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HVAC NEC 2023

HVAC Certification Practice Test 12

This test covers the difference between TXV and fixed-orifice metering devices, and how superheat and subcooling readings guide diagnosis of overcharge, undercharge, and restriction problems.

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HVAC Practice tests

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Frequently Asked Questions

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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: Low superheat generally points toward overcharge or flooding; high superheat points toward undercharge or restriction. Learn the pattern, not just the definitions.

25

Questions

25 min

Time

70%

Pass

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HVAC Certification Practice Test 12 — Question List

Question 1: How does a TXV (thermostatic expansion valve) control refrigerant flow?

A sensing bulb on the suction line feeds back superheat information, and the valve adjusts flow to hold superheat near a target
It uses a fixed-size hole with no moving parts
It only responds to outdoor air temperature
It meters refrigerant based on compressor amperage

Question 2: How does a fixed orifice (piston) metering device control refrigerant flow?

It uses a fixed-size opening and meters based on the pressure differential across it, with no active feedback
It uses a sensing bulb just like a TXV
It electronically adjusts based on indoor humidity
It only operates during the defrost cycle

Question 3: Which metering device's superheat reading is expected to shift more as outdoor temperature and load change?

TXV
Fixed orifice
Both vary identically
Neither varies with load

Question 4: A system shows superheat well below the normal range. What does this commonly indicate?

Overcharge or a flooding evaporator
Undercharge only
A failed run capacitor
A tripped high-pressure switch

Question 5: A system shows superheat well above the normal range. What does this commonly indicate?

Overcharge only
Undercharge or a restriction
A failed crankcase heater
A stuck-open reversing valve

Question 6: A system shows subcooling well below the normal range. What does this commonly indicate?

Overcharge
Undercharge
A dirty air filter only
A miswired thermostat

Question 7: A system shows subcooling well above the normal range. What does this commonly indicate?

Undercharge only
Overcharge or a restriction in the liquid line
A weak start capacitor
Reversed phase rotation

Question 8: A technician measures superheat at 28°F (high) and subcooling at 5°F (low) on a fixed-orifice system. What does this pattern most likely indicate?

The system is overcharged
The system is undercharged
The compressor windings are grounded
The thermostat needs new batteries

Question 9: A technician measures superheat at 3°F (low) and subcooling at 22°F (high) on a fixed-orifice system. What does this pattern most likely indicate?

The system is undercharged
The system is overcharged
The condenser fan motor has failed
The crankcase heater is missing

Question 10: On a TXV system with a moderate undercharge, why might superheat still read close to normal while subcooling reads low?

The TXV actively adjusts to hold superheat near target, so subcooling is often the more reliable charge indicator on a TXV system
TXV systems are immune to undercharge entirely
Subcooling cannot be measured on a TXV system
A TXV always shows zero superheat regardless of charge

Question 11: A technician finds high subcooling right before a partially clogged filter-drier, but a much lower reading measured just after it. What does this temperature/pressure drop across the drier suggest?

A restriction in the filter-drier is causing a pressure and temperature drop across it
The filter-drier is oversized for the system
This is completely normal and expected on every system
The compressor is drawing locked rotor amps

Question 12: A fixed-orifice system shows abnormally high superheat and normal subcooling. Besides undercharge, what else should the technician check?

A restriction limiting refrigerant flow to the evaporator, such as a clogged filter-drier or partially closed valve
Whether the thermostat batteries are low
Whether the disconnect is more than 50 feet away
Whether the condensate drain is clogged

Question 13: A fixed-orifice system reads low subcooling and high superheat, both pointing toward undercharge. What is the general corrective action?

Add refrigerant gradually while re-checking readings until both values move into the normal range
Add refrigerant until the high-pressure switch trips
Recover all refrigerant and leave the system empty
Increase the thermostat setpoint instead of adjusting charge

Question 14: A fixed-orifice system reads high subcooling and low superheat, both pointing toward overcharge. What is the general corrective action?

Add more refrigerant to compensate
Carefully recover a small amount of refrigerant and re-check readings until both values move into the normal range
Replace the compressor immediately
Increase the fan speed only

Question 15: What general symptom pattern would suggest a malfunctioning or "hunting" TXV rather than a simple charge issue?

Superheat readings that swing unstable or erratic over a short period, rather than settling near a consistent value
A perfectly steady superheat reading
The compressor never starting at all
The condenser fan cycling on a pressure switch

Question 16: Why does the TXV's sensing bulb need good physical contact with the suction line?

Poor contact gives the valve an inaccurate temperature signal, causing incorrect metering
The bulb's position has no effect on valve operation
The bulb only matters during the defrost cycle
The bulb is purely decorative on modern systems

Question 17: On a hot day with high indoor load, why might a fixed-orifice system show lower superheat than it did on a mild day?

The fixed orifice has no active feedback, so its superheat shifts with changing load and outdoor conditions
Fixed orifice superheat never changes under any condition
This only happens on three-phase systems
It indicates the crankcase heater has failed

Question 18: A system shows superheat and subcooling both within their normal ranges. What does this generally suggest about the refrigerant charge?

The system is definitely overcharged
The charge is likely correct
The system is definitely undercharged
No conclusion can be drawn from these readings

Question 19: How can a technician generally distinguish a true undercharge from a restriction, when both can cause high superheat?

Subcooling readings and other clues (like a pressure/temperature drop across a specific component) help tell them apart
There is no way to distinguish the two
Only the compressor amperage tells them apart
Only the outdoor ambient temperature tells them apart

Question 20: True or False: A properly functioning TXV keeps superheat closer to a consistent target across varying conditions than a fixed orifice does.

True
False
Only true on R-22 systems
Only true when the compressor is off

Question 21: A liquid line shows bubbles in a sight glass along with low subcooling. What does this most likely indicate?

The liquid is partially flashing to vapor, commonly due to low charge or insufficient subcooling
The system is definitely overcharged
The compressor windings are shorted to ground
This is a normal sight-glass appearance under all conditions

Question 22: Persistently low superheat over time can shorten compressor life. Why?

It increases the risk of liquid refrigerant reaching and damaging the compressor over repeated cycles
Low superheat has no effect on the compressor at all
It only affects the condenser fan bearings
It only matters on three-phase systems

Question 23: When charging a new TXV system, which reading is generally the primary target to charge to?

Superheat only, ignoring subcooling entirely
Subcooling, per the manufacturer's specified target
Outdoor ambient temperature alone
Compressor amperage alone

Question 24: When charging a fixed-orifice system, which reading is generally the primary target to charge to?

Subcooling only, ignoring superheat entirely
Superheat, per the manufacturer's specified target or a superheat charging chart
Indoor humidity alone
Condenser fan amperage alone

Question 25: A fixed-orifice system reads superheat of 30°F and subcooling of 6°F. Which two corrective directions does this pattern suggest, together?

The pattern points to overcharge; recover refrigerant
The pattern points to undercharge; the technician should add refrigerant gradually while re-checking both readings
The pattern is normal and needs no action
The pattern points to a failed run capacitor only

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