what does anyone think of these? ok to use? i currently have a brick chimney but hope to knock out the chimney breast in the bedroom and attic in a couple of years time (with the help of structural engineer etc)

Produce lots of water vapour that would normally go up the flue , can lead to excessive condensation.

thanks, bighead. was thinking about putting in an airbrick in the wall anyway when we knock out the chimney upstairs, so that may cover it.

You'd have to put an airbrick in anyway. Even the manufacturers state that they don't give a condensation problem because they are normally part of a house already heated by a central heating system - if it's going to be the prime source of heat in that room then you will have condensation problems.

Sorry to be a prophet of doom on 1st post but should warn you these things can be highly dangerous. Already one reported Carbon Monoxide death from a faulty installation. These have been discussed on diynot forum, a lot of gas engineers will not install them because of the inherent danger. National gas firm wont install them either. I would seriously think again before fitting one.