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Richard Popenhagen

I have played around a bit with different bathroom fans over the years and from my own personal experience going from some pretty average fans to much better, my ballpark recommendations are minimum size of 150mm (15cm) diameter including the ducting. Some fans, although appearing quite large, only have 100mm ducting on the exit.
Re airflow, yes an absolute minimum airflow of 250m3/ hour, which equates to about 70 litres a second, however more is better.
As an example I have attached some photos shot this winter in our bathroom. The first one is on exiting the shower the RH in the bathroom is 83%. Seven minutes later the fan has dropped the RH down to the magic 60% RH mark. Another five minutes later the RH is down to 48%, not bad for a bathroom on a cold winter’s morning!
This is where I need to fess up, our current bathroom fan is very powerful, although still only 150mm diameter its rated specification is 597m3/ hour (166 litres per second). It is so powerful that if you don’t crack open the bathroom window, we struggle to shut the bathroom door against the air pressure.
The point I want to reinforce is that you are better off with a very powerful bathroom fan, where you can reduce the moisture load quickly, rather than an undersized fan often which won’t have reduced the moisture adequately even after 40 minutes plus running time. Do it hard and do it FAST!
Cheers Richard