Bathroom fan - inline fan fitted outside on external wall in a sealed box

[Muppetatwork]

Hi there, I have a small downstairs bathroom, just over 6' x 6' x 9' and all the walls are tiled. The existing fan is a cheap Manrose Gold in external wall diagonally opposite to the shower. Anytime the kids take a shower the water is literally running down the walls. Also the fan tube goes to the corner of a large airbrick on the outside wall. Doesn't go through it, just rests against it so I guess a fair bit of the moist air is just going into the cavity.

Wondering if it's possible/ok to put an inline fan (Manrose MF100T) in a box on the outside of the external wall and run the ducting back up and into the hole where the existing fan is?
The benefits would be:
Minimal mess in the bathroom and easy to carry out work.
Channeling the ducting through the airbrick in a pipe stops moist air ending up in the cavity.
The fan noise would be outside so nice and quiet.
Easy to access for maintenance.

The existing isolator is postioned about 6 inches from the fan and would be easy to replace and run the cable - is it ok to do this regs/rules?.
Regards sitting the fan in a box (I would waterproof the box) - does it need ventilation to stop overheating, e.g, grill on the underside, a lot of airspace in the box?

The instal instructions only show it being posititioned horizontally - wonder if its ok to fit vertically to avoid any condensation issues?

Thanks in advance

 

[ericmark]

My first action, would be to try only turning on the fan after they have finished in the shower. We really do not want to drag the water vapour out of the shower cubical.

Rules wise it is silly, it does not really say a fan must be used, it just says it must be there to use, so a simply pneumatic timer you press as leaving the shower is enough.