RMGA Gas Appliance Installation and Service 2 — Questions and Answers
Question 1: What is the function of a gas appliance's thermocouple or thermopile?
- To ignite the main burner
- To sense pilot flame presence and hold the gas valve open only when the pilot is lit (Correct answer)
- To measure flue gas temperature
- To regulate gas pressure to the burner
Correct answer: To sense pilot flame presence and hold the gas valve open only when the pilot is lit
A thermocouple generates a small electrical current when heated by the pilot flame, which holds the safety valve open; if the pilot goes out, the valve closes and stops gas flow.
Question 2: What causes 'yellow tipping' or 'lifting' of a natural gas burner flame?
- Excessive gas pressure only
- Improper air-to-gas ratio—yellow tipping indicates incomplete combustion, lifting indicates too much primary air (Correct answer)
- Faulty thermocouple
- High humidity in the ambient air
Correct answer: Improper air-to-gas ratio—yellow tipping indicates incomplete combustion, lifting indicates too much primary air
Yellow tipping results from incomplete combustion due to insufficient primary air, while flame lifting is caused by excess primary air or high gas pressure, both indicating a need for burner adjustment.
Question 3: What is the purpose of a draft diverter (draft hood) on a natural gas furnace or water heater?
- To increase the draft in the flue to improve combustion efficiency
- To isolate the appliance combustion zone from flue pressure variations and allow dilution air to mix with flue gases (Correct answer)
- To reduce gas pressure entering the burner
- To prevent backflow of combustion gases into the appliance
Correct answer: To isolate the appliance combustion zone from flue pressure variations and allow dilution air to mix with flue gases
A draft hood protects the burner from flue pressure variations and allows dilution air to enter the venting system, preventing downdrafts from extinguishing the burner.
Question 4: What is the minimum clearance required above a gas range burner grate to combustible overhead cabinets under NFPA 54?
- 12 inches
- 18 inches
- 24 inches
- 30 inches (Correct answer)
Correct answer: 30 inches
NFPA 54 and most appliance manufacturers require a minimum of 30 inches clearance between the cooking surface and combustible overhead cabinets.
Question 5: What type of venting material is prohibited for natural gas appliance Category I venting applications?
- Type B double-wall vent pipe
- Single-wall galvanized sheet metal vent pipe
- PVC or CPVC plastic pipe (Correct answer)
- Type L vent pipe
Correct answer: PVC or CPVC plastic pipe
PVC and CPVC plastic pipe are not approved for Category I natural gas appliance venting because flue gas temperatures can exceed the material's rated temperature limit.
Question 6: What is the correct action if a technician detects a gas leak at a union fitting on a gas range supply line?
- Tighten the union while gas is flowing and test with soapy water
- Shut off gas at the appliance valve, evacuate if necessary, and repair or replace the leaking fitting before restoring service (Correct answer)
- Apply leak stop compound to the union and restore service
- Increase gas pressure to re-seat the union seal
Correct answer: Shut off gas at the appliance valve, evacuate if necessary, and repair or replace the leaking fitting before restoring service
Gas must be shut off before any repair to a leaking fitting to eliminate the ignition hazard; the fitting must be properly repaired and leak-tested before service is restored.
RMGA Gas Appliance Installation and Service 2
What is the function of a gas appliance's thermocouple or thermopile?