What is a common indication that a furnace's heat exchanger may be damaged?

A yellow burner flame is a significant indication that a furnace's heat exchanger may be damaged or functioning improperly. In a properly operating furnace, the burner flames should be blue, which indicates that the gas is combusting efficiently and completely. When the flames turn yellow, it suggests incomplete combustion, which can occur due to various issues within the furnace, including a damaged heat exchanger. This incomplete combustion can release carbon monoxide, a dangerous gas, signaling that further investigation and potential repairs are necessary for safety.

In this context, while other options like increased energy bills, soot buildup, and strange noises can also indicate issues with the furnace, they are often less directly associated with heat exchanger damage. Increased energy bills may reflect overall inefficiencies, soot buildup generally indicates a combustion problem, and strange noises could be caused by multiple mechanical issues. However, yellow flames specifically point towards combustion and heat exchanger problems, making it a critical indicator of potential damage.