DIYbanter - View Single Post - Toilet extractor fan solution after extension

wrote:
We've had the suggestion that ducting into a cavity from a toilet only
room (not a bathroom or shower with moist air) can be acceptable if you
use a fan with a carbon filter for odour control.

No. The purpose of the extractor fan is to extract the air to the
outside, not pump it into the wall cavity and hope it makes its way
outside somehow.

When you flush the toilet, aerosolised fecal matter laden with bacteria
is sprayed into the air. It is this which you need to extract to the
outside, rather than 'odour' per se. Using a carbon filter for odour
control is only of relevance if you are using a recirculating fan; using
one would be an admission that your fan is not extracting from the room
space.

If you can't take the long way across the extension can you go up
through the roof? Or even down and under the floor? Oblong ducting will
run parallel to joists quite easily.

Owain