Furnace Pilot Light Yellow Flame

Dust, rust, or spider webs in the burner ports or pilot orifice cause incomplete combustion. Flame can't get enough oxygen at the burner ports, so it burns yellow (carbon-laden flame instead of clean blue combustion).

Visual inspection of burners (power OFF). Look for visible debris, rust, or yellow-tipped flames. Pilot orifice (small opening) may be visibly dirty.

Combustion air supply is restricted — closed combustion air intake, lint/dust around the unit, or air shutters out of adjustment. Without proper air mix, gas burns incompletely (yellow flame, CO production).

Check that combustion air intake is clear (some furnaces draw from inside, some from outside). Look for laundry lint, dust, or stored items near the furnace.

Air movement near burners distorts flame shape, mixing combustion air unpredictably. Flame appears yellow, orange, or wavering. Can be caused by return air leaks near burner area.

Light incense near the burner area. Significant air movement = draft. Look for nearby return duct gaps or vent terminations.

Cracked heat exchanger lets blower air interact with the combustion flame, distorting it. Yellow or wavering flames during blower operation are a CLASSIC sign of heat exchanger crack. DANGEROUS — CO leak risk.

Watch flame closely when the blower kicks in. If flame distorts or color shifts dramatically (blue to yellow), crack is likely. Yellow flame on a furnace with rust streaks on the heat exchanger is highly suspicious.

Insufficient gas pressure produces a weak, wavering flame that often burns yellow. Different from no-gas situations — flame exists but is anemic.

Other gas appliances also seem weak. Flame is small and unstable. Tech measures gas pressure with a manometer.

If furnace was converted between natural gas and LP propane (or vice versa), wrong orifice size produces poor combustion. Yellow flames.

Was the furnace recently moved or converted between fuel types? Check the data plate on the unit and compare to the gas type you have.