I have a very old, very large Bryant furnace (circa 1938) in my
basement. Last spring, near the end of the heating season, I
noticed a gassy smell when the furnace first fired.

Today I had the unit examined by a technician from a local
(reputable) repair company. When we turned the furnace on, his
CO meter went insane as we approached the front of the unit.

Now, the trouble is that he couldn't find the source of the
"leak." It seemed to him that the burned gases weren't reaching
the flue properly. It could be that there's a blockage in the
heat exchanger or the exhaust pipes. Or there could be a crack
in the heat exchanger itself. (It's just not possible to get a
full view of the exchanger without taking part of the furnace
apart.)

I'm trying to decide on options. Should I just get a new
furnace? Or should I pay the furnace company a fair bundle to
disassemble the unit and look for a block or leak? Will
exchangers that are approaching their 70th birthday even come
apart, so we can look for blockage?

If a new unit is need, what's the ballpark price in the
Pittsburgh region? (The old, 1938, ratings were 200,000 BTS in
and 160,000 BTUs out. But several furnace folks have told me
that a new unit wouldn't need to be anywhere near as robust,
because newer furnaces lose less of their heat up the chimney.)

Thanks.

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Brian E. Clark