Ventilation is key in controlling moisture and pollutant levels in any building, if the ventilation is not sufficient it can lead to an unpleasant living environment and in the longer-term problems with the building and/or health issues.

Although extract vents claim a level of air flow based on laboratory testing this does not guarantee they will achieve this in practice due to various issues but most commonly the installation. Therefore, Approved Document F (Ventilation) requires background ventilation to be tested after installation meets the requirements, for example for intermittent extraction:

The testing is done using a hood placed over the extraction fan which channels the air the fan draws through a calibrated anemometer which measures the air flow.

The testing is best completed in combination with air or sound testing to minimise the costs.