And while 78% AFUE is a significant improvement over a 20 year old furnace you may have in your home, it's not classified as high efficiency by the DOE.
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The measure used to determine how well a furnace was running is known as an Annual Fuel Utilization Efficiency (AFUE) rating.
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This will lower the amount of gas you require to run the furnace, making even a furnace with a slightly lower AFUE rating more efficient.
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Today's high efficiency furnaces are available with an AFUE of up to 96.7%, which translates into a serious savings on your gas bill each winter.
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You may find that purchasing a unit with one or two percentage points lower AFUE rating will actually save you more after the installation costs have been taken into consideration.
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You need to see the actual AFUE of the furnace you are considering.