CSLB C-20 HVAC ContractorHeating SystemsEasy
A homeowner's older standing-pilot gas furnace has a pilot flame that lights but the main burner never ignites, and the pilot eventually goes out. What component is the most likely cause?
- ABlocked flue baffle
- BOversized gas orifice
- CFaulty thermocouple
- DCracked heat exchanger
Show answer & explanationAnswer & explanation
Correct answer: C. Faulty thermocouple
The thermocouple generates a small millivolt signal when heated by the pilot flame, which holds the gas valve's safety solenoid open. If the thermocouple is weak, dirty, or misaligned, the valve senses no flame and shuts off gas to both pilot and main burner as a safety measure.
Why the other options are wrong
- A. A blocked flue baffle affects draft, not the pilot-to-main-valve safety circuit.
- B. An oversized orifice would cause excess flame, not loss of pilot signal.
- D. A cracked heat exchanger affects combustion products, not pilot/gas valve operation.
Thermocouple Safety Circuit
A thermocouple produces a small millivolt current when heated by the pilot flame, holding the gas valve's safety solenoid open to allow gas flow.
- Generates ~20-30 millivolts when properly heated
- Loss of flame breaks the circuit and shuts gas valve
- Common failure point in standing-pilot furnaces
Memory trick: No heat on the tip, no juice, no gas.