Old Carrier furnace and heat-exchanger

'owdy,

I belong to a little brick bungalow in the US midwest. Back in
'84, I had new heat/AC equip. installed.

The furnace, a middle-of-the-line Carrier (but made by Bryant)
is a standard forced-air-system unit, maybe 120k btu, with an
elec. vent damper. Has given good service, 'tho I had to
intall a new gas valve in the 90's.

Every late summer I clean the rust out of it. And I -try-
to get a peek at the heat-exchanger with one of those
little hinged mirrors like the dentist uses. Can actually
see very little.

About how long might such a furnace last with average
(midwest) usage?

The part that usually fails in late life is indeed the
heat-exchanger? How can I tell when its failing? I
assume I'd need to replace the unit and budgeting
could be b-b-bitch.

Thanks,
Puddin'
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