I still don't understand my oil furnace!

I'm going to buy a new furnace in the next 9 months but I'd still like
to understand the one I have now. Maybe the new one will have the same
parts.

When the flame goes on, no noticable air comes out of any of the
connections, but (I've patched all this now, but it used to be that)
when the air circulating fan comes on a minute or two later, air blows
out from lots of places. Is this normal?

1) One of them was under the edge of the flu collector. I know that
wasn't normal, and I'm not asking about that. I caused it when I took
the part off and when putting it back on, I had a surprisingly hard
time positioning it correctly and the sealant I'd put on got spread
around too much and the amount wasn't enough everywhere.***

2) Another place was just above the furnace end of the flue collector,
above it but not related to it, at the very top of the furnace. Only
on the right half for some reason, I couldn't see it and I didn't
bother to get a mirror, but I could feel a slot at least a quarter
inch front to back and 6 or 8 inches wide, where it seemed the furnace
flange for the air output duct was somehow not as wide front to back
as the duct itself was. How could that be! And why only one one
half? Anyhow, I also filled that with hi-temp silicone.

3) And most interesting, after I sealed those two areas, lots of air
was still blowing out when the blower was on, from around, I don't
know what you call it, the fire window tube? There is a "tube",
about 5" in diameter, with a spring loaded metal door, which one can
lift up with a tool (because it's hot) and see the fire itself in the
fire box. (This tube is about 6 inches above the whole burner thingy,
the transformer, oil pump, electronics etc.) A few years ago, when
the furnace was mal-adjusted, soot piled up in this tube. The left
half of the circumference of this big tube, at the base of it, had air
blowing out when the burner and fan were on. Enough to be easily felt
with my fingers.

Since right on the other side of the wall was the fire box, was this
air exhaust air?

It didnt' smell like oil, or feel oily or dirty, but if the burner is
set up right and the combustion is complete, does the exhaust air from
an oil furnace have a smell?


And why would more air be coming out of the fire box when the air fan
is on then before it goes on? Isn't the firebox and flue and chimney
separate from the air in the cold and warm air ducts? I scraped and
vacuumed the 6 or 7 oblong tubes in the heat exchanger and they seemed
hard as iron, no rusting or leaks there. If there were leaks, where
would they be?

Thanks.


***which I posted about here a couple weeks ago, which had rusted
through because my humidifier dribbled water on it for years. I had
another layer welded onto three sides and then I used hi-temp silicone
sealant** to patch the smaller holes the welding caused. When I went
to screw the thing back to the furnace, I had a surprisingly hard time
positioning it correctly and the amount of sealant I'd put on wasn't
enough everwhere. I could have used "furnace wool", I think it is
called, which is like a gasket, but online the only thing I found was
a whole roll for 22 dollars, and I didn't take the time to hunt for it
locally. Partly because I'm not going to take this furnace apart
again.


**The hi-temp silicone in 4 oz. tubes is only available at auto parts
stores, not the big box hardware stores I've been to. Maybe the big
box tubes has the stuff in caulking tubes, I don't remember. But a
little of this goes a long way.)