Centrifugal fans: often fitted in the ceiling, these are designed for bathrooms that are not close to an external wall, but over 1.5m away.

If your bathroom has a toilet only, you are fine to use a window alone as your source of ventilation.Axial fans: these connect directly through an external wall and are used when the air doesn't need to be moved long distances.

In new-build bathrooms with a bath and shower, mechanical ventilation is required in the form of an extractor fan. This is because new-builds are built to be more airtight, so natural ventilation is much lower. Power source – This is the main power supply for the fan. It typically consists of a dedicated circuit breaker, but may also include other components such as a fuse box.

The last point means that the 15l/s, or 54m3/hr, refers to the measurement of how much old air is replaced by new air.