Humidstat fans have a sensor in them. This detects moisture in the air. When a set humidity point is reached the fan will switch on. There are two types of sensor – Traditional and Digital/Intelligent.
Traditional. These work by having a fixed humidity level which, when reached, the fan activates. Traditional humidistats work well but as natural levels of humidity rise and fall between summer and winter, they can become more or less sensitive to switch on periodically.
Digital/Intelligent. These have no fixed humidity point for activation. Instead they constantly measure humidity levels and only activate when man-made spikes in humidity - caused by showering, bathing or cooking - are detected. Consequently they are more responsive to run only when required