NATE Gas Heating 3 — Questions and Answers
Question 1: What is the minimum clearance from combustibles required for Type B vent pipe in most residential applications?
- 6 inches
- 1 inch (Correct answer)
- 3 inches
- No clearance required
Correct answer: 1 inch
Type B double-wall vent pipe requires only 1-inch clearance from combustible materials due to its insulating outer jacket.
Question 2: What does a cracked heat exchanger in a gas furnace risk introducing into the living space?
- Nitrogen dioxide only
- Carbon monoxide (CO) and other combustion byproducts (Correct answer)
- Natural gas from the supply line
- Refrigerant vapor
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 3: What is the purpose of the vent pressure switch on a two-stage condensing furnace?
- Verify combustion air is being supplied
- Confirm the inducer is operating at proper speed and the vent is clear before allowing ignition (Correct answer)
- Monitor return air static pressure
- Control secondary heat exchanger temperature
Correct answer: Confirm the inducer is operating at proper speed and the vent is clear before allowing ignition
The vent pressure switch proves adequate draft by sensing the negative pressure created by the inducer blower before the gas valve is energized.
Question 4: LP (propane) gas manifold pressure for residential furnaces is typically set to which value?
- 3.5 inches WC
- 7–11 inches WC (Correct answer)
- 10–14 inches WC
- 0.5 PSI
Correct answer: 7–11 inches WC
Propane furnaces require higher manifold pressure than natural gas, typically 10–11 inches WC, because propane has different combustion characteristics.
Question 5: Which type of furnace venting uses a dedicated fresh-air intake pipe running alongside the flue pipe?
- Natural draft venting
- Direct vent (sealed combustion) system (Correct answer)
- Power vent system
- Type B common vent
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 6: What is the correct procedure when a gas leak is suspected at a furnace?
- Turn on exhaust fans and relight the pilot
- Do not use any electrical switches; leave the building and call the gas company (Correct answer)
- Apply pipe dope and continue operating the furnace
- Increase gas pressure to identify the leak location
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.
NATE Gas Heating 3
What is the minimum clearance from combustibles required for Type B vent pipe in most residential applications?