When do plate humidifiers operate?

Plate humidifiers are designed to operate whenever the air handling unit's fan is running. This allows them to continuously add moisture to the air as it passes over the plates of the humidifier. The operation during fan activity ensures that the humidified air is distributed throughout the living space, maintaining a consistent humidity level.

In contrast, plate humidifiers do not function solely based on humidity demand or external temperature conditions, which is why the other options aren't applicable. For instance, they may not operate only when the humidistat indicates a need for humidity, as they are meant to work in conjunction with fan operation rather than just responding to humidity levels. They also do not limit their operation to specific seasons, such as the humid summer months, when humidity levels may already be sufficient. Finally, they aren’t activated simply by outside temperatures dropping, as their primary function is tied to the airflow of the system. This is what makes continuous operation while the fan is running the correct understanding of how plate humidifiers operate.