What might be the cause of excessive humidity in an air-conditioned space?

A malfunctioning dehumidistat is indeed a likely cause of excessive humidity in an air-conditioned space. The dehumidistat is designed to control the humidity level by regulating the operation of the air conditioning system. When it is not functioning correctly, it may fail to signal the system to reduce humidity levels, leading to a buildup of moisture in the air.

This malfunction can result in the air conditioning unit not running long enough or effectively to remove the moisture, thus allowing humidity levels to rise. It’s important to regularly check and maintain the dehumidistat to ensure that it operates properly and maintains a comfortable environment.

In contrast, the other options have their own implications but do not directly indicate a malfunction of the system's humidity control. Low temperature settings can lead to some moisture removal by the cooling process, while drafty windows and frequent door openings introduce outside air, which may contain higher humidity and affect indoor levels but are not mechanical failures in the HVAC system itself.