How does a faulty thermostat primarily affect an HVAC system?

A faulty thermostat primarily affects an HVAC system by causing incorrect temperature readings and inefficiency. The thermostat is the device that regulates the temperature by communicating the desired setpoints to the heating or cooling units. When it malfunctions, it might not accurately sense the current temperature or interpret the setpoint correctly, leading to the system operating under false assumptions. This can result in the HVAC system running longer than necessary or not running at all, creating fluctuations in temperature that lead to discomfort and excessive energy consumption.

In contrast, options about keeping the system running continuously or only affecting heating functions do not encompass the broader impact of a malfunctioning thermostat, as they might not accurately describe the extent of the problem. Moreover, suggesting that a faulty thermostat improves air quality does not align with its function; in fact, an inefficiency in the HVAC system could lead to poor air quality due to inadequate ventilation or filtration. Therefore, the correct answer highlights the significant role of the thermostat in maintaining efficiency and accurate temperature control.