Airvent 448381 Axial Extractor Fan with Backdraught Shutter - 100mm (Humidistat / Timer) This model incorporates a condensation (humidity) sensor which automatically switches the fan on when the condensation level builds up. Once the condensation clears the fan turns off. It can also be connected to work with the room light switch. Requires at least a permanent unswitched live and neutral connection. If the timer functionality is required then an additional switched live connection is required.