It's not something I claim to have known about before geoff mentioned
it, but it encouraged a bit of googling ...
It seems it stems from the burning of gas ionising it, so you've got
some free electrons available, if you try to pass A/C between the base
of the flame and the tip, it'll happen more readily in one direction
than the other, you detect the conduction varying with the phase (rather
than just conduction itself, or lack of it).
My Viessmann boiler threw a wobbly and tripped out. Press the re-set, and
it went through all the normal startup procedure (fans whirring, solenoids
clicking), but then tripped again, so I was hoping it was nothing major.
The fault code Eb, means 'heat draw off repeatedly too low during
calibration' The cure is to 'initiate heat draw off and trigger manual
calibration (see page 89)' which didn't help much as there's nothing on
page 89 about it. And when I did find it, it didn't seem to be applicable
to my version of the boiler as the calibration required you to use a code
number which didn't exist.
However, similar sort of faults mentioned the ionisation electrode so in
desperation I followed the service instructions for removing and cleaning
it - and to my delight it sorted things.
What does it do?
The same problem was on my Wiesmann vitodens w 300. The service team was checking it few times. No result. After I have cleaned the ionisation electrode and it's connector now all is ok. Thank you for good advice.