[nochaos]

I decided to think about this a bit more...

1) The 120 to 24V switchover for this hybrid HSI/ flame sensor is written on the schematics I found inside the furnace housing.

Now, my opinion on the 24V disappearing act seems to be erroneous as I realized that the relay inside the controller would trip when I placed the (crapy) multimeter terminal on the IGN connector. If the relay is so touchy-feely it must be either an issue of a messed up relay - about to crack, inside the controller, or (I guess) a matter of unstable flame sensor - maybe even dirty.

I am holding back from purchasing the controller till I:
- a) check the HSI/sensor and clean it up - what is a safe and effective way to do that?
- b) do the knocking test on the controller when the relay won't switch to 24V - it will be a matter of timing and listening...

2) I am tiny bit less desperate as I seem to have a way for keeping us warm by ensuring that I do not give the controller "ideas" - i.e hiking up the temp on the thermostat high enough to have the furnace work continuously.
When it will get very hot we will switch off and that should take us through the night.
I will sure have to wake up early, or at night to turn it back on....

But I should now be very close to solving this mystery.

So, is it the HSI/sensor, or is it the controller? I should be able to tell by tomorrow, after I return home from work...

Thanks for your help.
I will update again tomorrow,
Eugen