With a conventional gas furnace of an AFUE below 90 the technology used is pretty basic. With high efficiency furnaces however, it's a whole different story.
The high efficiency or condensing furnace having furnace efficiencies of AFUE 90 to 94 and higher is a pretty impressive piece of technology. These furnaces are different in three main areas:
the specially designed heat exchanger technology used to get every bit of heat from the combustion process,
the way air is brought to the furnace for combustion, and
how combustion gases are exhausted.
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