[EE]: Flame rectification

I was doing a cleaning of my gas-fired forced air furnace, and it got me thinking about how wonderfully designed it must be. It has a Honeywell S8610U "Intermittent Pilot" module that combines both high(15KV-ish) voltage ignition and low(80VAC-ish) voltage, low(5uA-ish) current flame detection on the same wire and electrode. Fantastic. Especially since this very module has been running since 1997, with very little maintenance, and has not missed a beat.

Has anyone reverse engineered the in and outs of this device and can describe the circuitry, and how flame detection doesn't die the first time the high voltage comes on? And how it can toggle between the two modes so quickly and reliably?

For those who haven't encountered it, the module runs on 24VAC, generates a recurring high voltage spark at twelve Hz, energizes a pilot gas valve, senses the pilot ignition on the same wire using flame rectification (in between the high voltage sparks) and energizes the main gas valve if there is a pilot flame present, then stops the HV and continues to monitor the flame. It sounds like bzz-click-bzz-thunk-whoosh when starting. Quite obsolete by today's standards, of course. It was built to last almost forever :)

Thanks! Bob