Re: [EE]: Flame rectification

At 07:44 AM 2025-12-24, you wrote:
>I have been working on (baffled by) my daughter's Payne high 
>efficiency furnace. The flame detector seems to be a simple steel 
>rod. It has only one electrical connector with about 90V on it. I 
>assumed, at first, that it was some kind of thermocouple referenced 
>to ground through its one mounting screw. However, the path from the 
>connector to ground seems to be open. Diagnostic instructions 
>indicate that it should carry .5 micro-amps or more. It looks like 
>the flame provides the path to ground. It it gets dirty it doesn't 
>work. Does anybody know anything about this?
>
>Allen Mulvey
>--

This is exactly what we are talking about. There is an AC voltage 
between the flame rod and ground.
The flame passes a small AC current (as would contamination) but 
unlike contamination, the
conductivity is not symmetrical with polarity (hence 'rectification')

But they are usually pretty reliable. Maybe the control box is failing?
You can clean the flame rod with Scotch-Brite, but I've not found it 
necessary, literally in decades.
That's with natural gas fuel in a York HE furnace. Maybe propane is dirtier.

BTW, there are 'universal' replacement control boxes available which 
you can configure
for the required restarts, purge times etc. by means of plug-in 
cards. Eg. Emerson. Not expensive, at
least not compared to a new furnace, especially if they give you the 
contractor price.

Best regards,
Spehro Pefhany