Flame rectification. Episode fifty-seven of the Gas Service Animated series.

Every modern burner has to prove its flame in under a few seconds, or the gas closes. The proof is one of the most elegant tricks in the trade: the flame itself becomes an electrical component. This is flame rectification — the microamp story behind every ignition lockout you've ever met.

Here's the trick. The control puts an alternating signal on the flame rod. A flame is a cloud of ions, so it conducts — but the rod is tiny and the grounded burner is huge, so current flows far more easily one direction than the other. The flame clips the AC into pulsing DC — it rectifies it. The board isn't looking for conduction; it's looking for that DC signature, because a short circuit conducts but doesn't rectify.

The numbers are tiny. Healthy flame current runs on the order of one to ten microamps DC — some systems spec half a microamp to four. Dropout sits under about one. You measure it in series: meter on DC microamps, in line with the flame-rod lead — or read it straight from the board's diagnostics if it offers them.

Now the killers, in the order you should suspect them. A glaze of silica and oxide on the rod — it insulates. A poor burner ground — rusty burners, a missing ground strap — no return path, no current. A cracked ceramic insulator leaking the signal away. And the sneaky one: reversed line polarity. Swap hot and neutral at the outlet or the board, and the AC reference collapses — the appliance lights, proves nothing, and locks out. Check polarity before you condemn parts.

Cleaning the rod is a religion with two denominations. Fine abrasive, gently, per the manufacturer — never coarse emery that pits the surface and glazes faster next time. And some makers say don't clean, replace. The manual settles it. Either way: the rod must sit in the flame, not at its edge.

On many direct-spark and hot-surface systems, one electrode does double duty — it sparks or glows to light the burner, then becomes the flame sensor. Same physics, same failure modes, one part. If a single-rod system lights and immediately dies, think sensing, not ignition — the flame was there; the proof wasn't.

So the lockout diagnosis runs: does it light at all? If it lights and drops out, measure the microamps. Low? Clean or replace the rod, prove the ground path, inspect the ceramic, verify polarity — then measure again. You're not guessing parts; you're restoring a number. The board wanted microamps; give it microamps.

Why this matters beyond furnaces: the same rectification proof runs boilers, tankless heaters, unit heaters, and rooftop equipment. Master one microamp measurement and you've learned the flame-proving system for most of the modern gas world.

Rectification in one breath. The flame clips AC into DC microamps — that's the proof. Healthy is a few microamps; dropout is under about one. The killers: glazed rod, bad ground, cracked ceramic, reversed polarity. Restore the number, don't guess the part. Next: the four ignition systems, in sequence.