Which component of a high efficiency gas furnace is typically not found in a mid efficiency furnace?

• A. Secondary heat exchanger
• B. Induced draft venting
• C. Heat exchanger coil
• D. Electronic air cleaner

Answer: (A)

In a high-efficiency (condensing) gas furnace, an extra heat-recovery stage is used to squeeze out as much heat as possible from the combustion gases. This is the secondary heat exchanger. As the exhaust gas is cooled further, water vapor condenses, releasing latent heat that would otherwise be lost. This design pushes overall efficiency higher, but it requires materials that resist the acidic condensate and a venting setup suited for cooler, moist exhaust (often PVC). Mid-efficiency furnaces, on the other hand, use only a single primary heat exchanger and do not condense the exhaust, so they do not include a secondary heat exchanger. The other components—such as induced-draft venting or a basic heat exchanger—can appear in various configurations, but the secondary heat exchanger is the feature that distinguishes high-efficiency units and is not found in mid-efficiency models.