What sign indicates that humidifier pads need replacing?

The choice indicating decreased humidity levels and poor moisture output is correct because these symptoms are typically associated with worn-out or saturated humidifier pads. When humidifier pads become clogged with minerals or debris, they lose their effectiveness in absorbing water, which subsequently leads to a decreased ability to add moisture to the air. This results in lower humidity levels in the indoor environment, as the system is unable to function properly and provide the desired moisture output.

In contrast, while increased noise, higher energy consumption, and foul odors can indicate various issues with the humidifier, they are not specific indicators of the need for replacing the pads. Increased noise could suggest mechanical issues or lack of lubrication rather than pad condition. Higher energy consumption might arise from other factors, such as overall system inefficiency or other components failing. Foul odors could be a sign of mold or mildew, but again, this does not directly point to the need for pad replacement in all cases. Thus, focusing on the performance in terms of humidity output is the best indicator for pad condition.