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NATE_AC_SERVICE NEC 2023

NATE AC Service Practice Test 2

This test compares thermostatic expansion valves (TXV) with fixed-orifice metering devices. You will practice diagnosing overcharge, undercharge, and restriction scenarios on both valve types.

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NATE AC 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: A TXV actively holds superheat near a target using a sensing bulb. A fixed orifice has no such feedback, so its superheat shifts more with load.

30

Questions

30 min

Time

70%

Pass

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NATE AC Service Practice Test 2 — Question List

Question 1: What does a thermostatic expansion valve (TXV) do?

It meters refrigerant flow and actively adjusts to hold superheat near a target
It switches the compressor on and off based on room temperature
It measures airflow across the blower
It stores refrigerant during the off cycle only

NEC Reference: NULL

Question 2: What is a fixed-orifice metering device?

A valve that varies its opening based on suction pressure
A small, fixed-size opening with no moving parts or feedback control
A sensor that reports refrigerant charge to the thermostat
An electronic valve controlled by a circuit board

NEC Reference: NULL

Question 3: Where is the TXV sensing bulb normally mounted?

Inside the compressor shell
On the liquid line at the condenser outlet
On the suction line near the evaporator outlet
On the outdoor unit cabinet

NEC Reference: NULL

Question 4: True or False: A TXV system generally holds a steadier superheat across changing load conditions than a fixed-orifice system.

False, fixed orifice is always steadier
False, both are identical in this respect
True
True, but only in heating mode

NEC Reference: NULL

Question 5: On an overcharged TXV system, what usually happens to superheat compared to a fixed-orifice system with the same overcharge?

The TXV superheat drops much more than the fixed orifice
Both react identically since overcharge affects each the same way
The TXV holds superheat closer to normal; the fixed orifice shows a bigger drop
Neither system shows any change with overcharge

NEC Reference: NULL

Question 6: A TXV system is undercharged. What happens when the charge gets low enough?

Superheat drops to zero and stays there
Subcooling rises sharply while superheat stays low
Nothing changes, TXVs are unaffected by undercharge
Superheat rises and subcooling drops

NEC Reference: NULL

Question 7: A technician finds high superheat and normal to high subcooling on a fixed-orifice system. What does this pattern most likely point to?

A restriction such as a clogged filter drier
Overcharge of refrigerant
A failed run capacitor
Low outdoor ambient temperature

NEC Reference: NULL

Question 8: What is "hunting" on a TXV system?

The valve repeatedly overshooting and undershooting its target superheat
The compressor short-cycling on a pressure switch
The condenser fan cycling with the compressor
The blower motor changing speeds automatically

NEC Reference: NULL

Question 9: True or False: A loose or poorly insulated TXV sensing bulb can cause inaccurate superheat control.

False, bulb mounting does not affect performance
False, only fixed-orifice systems are sensitive to this
True, but only during heating operation
True

NEC Reference: NULL

Question 10: On a hot day with high indoor humidity, why might a fixed-orifice system's superheat be higher than it was on a mild, dry day?

The refrigerant type changes with humidity
Higher load conditions shift superheat because the fixed orifice cannot adjust
The compressor automatically increases displacement
Humidity has no possible effect on superheat

NEC Reference: NULL

Question 11: Which reading is typically used as the primary charging reference on a TXV system, since the valve actively manages superheat?

Subcooling
Compressor amp draw
Indoor humidity
Blower CFM

NEC Reference: NULL

Question 12: Which reading is typically used as the primary charging reference on a fixed-orifice system?

Indoor return air humidity
Superheat
Condenser fan amp draw
Thermostat cycle rate

NEC Reference: NULL

Question 13: A TXV system shows very low superheat that will not come up, even after the technician removes some refrigerant. What might be stuck or damaged?

The condenser fan motor
The indoor thermostat
The TXV, stuck open or with a lost bulb charge
The outdoor disconnect switch

NEC Reference: NULL

Question 14: True or False: A fixed-orifice metering device is generally simpler and less expensive than a TXV because it has no moving parts.

False, TXVs are always cheaper
False, both cost the same
True, but only for commercial units
True

NEC Reference: NULL

Question 15: A technician reads 5 degrees F superheat and 22 degrees F subcooling on a fixed-orifice system. What condition does this best match?

Undercharge
A dirty evaporator coil
A weak indoor blower
Overcharge

NEC Reference: NULL

Question 16: A technician reads 28 degrees F superheat and 5 degrees F subcooling on a fixed-orifice system. What condition does this best match?

Undercharge
Overcharge
A failed contactor
Normal operation

NEC Reference: NULL

Question 17: What can happen if a TXV is oversized for the system it is installed on?

It will always undercharge the evaporator
It has no effect as long as refrigerant type matches
It will improve efficiency with no downside
It can overfeed the evaporator and hunt, causing unstable superheat

NEC Reference: NULL

Question 18: A clogged liquid-line filter drier is suspected. What is a simple field check a technician can perform?

Check the thermostat batteries
Feel or measure for a temperature drop across the drier
Measure indoor humidity only
Check the condenser fan blade balance

NEC Reference: NULL

Question 19: True or False: A TXV must be matched to the refrigerant type used in the system to control superheat correctly.

False, any TXV works with any refrigerant
True
False, only fixed orifice devices are refrigerant-specific
True, but only for R-22 systems

NEC Reference: NULL

Question 20: Outdoor temperature drops sharply on a mild evening. A fixed-orifice system's superheat rises noticeably, while a nearby TXV system's superheat stays about the same. Why?

The fixed-orifice system has a leak and the TXV does not
The TXV actively adjusts to compensate; the fixed orifice cannot
The TXV system has more refrigerant charge in general
Outdoor temperature does not affect either device

NEC Reference: NULL

Question 21: What is the purpose of the external equalizer line on many TXVs?

It lets the valve sense true evaporator outlet pressure for accurate control
It equalizes voltage to the compressor motor
It balances airflow between two evaporator coils
It vents refrigerant to atmosphere during defrost

NEC Reference: NULL

Question 22: On a TXV system, why should a technician avoid relying on superheat alone to judge whether the system is overcharged?

Because superheat readings are illegal to use on TXV systems
Because gauges cannot read superheat on TXV systems
Because the TXV keeps superheat near normal even with overcharge; subcooling shows it better
Because superheat only matters for fixed orifice systems

NEC Reference: NULL

Question 23: True or False: Some fixed-orifice piston metering devices can be removed and swapped for a different size to better match a specific indoor and outdoor unit combination.

False, piston size can never be changed
False, this only applies to TXVs
True, but only in commercial systems
True

NEC Reference: NULL

Question 24: A technician confirms correct refrigerant charge by weight, but superheat still reads chronically far outside the normal range on a fixed-orifice system. What should the technician suspect?

A wrong-size piston or another metering device problem
A blown fuse in the disconnect box
A miswired thermostat
A cracked heat exchanger

NEC Reference: NULL

Question 25: True or False: Subcooling can be checked on both TXV and fixed-orifice systems, since it depends on the condenser, not the metering device type.

False, subcooling only applies to TXV systems
False, subcooling only applies to fixed-orifice systems
True, but readings are not comparable between types
True

NEC Reference: NULL

Question 26: What problem can moisture inside the refrigerant system cause at a TXV in cold weather?

It increases compressor voltage draw
It can freeze inside the valve and restrict refrigerant flow
It causes the condenser fan to run backward
It has no effect on the metering device

NEC Reference: NULL

Question 27: A technician finds a TXV sensing bulb clamped loosely to the suction line with no insulation. Superheat readings have been unstable. What is the correct fix?

Replace the compressor
Increase the refrigerant charge by 2 pounds
Re-clamp the bulb tightly and insulate it from ambient air
Remove the bulb completely since it is not needed

NEC Reference: NULL

Question 28: True or False: Some heat pump systems use a fixed orifice for one mode of operation and a TXV (or a check valve/TXV combination) for the other mode.

False, heat pumps never mix metering device types
False, only straight cooling systems use fixed orifices
False, TXVs cannot be used on heat pumps
True

NEC Reference: NULL

Question 29: Which type of system typically requires the technician to reference a superheat target chart based on outdoor temperature and indoor wet-bulb temperature?

A fixed-orifice system
A TXV system only
Neither system type needs a chart
Only electronic expansion valve systems

NEC Reference: NULL

Question 30: Which statement best summarizes the key difference between a TXV and a fixed orifice?

A fixed orifice adjusts flow electronically while a TXV never adjusts
There is no meaningful difference between the two devices
A TXV actively adjusts flow to hold target superheat; a fixed orifice has no feedback control
A fixed orifice is only used on commercial rooftop units

NEC Reference: NULL

EXAM CARD NATE_AC_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)