Gas burner will not ignite

Every gas water heater lights in the same order: gas must arrive at the correct pressure, the ignition source must spark or hold a pilot, the flame must be sensed, and the control must keep the gas valve open. A failure at any link stops the burner, and the symptoms differ: a pilot that will not light at all points at gas supply or the pilot assembly; a pilot that lights and goes out points at the thermocouple or thermopile; a burner that never fires with the pilot lit points at the thermostat or gas control.

Tankless units report the same chain as codes: Rinnai's code 11 is No Ignition and code 12 is No Flame, and the checks the manual lists run from the gas supply to the igniter, flame rod and gas solenoids. A code is the starting point for the diagnosis, never the diagnosis itself.

Before calling, confirm the obvious supply items: the gas valve at the heater is open, other gas appliances work, the propane tank has gas, and the unit has power (a display lit or an LED pattern visible). On a standing-pilot tank, the manufacturer's lighting procedure in the manual is a homeowner step, and a thermocouple that fails right after lighting is the classic cause of a pilot that will not stay lit.

Stop at the point the manual stops: gas pressure adjustment, orifice changes, igniter and flame-rod work, and opening the gas train are licensed-gas-fitter work.

Repeatedly resetting a tankless unit or relighting a stubborn pilot does not fix the cause and, on some controls, extends lockout timing. The useful information for the technician is the code, the flash pattern, whether the unit fires at all, and what changed recently: a new gas line, a tank swap, or a propane fill are all classic ignition-failure triggers.