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NATE_AC_SERVICE NEC 2023

NATE AC Service Practice Test 3

This test focuses on refrigerant charging methods, including liquid charging into the high side and vapor charging into the low side. You will practice charging safety rules and proper technique.

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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: Never add liquid refrigerant to the low side of a running compressor. Liquid entering the suction line can slug the compressor and cause serious damage.

30

Questions

30 min

Time

70%

Pass

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NATE AC Service Practice Test 3 — Question List

Question 1: What is "liquid charging"?

Adding refrigerant in liquid form into the high side, usually with the compressor off
Adding refrigerant vapor into the low side with the compressor running
Draining oil from the compressor crankcase
Bleeding air out of the service hoses only

NEC Reference: NULL

Question 2: What is "vapor charging"?

Removing refrigerant vapor from the high side
Adding refrigerant in vapor form into the low side with the compressor running
Adding oil to the compressor through the suction port
Pressurizing the system with nitrogen for a leak test

NEC Reference: NULL

Question 3: Why should a technician never add liquid refrigerant directly into the low (suction) side of a running compressor?

It voids the manufacturer warranty automatically
It can cause liquid slugging and damage the compressor
It causes the condenser fan to reverse direction
It has no real risk, it just wastes refrigerant

NEC Reference: NULL

Question 4: True or False: Liquid charging is done through the high-side port, typically with the compressor off.

True
False, liquid charging is done through the low side only
False, liquid charging always requires the compressor running
True, but only for R-22 systems

NEC Reference: NULL

Question 5: Why does vapor charging generally take longer than liquid charging for the same amount of refrigerant?

Vapor is less dense than liquid, so it takes longer to add the same weight
Vapor charging requires a bigger compressor
Vapor charging is actually faster, not slower
There is no real time difference between the two methods

NEC Reference: NULL

Question 6: A technician needs to add a small amount of refrigerant to fine-tune subcooling on a running system. Which method is appropriate?

Liquid charging into the low side with the compressor running
Vapor charging into the low side with the compressor running
Draining refrigerant from the oil sump
Adding refrigerant through the compressor discharge line

NEC Reference: NULL

Question 7: To liquid charge from a refrigerant cylinder, how should the cylinder typically be positioned?

Upright, or as needed per the valve design, to draw liquid
Upside down only
Lying on its side at all times
Orientation never matters for any cylinder

NEC Reference: NULL

Question 8: True or False: When a sealed refrigerant system must be opened for repair, refrigerant should be recovered first rather than vented to the atmosphere.

False, venting is allowed for small amounts
False, recovery is optional for residential systems
False, only R-22 must be recovered
True

NEC Reference: NULL

Question 9: While vapor charging to raise subcooling, what should the technician do between small refrigerant additions?

Pause, let the system stabilize, then re-check readings
Add the entire remaining cylinder at once
Immediately shut off the system entirely
Switch the thermostat to heat mode

NEC Reference: NULL

Question 10: What tool helps a technician track the exact weight of refrigerant added during charging?

A clamp-on ammeter
A refrigerant scale
A combustion analyzer
A megohmmeter

NEC Reference: NULL

Question 11: After replacing a compressor and evacuating the system, what should the technician do before starting the equipment to add the full refrigerant charge?

Guess the charge amount based on outdoor temperature only
Weigh in the full factory charge as liquid into the high side, then fine-tune
Add refrigerant only as vapor into the low side for the entire charge
Skip evacuation and charge directly with air still in the system

NEC Reference: NULL

Question 12: True or False: Repeatedly topping off a system with a known leak instead of repairing the leak is considered poor practice.

False, topping off is always an acceptable long-term solution
False, leaks are expected and do not need repair
True
False, only commercial systems need leak repair

NEC Reference: NULL

Question 13: What personal protective equipment should a technician wear while handling refrigerant during charging?

Safety glasses and gloves
A dust mask only
Rubber boots only
No PPE is needed for refrigerant handling

NEC Reference: NULL

Question 14: A technician accidentally adds too much refrigerant while charging and subcooling is now well above target. What is the correct next step?

Vent refrigerant to the atmosphere until the pressure drops
Leave it as is, overcharge is not a real concern
Increase the thermostat setpoint instead of adjusting charge
Carefully recover a small amount of refrigerant using a recovery machine

NEC Reference: NULL

Question 15: True or False: Whenever available, a technician should charge to the manufacturer's specified target (charge weight, superheat, or subcooling) rather than a generic rule of thumb.

False, generic rules of thumb always override manufacturer data
False, manufacturer specs are never accurate
False, charge weight is never listed by manufacturers
True

NEC Reference: NULL

Question 16: When vapor charging a TXV system to correct a charge problem, which reading should the technician watch most closely as the primary target?

Indoor humidity
Subcooling
Compressor start capacitor rating
Duct static pressure

NEC Reference: NULL

Question 17: Why should a technician briefly purge air from charging hoses before connecting them to the system?

To prevent air and moisture from entering the system
To make the hoses easier to coil up afterward
To reduce the cost of the refrigerant used
It is a cosmetic step with no technical purpose

NEC Reference: NULL

Question 18: A technician finds a system completely out of refrigerant. What should happen before charging the full amount back in?

Charge the system immediately without further checks
Replace the thermostat before charging
Find and repair the leak, then evacuate before charging
Add only enough refrigerant to run for one day

NEC Reference: NULL

Question 19: True or False: A technician should confirm the refrigerant type before charging, since gauge readings and pressure-temperature relationships differ between refrigerants.

False, all refrigerants share the same pressure-temperature chart
True
False, refrigerant type only matters for heating equipment
False, gauges automatically detect refrigerant type

NEC Reference: NULL

Question 20: A technician mistakenly opens the liquid valve on a charging cylinder while connected to the low-side port with the compressor running. What is the immediate risk?

The condenser fan will simply slow down
Nothing, this is a normal and safe practice
The thermostat will shut off automatically with no damage
Liquid slugging of the compressor, which can cause severe damage

NEC Reference: NULL

Question 21: Why should refrigerant cylinders be kept out of direct sunlight and away from heat sources during charging?

It has no effect on the cylinder at all
It makes the refrigerant change color permanently
Heating raises cylinder pressure, which is a safety hazard
It only matters for empty cylinders

NEC Reference: NULL

Question 22: On a brand-new split-system installation with long refrigerant line sets, what typically must be added to the factory-installed charge?

Nothing, factory charge is always correct regardless of line length
An additional amount based on extra line length, per manufacturer chart
Only nitrogen, no additional refrigerant
Twice the factory charge amount in every case

NEC Reference: NULL

Question 23: True or False: A technician should record the amount of refrigerant added or removed, along with final superheat and subcooling readings, on the service ticket.

False, documentation is unnecessary for routine charging
False, only the total bill amount needs to be recorded
False, this is only required for commercial jobs
True

NEC Reference: NULL

Question 24: A technician cannot find a leak right away but the customer needs cooling restored today. What is an acceptable temporary approach, with a clear plan to follow up?

Refuse to add any refrigerant under any circumstances
Overcharge the system heavily to delay the next visit
Add a documented amount to restore operation and schedule a follow-up leak search
Tell the customer the system cannot be fixed

NEC Reference: NULL

Question 25: Before charging refrigerant by weight, what should the technician do with the refrigerant scale?

Place the cylinder on the scale and note or zero the starting weight
Fill the cylinder completely before charging
Disconnect the scale, it is not needed for charging
Heat the cylinder to raise the reading

NEC Reference: NULL

Question 26: For vapor charging, how should the refrigerant cylinder valve be positioned so that only vapor leaves the cylinder?

Upside down so liquid pours out first
On its side, rolling slowly
Upright, so vapor is drawn from the top
Orientation does not matter for vapor charging

NEC Reference: NULL

Question 27: True or False: Recovery cylinders must never be filled beyond their rated safe fill level, since overfilling is a serious safety hazard.

False, recovery cylinders have no fill limit
False, only DOT inspectors need to worry about fill level
False, overfilling only matters in cold weather
True

NEC Reference: NULL

Question 28: While liquid charging into the high side with the system off, why is it good practice to add refrigerant slowly rather than all at once?

Slow addition prevents the refrigerant from changing type
It has no real benefit, speed does not matter
Adding too quickly can spike pressure and reduce control over the final amount
It is required only for R-410A systems

NEC Reference: NULL

Question 29: On a standard manifold gauge set, what do the blue and red hose connections typically represent?

Blue is high side; red is low side
Both represent the same side, color has no meaning
Blue is low side; red is high side
Red and blue are only used for heating equipment

NEC Reference: NULL

Question 30: Which statement about refrigerant charging safety is correct?

Liquid can safely be added to the low side any time
Vapor should never be used for charging
Cylinder orientation never matters for charging method
Liquid goes to the high side; vapor is used for low-side charging on a running compressor

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)