
True
False, frost only builds up in cooling mode
False, frost has nothing to do with which mode the system is in

Question 9: A heat pump seems to be stuck running defrost continuously, well beyond what seems normal. What should a technician suspect?

The outdoor coil is too clean
The indoor blower is oversized
A defrost control or coil sensor fault causing the system to stay in or repeatedly re-enter defrost
The thermostat batteries are fully charged

Question 10: A heat pump's outdoor coil is heavily coated in ice, but the system never initiates a defrost cycle. What does this suggest?

The system is working exactly as designed
The reversing valve is oversized
The indoor air filter needs replacement
The defrost control or its sensor has likely failed to detect the frost condition

Question 11: From a refrigerant-flow standpoint, what mode is the system actually running in during a defrost cycle?

Cooling mode, temporarily
Heating mode, temporarily
Emergency heat mode
The compressor stops, so no mode applies

Question 12: Why is auxiliary heat commonly used during defrost even in relatively mild weather?

Because the compressor cannot run during defrost
Because the indoor coil is briefly acting as the evaporator, making supply air feel cold without it
Because auxiliary heat is required by all thermostats at all times
Because the outdoor fan needs extra power during defrost

Question 13: What mainly determines how often a heat pump needs to run defrost cycles?

The brand of thermostat installed
The size of the indoor blower
Outdoor temperature and humidity conditions that affect how quickly frost builds up
The color of the outdoor unit cabinet

Question 14: True or False: A heat pump running in a very dry, cold climate needs defrost cycles just as often as one running in a cold, humid climate.

True, humidity has no effect on frost
True, but only for older equipment
False, dry climates need defrost more often
False, defrost is needed more in cold, humid weather since more moisture is available to freeze on the coil

Question 15: Which two general approaches do defrost controls commonly use to decide when to start a defrost cycle?

Sensing coil temperature and/or tracking accumulated compressor run time
Reading the home's thermostat setpoint and the time of day only
Measuring indoor humidity and blower speed
Checking the outdoor unit's serial number

Question 16: A homeowner notices the air from their vents feels cool for a couple minutes during winter, then warms back up. What is a likely, normal explanation?

The furnace filter needs replacement
The system was running a normal defrost cycle
The thermostat is broken
The refrigerant charge is critically low

Question 17: True or False: Letting the outdoor fan keep running during defrost would help melt the frost faster.

True, more airflow always helps melt ice
True, but only in mild weather
False, it would blow cold outdoor air across the coil and slow down melting
False, because the fan cannot physically keep running at that point

Question 18: What component typically senses that the outdoor coil is clear of frost, allowing defrost to end?

The indoor thermostat
The compressor overload protector
The indoor blower relay
A coil temperature sensor (thermistor)

Question 19: If the coil sensor fails to indicate the coil is clear, what typically ends the defrost cycle as a backup?

A maximum defrost time limit built into the control
The homeowner must manually end it
The compressor automatically shuts off permanently
There is no backup; defrost would run forever

Question 20: A technician manually forces a defrost cycle to test the system. Which sequence of events would indicate normal operation?

The compressor shuts off and the indoor blower speeds up
The reversing valve shifts to cooling mode, the outdoor fan stops, and the coil begins to warm
The outdoor fan speeds up while the reversing valve stays in heating mode
The thermostat display goes blank until defrost ends

Question 21: True or False: During defrost, the indoor coil temporarily becomes cold because it is acting as the evaporator, which is why some systems run auxiliary heat at the same time.

False, the indoor coil stays hot during defrost
False, the indoor coil has no role during defrost
True
False, only the outdoor coil is active during defrost

Question 22: True or False: The compressor shuts off completely during a defrost cycle.

True, the compressor must be off for defrost to work
True, but only in very cold weather
True, defrost only uses the auxiliary heat strips
False, the compressor keeps running during defrost to pump hot gas to the outdoor coil

Question 23: A homeowner sees what looks like steam or light smoke rising from the outdoor unit right after a defrost cycle ends on a cold day. What is the most likely explanation?

This is normal — it is steam from melted frost/ice evaporating off the warm coil
The unit is on fire and needs immediate shutdown
The refrigerant is leaking and burning off
The compressor windings are overheating

Question 24: Why is frost buildup a bigger operational concern for heat pumps than for standard air conditioners?

Heat pumps use a different refrigerant that freezes more easily
A heat pump's outdoor coil runs cold as the evaporator during heating mode in cold weather, while an AC's outdoor coil is always the warm condenser
Heat pumps do not have outdoor fans
Air conditioners never operate in cold weather at all

Question 25: A heat pump is defrosting very frequently even though the weather is mild and dry — conditions that normally would not cause much frost. What should a technician suspect?

This is completely normal behavior in mild weather
The compressor is oversized for the home
A defrost control or sensor problem causing unnecessary defrost cycles
The thermostat fan setting is set to Auto

Question 26: A heat pump's outdoor coil is visibly caked in ice after days of cold, humid weather, but a check shows the defrost control has not initiated a single cycle. What is the most likely issue?

The system is operating normally for these conditions
The compressor capacitor is weak
The indoor thermostat needs new batteries
The defrost control or its sensing element has failed to detect the frost condition

Question 27: True or False: A defrost cycle can end either because a sensor confirms the coil is clear, or because a maximum time limit is reached, whichever happens first.

True
False, only the sensor can end defrost
False, only the time limit can end defrost
False, defrost never has a way to end automatically

Question 28: What is a potential downside if a defrost control malfunctions and lets a defrost cycle run far longer than needed?

The outdoor unit will be permanently damaged within seconds
Extra energy use and a longer period of cooler supply air indoors than necessary
The reversing valve will be destroyed instantly
The compressor will run in reverse

Question 29: A heat pump's auxiliary heat strips have failed, and the system enters a normal defrost cycle in cold weather. What will the occupants most likely notice?

No difference at all, since defrost doesn't use auxiliary heat
The system will refuse to enter defrost at all
A noticeable blast of cool air during the defrost cycle, since the failed strips can't offset the cold indoor coil
The outdoor unit will shut down permanently

Question 30: Which statement best summarizes what happens during a heat pump defrost cycle?

The compressor shuts off while the outdoor fan runs at full speed
The system permanently switches to cooling mode until manually reset
The indoor coil is disconnected from the refrigerant circuit
The reversing valve temporarily shifts to cooling mode, the outdoor fan stops, and the system returns to heating once the coil is clear or a time limit is reached

EXAM CARD NATE_HEATPUMP_SERVICE

Questions30

Time limit30 min

Pass score70%

Code editionNEC 2023

DifficultyIntermediate

Last reviewedJun 2026

Timer starts immediately. Progress is auto-saved.

Source: NEC 2026 (NFPA 70)