John White wrote:

John Rumm wrote:

John White wrote:

I've put in a few of these recently and would strongly recommend them.
They have the great advantage of only switching on when they are
needed.

Use can a double pole switch as an isolator, and wire it straight into
the lighting circuit.

IIRC, the ones I fitted required fusing at 3A which meant I needed a FCU
as well...

Quite right. I meant to say "Use a double pole, switched FCU as an
isolator, and ...".

I really should read what I write before I post it. :-(

I would put a new flue through the wall away from the cast iron grill,
and then leave the grill to ventilate the cavity.

This air brick sounds more like the type that we have in our bathroom -
it is simply connected through the wall to a louvered vent on the inside
and serves as permenent room ventilation.

Fair enough. I was going by the OP's reference to a cavity.

John
--
John White,
Electrical Contractor

Hi All,

Thanks for the many responses. I'll try and answer the various points
raised.

- it is a cavity wall (I checked before I bought the house).
- the fan is rated at 15w so connecting it to the lighting circuit
(using a dp switch) is ok (AFAIK).

Basically there is a cast iron grill on the inside wall and one on the
outside wall. My plan as it stands (unless someone persuades me any
differently) is to install the fan on the inside and duct it to the
outside grill so as not to encourage the damp into the cavity.

This would still leave a 3" X 5" area of the grill unobstructed by the
fan.

I'm reluctant to put another whole through the wall as it feels a
little ott. At present most of the ventilation to the bathroom is via
this grill which is currently open to the cavity.

What do you think?