The four ignition systems. Episode fifty-eight of the Gas Service Animated series.

Every gas appliance answers the same three questions — is there a call, can I light, can I prove it — but four different ignition systems answer them four different ways. Know the sequence and the fault finds itself.

System one: the standing pilot. The pilot burns around the clock; the thermocouple holds the safety as long as it proves. A call for heat simply opens the main valve, and the pilot lights the burner. Its failures are last episode's story — pilot outages, weak thermocouples — and its relight follows the valve-knob sequence on the label, every time.

System two: the intermittent pilot. No standing flame — on a call, the board sparks and opens the pilot valve together. The pilot must prove, by rectification, before the main valve is allowed to open; then the pilot lights the main. Fail to prove and it retries, then locks out. It's the transition species: pilot safety logic, electronic proof.

System three: direct spark. No pilot at all. The board runs a prepurge — clearing the box with air — then fires the spark and opens the main valve together. The flame has a trial for ignition, typically four to seven seconds, to appear and prove. Miss it, and the board recycles for another try; miss enough tries, and it locks out. Everything happens fast and at the main burner.

System four: hot surface. Prepurge, then the igniter glows — a resistance element heating to orange before the valve opens onto it. Old silicon-carbide igniters run roughly forty to ninety ohms and are famously fragile — never touch the element; oils and fingers kill them. Newer silicon nitride runs about eleven to twenty ohms, heats faster, and survives handling. Ohm the igniter against its spec and half your no-lights are solved at the bench.

The timing you don't see: prepurge before the attempt, interpurge between attempts — deliberate air changes flushing unburned gas before any spark or glow. When a board seems to 'wait forever' before trying, it isn't stalling; it's purging. Respect it — it's the reason failed attempts don't become events.

And when all retries fail: lockout. Soft lockout resets itself — commonly after about an hour — which is why 'it works again by the time I get there' is a real phenomenon, not a ghost. Hard lockout takes a power cycle or a reset. The fault light tells you the subsystem in code. But the rule from the no-gas season stands: reset once to observe, never repeatedly — a lockout is evidence, and clearing it without a cause is erasing the evidence.

Line the four up and the diagnosis writes itself. No light on a standing pilot: think thermocouple. Lights-then-dies on any electronic system: think proof — microamps. No attempt at all: think call and safety chain. Attempt but no flame: think igniter, gas, or trial timing. Four systems, one method.

Ignition in one breath. Standing pilot holds by thermocouple. Intermittent pilot sparks, proves, then mains. Direct spark lights the main inside a seconds-long trial. Hot surface glows first — ohm the igniter. Purges are deliberate, lockouts are evidence, and the sequence tells you where it died. Next: the meter that ties it all together.