"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?

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