What could be an effect of an over-firing furnace?

An over-firing furnace occurs when it operates at a level higher than its designed capacity, often leading to a variety of problems. In this context, the best choice relates to a malfunction of the safety controls, specifically the limit switch.

The limit switch is a safety device that monitors the temperature within the furnace and shuts off the burner when the temperature exceeds a preset limit. If the furnace is over-firing, it can cause the temperature to rise too quickly or exceed the limit for too long, eventually leading to the limit switch opening. When this happens, the furnace stops the combustion process to prevent overheating, which is a critical safety mechanism designed to protect both the appliance and the home from potential fire hazards or damage.

The other choices do not directly relate to the immediate consequences of an over-firing situation. For instance, increased energy efficiency is contrary to the effects of over-firing, as excessive heat production typically leads to higher energy consumption. Higher indoor humidity is also usually a sign of insufficient dehumidification rather than an over-firing issue. Improved airflow does not inherently result from an over-firing furnace; in fact, it may create various airflow problems due to improper balance or system strain. Thus, the other options do not align with the