I can't speak to water tube boilers specifically, a lot of spark ignition
systems use flame rectification to sense flame.
When a flame exists, there is ionization of the gas between the ignition rod
and the burner. If you apply a positive voltage to the burner, there is a
current typically of a few uA from the ignition rod to the burner. If the
physically much smaller ignition rod has a positive voltage on it, most of
the negative ions are drawn toward it but miss it (much like current flow
past the grid or screen in a vacuum tube).
Rogue Engineering Inc.
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