NATE Gas Heating 2 — Questions and Answers
Question 1: What is the AFUE rating and what does it measure for a gas furnace?
- Annual Fuel Utilization Efficiency; measures what percentage of fuel energy is converted to heat (Correct answer)
- Airflow Factor Under Extreme conditions
- Average Furnace Unit Efficiency; measures BTU output
- Annual Flue Utilization Effectiveness
Correct answer: Annual Fuel Utilization Efficiency; measures what percentage of fuel energy is converted to heat
AFUE expresses as a percentage how much of the fuel's energy is converted to usable heat; a 96% AFUE furnace wastes only 4% up the flue.
Question 2: What type of venting is required for a 96% AFUE condensing gas furnace?
- Type B double-wall vent
- Category IV stainless steel vent
- PVC plastic pipe for both flue and combustion air (Correct answer)
- Masonry chimney with liner
Correct answer: PVC plastic pipe for both flue and combustion air
High-efficiency condensing furnaces produce cool, moist flue gases that must be exhausted through PVC pipe; Category IV designation.
Question 3: What does the flame sensor do in a modern electronic ignition furnace?
- Measures combustion air temperature
- Passes a small DC current through the flame to prove ignition and keep the gas valve open (Correct answer)
- Controls burner gas pressure
- Monitors heat exchanger temperature
Correct answer: Passes a small DC current through the flame to prove ignition and keep the gas valve open
The flame sensor uses flame rectification—the ionized flame allows DC current to flow—proving the burner is lit and preventing gas buildup.
Question 4: What is a condensate drain trap used for on a high-efficiency furnace?
- Trap combustion gases before they exit
- Prevent flue gases from backing up through the condensate drain while allowing water to drain freely (Correct answer)
- Filter particulates from the condensate water
- Reduce drain line vibration
Correct answer: Prevent flue gases from backing up through the condensate drain while allowing water to drain freely
The condensate trap maintains a water seal that blocks furnace gases from escaping through the drain while permitting condensate to flow out.
Question 5: What color should the burner flame be on a properly adjusted natural gas furnace?
- Yellow/orange with tips
- Mostly blue with slight yellow tips (Correct answer)
- Pure white
- Red/orange
Correct answer: Mostly blue with slight yellow tips
A properly burning natural gas flame should be predominantly blue, indicating complete combustion, with minor yellow tips acceptable.
Question 6: A furnace high-limit switch trips repeatedly. What is the most common cause?
- Gas valve is stuck open
- Dirty air filter or restricted airflow causing the heat exchanger to overheat (Correct answer)
- Faulty thermostat
- Low gas pressure
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.
NATE Gas Heating 2
What is the AFUE rating and what does it measure for a gas furnace?