Bathroom Extractor

I want to fit a bathroom extractor fan.
There is an existing "passive" internal vent and corresponding external air brick.
Question: Can I pass the ventilation hose through the cavity and mount the external vent onto the air brick? I realise I will have to make a 100mm hole in the air brick.
Are there any regulations as to the position of the external vent, e.g. wrt to windows, soffit board, etc. ?

You can do what you suggest providing:

1. If the wall has a cavity, you must pass the duct or liner through it and ensure it is sealed, otherwise you will be blowing damp air into the cavity. If there is any of the original vent still exposed after installation, this should also be covered to prevent the damp air getting back into the building.

2. Most importantly, the fan you buy must be suitable for the Zone of the bathroom in which it is located, which may mean SELV (fed via a safe, isolated low voltage transformer) the link below is for bathroom lighting, but the zoning info applies.

http://www.bathroomexpress.co.uk/light.htm

It will also need to have a special switch that isolates the switched live, neutral and permanent live (if applicable) for maintenance and cleaning. This switch needs to be located in a safe position. It's usually found outside the bathroom just above the door.

3. The duct should slope slightly downwards towards the outside so that any condensation in the duct runs outside not back in to the fan for obvious reasons.

4. That you will not cause a problem by loosing the airbrick (ie it's not there because there is an open flued heating appliance in the same room) this is unlikely in a bathroom but not unknown.

Thanks for the reply.
1) So I need to remove the existing external vent? Won't this affect the cavity wall ventilation. It's a bungalow.
2) The fan will be in zone 3. Understood about the isolator, but can it be next to the fan itself with a pull cord. I think I have seen some that have a pull cord?
3) understood
4) n/a

2) That'll be an on-off switch, not an isolator. If this fan has a timed-overrun, then it will have a permanent live feed as well as a switched one, and a normal pull-cord switch will not isolate it - you will need a 3-pole one for that.

Thanks.

If the vent you refer to is one that goes right through the house, ie form the outside in, it should be sealed from the cavity already. Especially if you have cavity wall insulation otherwise it would come out of the vent, or fall and block it. Whatever the case, you don't want warm damp air from your bathroom condensing in the cavity.

I've never seen a cavity wall with a vent specifically for venting the cavity, they're usually just open at the top (unless full of insulation) and might have small weep vents in the mortar at the bottom. If you're still worried, stick a post on the building forum, they're more likely to be able help with this aspect of the job.