NATE (North American Technician Excellence) Certification — Questions and Answers

Question 1: What is the minimum recommended flue gas temperature for an oil furnace to prevent condensation in the flue?

150°F
350°F (Correct answer)
250°F
450°F

Correct answer: 350°F

Flue gas temperatures above 350°F prevent sulfuric acid condensation from oil combustion products, which would corrode the flue system.

Question 2: A furnace high-limit switch trips repeatedly. What is the most common cause?

Dirty air filter or restricted airflow causing the heat exchanger to overheat (Correct answer)
Faulty thermostat
Low gas pressure
Gas valve is stuck open

Correct answer: Dirty air filter or restricted airflow causing the heat exchanger to overheat

Restricted airflow from a clogged filter or blocked supply/return prevents adequate heat dissipation, causing heat exchanger temperatures to exceed the limit switch setpoint.

Question 3: Which type of furnace venting uses a dedicated fresh-air intake pipe running alongside the flue pipe?

Direct vent (sealed combustion) system (Correct answer)
Power vent system
Type B common vent
Natural draft venting

Correct answer: Direct vent (sealed combustion) system

Direct vent sealed-combustion furnaces draw all combustion air from outside through a dedicated intake, preventing depressurization of the home.

Question 4: What is the purpose of a liquid line solenoid valve in a refrigeration system?

Prevent liquid refrigerant from returning to the compressor
Control condensing fan speed
Regulate refrigerant pressure in the liquid line
Stop refrigerant flow to the evaporator when the thermostat is satisfied or during pump-down (Correct answer)

Correct answer: Stop refrigerant flow to the evaporator when the thermostat is satisfied or during pump-down

A liquid line solenoid valve closes on a thermostat-off signal, stopping refrigerant flow and allowing the compressor to pump down the evaporator.

Question 5: A high-efficiency split system AC rated at SEER 18 performs better than a unit rated at which of the following?

SEER 19
SEER 20
SEER 16 (Correct answer)
SEER 21

Correct answer: SEER 16

SEER (Seasonal Energy Efficiency Ratio) measures cooling efficiency; higher numbers indicate better efficiency.

Question 6: What does a TXV (thermostatic expansion valve) use to control refrigerant flow?

Condenser outlet temperature
Liquid line subcooling
Discharge pressure
Suction line superheat sensed by a bulb (Correct answer)

Correct answer: Suction line superheat sensed by a bulb

A TXV senses evaporator outlet superheat via a sensing bulb and adjusts flow to maintain a set superheat.

Question 7: What is the refrigeration effect and how is it calculated?

Heat absorbed in the evaporator; calculated as evaporator outlet enthalpy minus evaporator inlet enthalpy (Correct answer)
Work input to the compressor
Heat gained by subcooling the liquid line
Heat rejected at the condenser; calculated as condenser enthalpy minus compressor inlet enthalpy

Correct answer: Heat absorbed in the evaporator; calculated as evaporator outlet enthalpy minus evaporator inlet enthalpy

The refrigeration effect is the heat absorbed per pound of refrigerant in the evaporator, equal to the enthalpy difference across the evaporator.

Question 8: What is a dual-fuel heat pump system?

A heat pump that uses both R-410A and R-32 refrigerants
A heat pump paired with a gas furnace that automatically switches to gas heat below a set outdoor temperature (Correct answer)
A system with two compressors for redundancy
A heat pump that can use solar and grid electricity

Correct answer: A heat pump paired with a gas furnace that automatically switches to gas heat below a set outdoor temperature

Dual-fuel systems use the heat pump for mild weather and switch to the gas furnace when outdoor temperatures drop below the economic balance point.

Question 9: Which component in an air conditioning system is responsible for rejecting heat to the outdoors?

Compressor
Condenser (Correct answer)
Expansion valve
Evaporator

Correct answer: Condenser

The condenser releases heat absorbed from the indoor air to the outdoor environment.

Question 10: What component in an induced-draft furnace proves that combustion airflow is established before the gas valve opens?

Pressure switch (Correct answer)
Limit switch
Thermocouple
Rollout switch

Correct answer: Pressure switch

The pressure switch senses negative pressure created by the inducer motor to confirm draft is established before allowing ignition.

Question 11: What is the minimum SEER2 requirement for a new split-system AC unit installed in the US Southeast after January 1, 2023?

14 SEER2 (Correct answer)
16 SEER2
15 SEER2
13 SEER2

Correct answer: 14 SEER2

The US Department of Energy set the minimum SEER2 efficiency standard at 14 for split systems in the Southeast and Southwest regions effective January 2023.

Question 12: What condition does a high CO2 reading combined with a high smoke number indicate on a combustion analysis?

Too little combustion air (rich mixture causing incomplete combustion) (Correct answer)
Too much combustion air (lean mixture)
Perfect combustion efficiency
Low oil pump pressure

Correct answer: Too little combustion air (rich mixture causing incomplete combustion)

High CO2 with high smoke indicates insufficient air, producing a rich mixture with incomplete combustion and carbon soot.

Question 13: Which tool is used to measure the wet-bulb temperature when evaluating air conditioning performance?

Manifold gauge set
Clamp meter
Sling psychrometer (Correct answer)
Anemometer

Correct answer: Sling psychrometer

A sling psychrometer uses a wet-bulb and dry-bulb thermometer to measure relative humidity and wet-bulb temperature.

Question 14: What is the maximum recommended duct velocity for residential supply ducts to minimize noise?

1,200–1,500 FPM
2,000+ FPM
700–900 FPM (Correct answer)
400–600 FPM

Correct answer: 700–900 FPM

Residential supply ducts typically target 700–900 FPM to balance duct size with acceptable noise levels.

Question 15: What does a cracked heat exchanger in a gas furnace risk introducing into the living space?

Natural gas from the supply line
Carbon monoxide (CO) and other combustion byproducts (Correct answer)
Refrigerant vapor
Nitrogen dioxide only

Correct answer: Carbon monoxide (CO) and other combustion byproducts

Cracks in the heat exchanger allow combustion gases including deadly carbon monoxide to mix with circulated air and enter the living space.

Question 16: What is the recommended insulation R-value for ducts installed in unconditioned attics in most US climate zones?

R-11
R-8 (Correct answer)
R-6
R-4

Correct answer: R-8

Most US energy codes and ACCA guidelines recommend R-8 duct insulation for ducts in unconditioned attic spaces to minimize heat gain or loss.

Question 17: What type of duct fitting causes the most turbulence and pressure drop?

Square elbow without turning vanes (Correct answer)
Long-radius elbow
Straight section
Conical reducer

Correct answer: Square elbow without turning vanes

Sharp square elbows cause severe turbulence and high pressure drop; long-radius elbows or elbows with turning vanes greatly reduce this loss.

Question 18: What is the correct procedure when a gas leak is suspected at a furnace?

Apply pipe dope and continue operating the furnace
Increase gas pressure to identify the leak location
Turn on exhaust fans and relight the pilot
Do not use any electrical switches; leave the building and call the gas company (Correct answer)

Correct answer: Do not use any electrical switches; leave the building and call the gas company

Any electrical switch operation could ignite accumulated gas; evacuate immediately and contact the gas utility from outside.

Question 19: A technician finds a blown fuse in the control circuit. After replacing it, the fuse blows again immediately. What is the BEST next step?

Install a higher-rated fuse
Replace the transformer
Restart the system immediately
Check for short circuits or grounded wiring in the control circuit (Correct answer)

Correct answer: Check for short circuits or grounded wiring in the control circuit

A repeatedly blown fuse indicates a short circuit or grounded wire in the control circuit that must be located and repaired.

Question 20: What does a #2 fuel oil supply line's two-pipe system accomplish compared to a single-pipe system?

Returns unused oil to the tank, preventing vapor lock and allowing the tank to be below the burner (Correct answer)
Provides redundancy if the primary supply fails
Filters water from the oil supply
Increases oil flow rate to the burner

Correct answer: Returns unused oil to the tank, preventing vapor lock and allowing the tank to be below the burner

A two-pipe system continuously circulates oil back to the tank, preventing air and vapor buildup and allowing a below-burner tank installation.

Question 21: What is the standard gas manifold pressure for natural gas residential furnaces in the US?

3.2–3.8 inches WC (Correct answer)
7–14 inches WC
10–12 inches WC
0.5–1.0 PSI

Correct answer: 3.2–3.8 inches WC

Natural gas furnace manifold pressure is typically set at 3.2–3.8 inches water column (around 3.5 inches WC nominal).

Question 22: What does a barometric draft control (draft regulator) do in an oil heating system?

Control combustion air to the burner
Prevent flue gases from entering the living area
Maintain constant flue draft by admitting dilution air when draft exceeds setpoint (Correct answer)
Boost flue draft in low-pressure conditions

Correct answer: Maintain constant flue draft by admitting dilution air when draft exceeds setpoint

The barometric damper swings open to admit room air when draft is too strong, maintaining a stable negative pressure in the flue.

Question 23: What is the typical voltage and frequency of the secondary side of an HVAC control transformer in the United States?

24V AC / 60Hz (Correct answer)
48V AC / 60Hz
24V DC / 60Hz
12V AC / 60Hz

Correct answer: 24V AC / 60Hz

HVAC control transformers in the US typically output 24V AC at 60Hz to power the low-voltage control circuit.

Question 24: What is a bypass damper used for in a zoned duct system?

Relieve excess static pressure when zone dampers close by routing air back to the return side (Correct answer)
Provide fresh outdoor air to the return system
Control humidity in a single zone
Balance airflow between supply and return plenums

Correct answer: Relieve excess static pressure when zone dampers close by routing air back to the return side

When zone dampers close, the bypass damper opens to maintain minimum airflow across the coil and prevent equipment damage from excessive static pressure.

Question 25: When a heat pump is in heating mode and the emergency heat is energized, what happens?

Both the compressor and electric strips run simultaneously
The outdoor fan runs at high speed
Only electric resistance strips provide heat; the compressor is locked out (Correct answer)
The reversing valve switches to cooling mode

Correct answer: Only electric resistance strips provide heat; the compressor is locked out

Emergency heat bypasses the heat pump compressor and relies solely on electric resistance heat strips, typically used when the heat pump has failed.

Question 26: Which type of compressor uses scroll technology and is common in modern residential AC units?

Reciprocating
Screw
Scroll (Correct answer)
Centrifugal

Correct answer: Scroll

Scroll compressors use two interlocking spirals and are favored in residential equipment for efficiency and quiet operation.

Question 27: What refrigerant is most commonly used in modern residential air conditioning systems in the US?

R-134a
R-410A (Correct answer)
R-22
R-404A

Correct answer: R-410A

R-410A replaced R-22 as the standard residential AC refrigerant due to R-22's ozone-depleting properties.

Question 28: What does superheat measure in a refrigeration system?

Temperature rise of vapor above the saturation point at a given pressure (Correct answer)
Subcooling at the condenser outlet
Compressor discharge temperature
Temperature of liquid refrigerant leaving the condenser

Correct answer: Temperature rise of vapor above the saturation point at a given pressure

Superheat is the number of degrees the refrigerant vapor temperature exceeds the saturation temperature at that pressure.

Question 29: What type of compressor is most commonly used in large commercial refrigeration rack systems?

Centrifugal
Absorption
Reciprocating (semi-hermetic) (Correct answer)
Scroll

Correct answer: Reciprocating (semi-hermetic)

Semi-hermetic reciprocating compressors are widely used in commercial rack systems because they are serviceable and available in a wide range of capacities.

Question 30: What does a smoke tester (Bacharach pump) measure on an oil burner?

Stack gas CO2 percentage
Carbon soot level in flue gases by comparing a filter paper sample to a standard scale (Correct answer)
Combustion air temperature
Oil pump pressure

Correct answer: Carbon soot level in flue gases by comparing a filter paper sample to a standard scale

The smoke tester draws flue gas through filter paper; the resulting stain is compared to a numbered scale (0 = clean, 9 = heavy smoke).

Question 31: What MERV rating range is recommended for residential HVAC filters to balance filtration and airflow?

1–4
9–13
5–8 (Correct answer)
14–16

Correct answer: 5–8

MERV 5–8 filters provide adequate filtration for residential equipment while maintaining sufficient airflow and avoiding excessive pressure drop.

Question 32: What is the purpose of a hot gas defrost system in commercial refrigeration?

Prevent condensation on the case glass doors
Melt frost from evaporator coils by circulating hot discharge gas through them instead of using electric heaters (Correct answer)
Reheat supply air after cooling
Warm the compressor oil at startup

Correct answer: Melt frost from evaporator coils by circulating hot discharge gas through them instead of using electric heaters

Hot gas defrost diverts discharge gas from the compressor into the evaporator to melt frost quickly and efficiently without electric heaters.

NATE (North American Technician Excellence) Certification

The NATE certification exam validates HVAC technician knowledge and skills across specialty areas including air conditioning, heat pumps, gas heating, oil heating, and air distribution systems. Technicians must demonstrate competency in installation and service/troubleshooting to earn industry-recognized NATE credentials.

Questions

35 min

Time Limit

70%

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