Ducane CMPE100U4B 100,000 BTU 4 Ton 90 Upflow Furnace NIB

High efficiency furnaces are also known as "condensing" furnaces. This is because the gases generated by the furnace's initial combustion are forced through a series of pipes, where they give up more and more of their heat. Eventually, these gases cool enough that their water vapor condenses into liquid water, and is discarded. The extra efficiency of these furnaces is due to their ability to capture heat during this "condensing" phase.

One disadvantage of condensing furnaces is that they need a drain to discard the (often quite acidic) condensed water that is generated as the furnace extracts heat from water vapor.