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NATE Specialty Gas Heating Service Professional
Certification Exam | Latest Verified Questions and
Detailed Answers
OVERVIEW DESCRIPTION
This Comprehensive set of multiple choice questions is designed for the NATE
Specialty Gas Heating Service Professional Certification Exam, administered by North
American Technician Excellence (NATE). The exam validates the competency of
experienced HVAC technicians in servicing, maintaining, and troubleshooting
residential and light commercial gas furnaces up to 400,000 BTU/h. Its content is
organized around Knowledge Areas of Technician Expertise (KATEs), with a strong
emphasis on Service and Troubleshooting (approximately 43%) as well as Operational
Information, Installation, Safety, Instruments and Tools, and General Knowledge. The
questions assess diagnostic skills across high-efficiency condensing and conventional
systems, covering ignition systems, combustion analysis, venting, gas valves, safety
controls, and airflow, making it a focused assessment of real-world service capability
for seasoned professionals.
QUESTION 1
A technician is performing a combustion analysis on a natural gas furnace. The
measured oxygen (O₂) is 9% and the carbon monoxide (CO) reading is 400 ppm air-free.
Which of the following actions should be taken FIRST?
A) Increase manifold gas pressure.
B) Clean the burner orifices and check for sooting.
C) Replace the heat exchanger immediately.
D) Open the combustion air damper fully.
CORRECT ANSWER: B
EXPERT RATIONALE: High O₂ with elevated CO indicates incomplete combustion often
caused by dirty burners or improper air/fuel mixing. Cleaning burners and removing
blockage is the logical first step before adjusting pressure.
,2|Page
QUESTION 2
A condensing furnace with a 100,000 BTU/h input is operating with a temperature rise of
70°F. The rated temperature rise range is 40–70°F. Which blower speed adjustment
should the technician make?
A) Increase blower speed to lower temperature rise.
B) Decrease blower speed to lower temperature rise.
C) Leave blower speed as is since 70°F is within range.
D) Adjust gas manifold pressure to 3.2 inches W.C.
CORRECT ANSWER: A
EXPERT RATIONALE: At the maximum allowable rise, increasing blower speed will reduce
temperature rise into the mid-range, improving comfort and heat exchanger longevity.
QUESTION 3
When measuring the flame signal of a hot surface ignition system, the technician should
use a multimeter set to measure which parameter?
A) DC microamps
B) AC volts
C) Ohms
D) DC millivolts
CORRECT ANSWER: A
EXPERT RATIONALE: Flame rectification produces a small DC current, typically 1–5 µA,
measured in series with the flame sensor lead using a meter with microamp capability.
QUESTION 4
A furnace equipped with a direct spark ignition (DSI) system fails to light and the
ignition control locks out. The spark can be heard and seen at the igniter gap. Which of
the following is the most likely cause?
A) Defective igniter ceramic
,3|Page
B) No gas flow to the burners
C) Shorted flame rollout switch
D) Open high limit switch
CORRECT ANSWER: B
EXPERT RATIONALE: If the spark is present but no flame occurs, the gas valve may not
be opening due to a faulty valve, no power, or no gas supply, preventing ignition.
QUESTION 5
A standing pilot furnace has a pilot flame but the main burners will not ignite. The
technician measures 28 mV open-circuit at the thermocouple and 2 mV under load.
What does this indicate?
A) Thermocouple is good; the gas valve is defective.
B) Thermocouple is weak and should be replaced.
C) Pilot orifice is oversized.
D) The high limit is open.
CORRECT ANSWER: B
EXPERT RATIONALE: A healthy thermocouple should maintain 8–12 mV under load
when the pilot coil is energized. A drop to 2 mV shows the thermocouple cannot sustain
the power coil dropout threshold.
QUESTION 6
What is the purpose of a combustion air proving switch on an induced draft furnace?
A) Ensure the vent damper is fully open.
B) Verify that the inducer fan is operating and providing sufficient draft before ignition.
C) Detect flame rollout in the burner compartment.
D) Monitor the secondary heat exchanger temperature.
CORRECT ANSWER: B
EXPERT RATIONALE: The pressure switch closes only when the induced draft motor
, 4|Page
creates enough negative pressure, confirming adequate venting and combustion air
flow prior to ignition.
QUESTION 7
On a Category IV condensing furnace, the vent pipe must be:
A) Double-wall B-vent with negative draft.
B) Single-wall metal pipe sloped toward the furnace.
C) Airtight PVC, CPVC, or polypropylene with condensate drainage.
D) Galvanized steel pipe with a draft hood.
CORRECT ANSWER: C
EXPERT RATIONALE: Category IV appliances operate with positive vent pressure and
condensing flue gases, requiring a sealed, corrosion-resistant vent that handles
condensate.
QUESTION 8
During a preventive maintenance check, the technician measures the manifold gas
pressure on a natural gas furnace. The nameplate calls for 3.5 in. W.C. but the reading is
2.8 in. W.C. What effect will this have on unit operation?
A) Increased efficiency, higher CO₂.
B) Reduced input, potential delayed ignition.
C) Higher temperature rise, over-cycling limit.
D) No effect as long as flame is blue.
CORRECT ANSWER: B
EXPERT RATIONALE: Low manifold pressure reduces the burner input and can cause
lean combustion, longer ignition times, and possible flame instability.
QUESTION 9
Which safety control on a gas furnace is designed to de-energize the gas valve if flames
NATE Specialty Gas Heating Service Professional
Certification Exam | Latest Verified Questions and
Detailed Answers
OVERVIEW DESCRIPTION
This Comprehensive set of multiple choice questions is designed for the NATE
Specialty Gas Heating Service Professional Certification Exam, administered by North
American Technician Excellence (NATE). The exam validates the competency of
experienced HVAC technicians in servicing, maintaining, and troubleshooting
residential and light commercial gas furnaces up to 400,000 BTU/h. Its content is
organized around Knowledge Areas of Technician Expertise (KATEs), with a strong
emphasis on Service and Troubleshooting (approximately 43%) as well as Operational
Information, Installation, Safety, Instruments and Tools, and General Knowledge. The
questions assess diagnostic skills across high-efficiency condensing and conventional
systems, covering ignition systems, combustion analysis, venting, gas valves, safety
controls, and airflow, making it a focused assessment of real-world service capability
for seasoned professionals.
QUESTION 1
A technician is performing a combustion analysis on a natural gas furnace. The
measured oxygen (O₂) is 9% and the carbon monoxide (CO) reading is 400 ppm air-free.
Which of the following actions should be taken FIRST?
A) Increase manifold gas pressure.
B) Clean the burner orifices and check for sooting.
C) Replace the heat exchanger immediately.
D) Open the combustion air damper fully.
CORRECT ANSWER: B
EXPERT RATIONALE: High O₂ with elevated CO indicates incomplete combustion often
caused by dirty burners or improper air/fuel mixing. Cleaning burners and removing
blockage is the logical first step before adjusting pressure.
,2|Page
QUESTION 2
A condensing furnace with a 100,000 BTU/h input is operating with a temperature rise of
70°F. The rated temperature rise range is 40–70°F. Which blower speed adjustment
should the technician make?
A) Increase blower speed to lower temperature rise.
B) Decrease blower speed to lower temperature rise.
C) Leave blower speed as is since 70°F is within range.
D) Adjust gas manifold pressure to 3.2 inches W.C.
CORRECT ANSWER: A
EXPERT RATIONALE: At the maximum allowable rise, increasing blower speed will reduce
temperature rise into the mid-range, improving comfort and heat exchanger longevity.
QUESTION 3
When measuring the flame signal of a hot surface ignition system, the technician should
use a multimeter set to measure which parameter?
A) DC microamps
B) AC volts
C) Ohms
D) DC millivolts
CORRECT ANSWER: A
EXPERT RATIONALE: Flame rectification produces a small DC current, typically 1–5 µA,
measured in series with the flame sensor lead using a meter with microamp capability.
QUESTION 4
A furnace equipped with a direct spark ignition (DSI) system fails to light and the
ignition control locks out. The spark can be heard and seen at the igniter gap. Which of
the following is the most likely cause?
A) Defective igniter ceramic
,3|Page
B) No gas flow to the burners
C) Shorted flame rollout switch
D) Open high limit switch
CORRECT ANSWER: B
EXPERT RATIONALE: If the spark is present but no flame occurs, the gas valve may not
be opening due to a faulty valve, no power, or no gas supply, preventing ignition.
QUESTION 5
A standing pilot furnace has a pilot flame but the main burners will not ignite. The
technician measures 28 mV open-circuit at the thermocouple and 2 mV under load.
What does this indicate?
A) Thermocouple is good; the gas valve is defective.
B) Thermocouple is weak and should be replaced.
C) Pilot orifice is oversized.
D) The high limit is open.
CORRECT ANSWER: B
EXPERT RATIONALE: A healthy thermocouple should maintain 8–12 mV under load
when the pilot coil is energized. A drop to 2 mV shows the thermocouple cannot sustain
the power coil dropout threshold.
QUESTION 6
What is the purpose of a combustion air proving switch on an induced draft furnace?
A) Ensure the vent damper is fully open.
B) Verify that the inducer fan is operating and providing sufficient draft before ignition.
C) Detect flame rollout in the burner compartment.
D) Monitor the secondary heat exchanger temperature.
CORRECT ANSWER: B
EXPERT RATIONALE: The pressure switch closes only when the induced draft motor
, 4|Page
creates enough negative pressure, confirming adequate venting and combustion air
flow prior to ignition.
QUESTION 7
On a Category IV condensing furnace, the vent pipe must be:
A) Double-wall B-vent with negative draft.
B) Single-wall metal pipe sloped toward the furnace.
C) Airtight PVC, CPVC, or polypropylene with condensate drainage.
D) Galvanized steel pipe with a draft hood.
CORRECT ANSWER: C
EXPERT RATIONALE: Category IV appliances operate with positive vent pressure and
condensing flue gases, requiring a sealed, corrosion-resistant vent that handles
condensate.
QUESTION 8
During a preventive maintenance check, the technician measures the manifold gas
pressure on a natural gas furnace. The nameplate calls for 3.5 in. W.C. but the reading is
2.8 in. W.C. What effect will this have on unit operation?
A) Increased efficiency, higher CO₂.
B) Reduced input, potential delayed ignition.
C) Higher temperature rise, over-cycling limit.
D) No effect as long as flame is blue.
CORRECT ANSWER: B
EXPERT RATIONALE: Low manifold pressure reduces the burner input and can cause
lean combustion, longer ignition times, and possible flame instability.
QUESTION 9
Which safety control on a gas furnace is designed to de-energize the gas valve if flames
