There are two main ways to improve the efficiency of a gas furnace. The first is to increase the surface area of the heat exchanger. Most new high efficiency furnaces apply this principle. They're known as "recuperative" or condensing furnaces. As the name implies, these furnaces gain efficiency by recovering or recouping heat and condensing moisture from the exhaust gases.
They do this by adding a secondary condensing heat exchanger. In primary heat
exchangers, some heat is transferred as the hot gases pass over its surface.
A lot of heat slips by and is wasted up the chimney. By adding this second,
condensing heat exchanger, the exhaust gas has time to give up its heat, or
cool to the point that the water vapor contained in it condenses (or reverts
to its liquid state) giving up what is called its latent heat of vaporization,
970 BTU per pound.
In addition to increased energy efficiency, condensing furnaces offer the added advantage of not requiring a traditional chimney. Because the exhaust gases have cooled off so much passing through both heat exchangers, they can be horizontally vented directly to the "outside" making furnace location much more flexible. Condensing furnaces have a disadvantage: they require a drain to get rid of the condensate, (another word for the condensed water - which can be fairly acidic.)
The second way to increase furnace efficiency is to add a draft inducer. Most "conventional" furnaces use atmospheric burners, which allow air to move naturally through the combustion chamber. More efficient systems use fans to either force air into the combustion chamber or induce it out the flue. Power venting increases the flue gas velocity across the heat exchanger thereby increasing heat transfer.
Energy efficient furnaces and features cost more than conventional models, but the added investment can pay back in relatively short periods, (especially in areas of the country with long heating seasons or high fuel prices).
