What am I likely to find inside the oil furnace flue collector and
heat transfer tubes, rust or the products of combustion?

When I took off the metal box that connects my furnace to the flue,
because holes had rusted in it, of course it was raggedy. Even on the
sides that had no holes, like the inside bottom, there was a thick
layer over all of it, maybe a 16th of an inch or more (3 mm?) and the
layer came off when I hit or scraped it, or hit the edge of it where
an adjoining piece had fallen off.

What do you think this stuff is, rust or the products of combustion
that were gradually deposited on this 30 year old furnace? (In
Baltimore where the heating season is 4 to 5 months.)

An argument for deposits seems to me to be that underneath it all, the
metal is flat and smooth, not smooth like formica, but "textured" with
no dips or bumps that are bigger than the others. And it has no
layers, even though the stuff that comes off is a separate layer. If
it were rust coming off in a layer, I would not expect the part that
is left to be the same layer, the same thickness everywhere.

Also the pieces that came off would crumble if I tried to bend them.

The guy who welded it said it was rust, but he was in his 20's.

So, what do you think was flaking off?


Tonight when I was cleaning the heat transfer chambers, they too had
stuff that flaked off, a few pieces as big as a quarter, many as big
as a penny. They too might have been rust or combustion products.
Here too, the inside of the chambers seemed flat and smooth.

So, what do you think was flaking off?