Final answer:
The older house with an aging furnace is facing curable functional obsolescence, as it is a depreciation factor that can be remedied by replacing the furnace.
Step-by-step explanation:
If an older house begins to lose value due to an aging, inefficient furnace compared to newer homes in the neighborhood, and the furnace can be replaced, it can be said that the older house is suffering from curable functional obsolescence. This type of obsolescence occurs when an outdated feature of a property can feasibly be upgraded or improved, restoring value and function. This is in contrast to incurable obsolescence, where the depreciation factors cannot be remedied cost-effectively.