Bathroom Extractor Fan Size

[lxbn]

Hi,
Can anyone advise on what size extractor fan I would need for my bathroom size:
L=185cm
W=180cm
H=232cm

The bathroom has no windows and is in a flat in a purpose built block. The air is vented through a straight duct about 3m long.

I have had some condensation/slight mould problems with the extractor fan which is the original one when the block was built some years ago.

Any advice on what cfm, ltr psec, or make/model for my installation would be appreciated.
Thanks.

 

[stem]

The usual recommended air exchange rate for a bathroom is between 6 to 10 times per hour.

From the building regulations (Document F):
For bathrooms: an extract rate of no less than 54m3 /hour (15 l/sec.) For calculation purposes we recommend a minimum air change rate of 6 per hour.

Your bathroom volume is 1.85m x 1.8m x 2.32m = 7.73m3

So:
To change the air 6 times would require a fan of duty 47m3/hour (which is less than the 54m3/hour minimum requirement)
To change the air 10 times would require a fan of duty 77m3/hour

Because it is connected to a duct, I would recommend a centrifugal fan as opposed to an axial fan. Centrifugal fans generate a higher pressure, so they are better able to overcome the air resistance caused by the duct.

Also, check that the duct outlet is opening and unobstructed to let the extracted air vent outside properly.

Also of equal importance is that a free flow of replacement air coming into the room at the same rate as that being extracted will also be required. So a gap at the bottom of the door, or an inlet grille in the door itself may be required.

For the best effect, the fan should have been installed diagonally opposite the source of the replacement air so that the flow of air is drawn across the whole room, but in reality this can't always be achieved.

Condensation will form when warm moist air deposits its moisture on a cooler surface, so keeping the fabric of the room warmer or insulating any cold spots may be advantageous too.

 

[lxbn]

Thank you. That's just the kind of information I needed.
I've seen some fans on TLC's website so hopefully they have a product which will do the job.