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