Techs read a lazy flame as starved for air or momentum: low combustion air, poor draft, a dirty burner head, wrong gas pressure or a worn oil nozzle. It burns slowly and incompletely, leaving CO and soot, and the flame can impinge on tubes and refractory. A healthy gas flame is mostly blue with sharp edges, and a good oil flame is bright and stable without dark streaks.
Got a lazy flame, orange and rolling around the back of the furnace, check the air shutter and the vent before anything else.
Related terms
Running rich
Burning with too much fuel for the air available, so combustion is incomplete.
Excess air
Combustion air supplied beyond the exact amount needed to burn all the fuel, expressed as a percentage.
Air-free CO
Carbon monoxide in flue gas corrected to zero excess oxygen, so dilution with air cannot hide a CO problem.
Sooted up
Coated with soot on the fire side, from rich or poor combustion, so heat transfer and draft suffer.