Vent Axia VA100XH 251710 4 Inch Bathroom Fan with Humidistat, Pull Cord and Thermo-Electric Shutter 

Specification
Air Extraction : 107m3/hr
Air Extraction L/s : 30 L/s
Noise : 36dB(A)
Watts : 18W

The Vent Axia 251710 VA100XH is designed for use in domestic bathrooms and utility rooms. It extracts stale air and moisture at a rate of 25Ls - 10L/s more than the required airflow to meet Part F of the building regulations. It may be installed in the wall, ceiling or window (with window kit) of zone 3 of the bathroom.

The 251710 'XH' model features
1) thermo electric shutters - which prevents cold air entering the bathroom when the fan is not in use - and a 2) humidistat - which turns the fan on automatically when it detects steam or moisture. The RH (relative humidity) level which triggers the fan may be adjusted on install. This feature is great for landlords or any situation where users may forget to turn on the fan.