Which is a sign that a gas furnace may be inefficient?

High energy bills without changes in usage is indeed a significant sign that a gas furnace may be inefficient. When a furnace operates inefficiently, it tends to consume more fuel or energy than necessary to maintain the desired indoor temperature. This can result from various issues such as poor insulation, malfunctioning components, or age-related wear and tear, leading to increased operational costs even if other factors remain constant.

In contrast, some of the other options indicate normal or positive furnace performance. Quick heating times suggest that the furnace is effectively delivering heat, while consistent airflow indicates that the system is working properly to circulate air throughout the living space. Regular maintenance checks are also a sign of good practice, ensuring that the furnace operates efficiently and safely. Thus, high energy bills stand out as a clear indicator of potential inefficiency.