The area of extracting useful heat from the fuel combustion process is where the condensing furnace truly separates itself from the conventional furnace. As stated earlier, a condensing furnace extracts useful heat even after the combustion exhaust gases have "cooled" and condensed. This is accomplished by using two heat exchangers (unlike one for conventional furnaces), one for primary heat exchange and a secondary heat exchanger to handle the corrosive condensed exhaust gases of water and carbon dioxide (which form carbonic acid). The exhaust gases are depleted of heat until the water condensate drips out of the furnace's heat exchanger and the flue gases escape from a plastic PVC pipe instead of a chimney.
- Primary heat exchanger: The primary heat exchanger of a condensing furnace is similar to a conventional furnace. It removes the most amount of heat from the combustion flue gases. The primary heat exchanger is made of a specially coated steel tube system.
- Secondary condensing heat exchanger: The secondary heat exchanger is unique to the condensing furnace. This heat exchanger is made up of small tubes that receive the exhaust gases once they have gone through the primary heat exchanger. Here, more heat is extracted from the exhaust gases and as a result the gases are cooled to the point that they condense into water and carbon dioxide. Because water and carbon dioxide form a slightly acidic condensate called carbonic acid, the secondary heat exchanger must be made of stainless steel in order to resist corrosion.
- Condensate drain line: The carbonic acid condensate resulting from the gases going through the secondary heat exchanger must be drained. They are discharged by a plastic PVC pipe and usually discharge into a floor drain. It is critical to ensure integrity of all connections and to maintain proper slope for drainage because the condensate will attack and eat holes in the furnace body or any other metal with which it comes in contact.