"j j" wrote in message
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"CBHvac" wrote in message
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"j j" wrote in message
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our furnace is probably 50 years old. it's a big cast iron thing.

everyone we asked told us not to change it because new furnaces have
crappier construction.

Then you are asking the wrong people.
Granted, they dont make tanks anymore.

what determines the efficiency of a furnace?

The design, and the final AFUE rating.


is it the fact that it's a tank (are modern furnaces different?) or the
burner itself?

Both. Todays furnace isnt the one that you had when you grew up...(well..for
some it might still be..LOL) but seriously, huge design changes, computer
controls, secondary heat exchangers, better blowers...it all adds up.