Furnace Electrode

I have a question but first some background on the question. A friend showed me the starter from his old DuoTherm 65930 furnace. The starter has three thin probes; the first is attached to a insulated spark plug unit, which I assume is the lead from the capacitor. Very near to this one, is the second one which is attached to the frame. It has to be the ground, right? The third just looks to be the grounding probe for some solenoid. But here’s my question. The third probe, is raised above the burning area, therefore does the gas act as a conductor or is the flame the conductor? If either of the answers is yes, then it sure is something I never knew about in my 68 years of life!
Edward, (Victoria, BC) 

Wow Edward, you sure dug up an oldie-but-goodie! It just proves a point I’ve been preaching for years; that well maintained appliances and RVs can literally last for years and years!

What your friend showed you was the electrode assembly of the furnace. There’s an electrode assembly found in virtually every DSI (direct spark ignited) appliance. Some electrode assemblies only have two probes, but many of the older ones had three. As you guessed, one probe is the high voltage ignitor probe that receives the spark from the printed circuit board. When the voltage jumps the gap between this probe and the center, ground probe, it ignites the LP that comes into the burner. The third probe is called the “flame sense probe” and indeed it “senses” the presence of the flame by measuring the micro-amps produced by the flame and sends this current back to the printed circuit board, letting it know the burner is still lit. If the flame were to blow out for any reason, it’s this flame sense probe which tells the board to turn off the gas solenoid. This is but one of the safety devices incorporated in the LP appliances. It’s important that all three probes are kept clean and free of corrosion and carbon buildup. It’s something that must be checked annually.

Also important is the gap spacing between the three probes. Though some appliances may differ, a general rule of thumb is that the gap between the ground probe and the flame sense probe be twice as much as the gap between the ground probe and the spark probe. You certainly do not want that spark jumping to the flame sense probe. You’ll be replacing the printed circuit board if that happens! I’ve always asserted that if the electrode assembly is kept clean and gapped correctly and the LP pressure is set correctly, that the majority of furnace operational problems would be eliminated.