Hi, my daughter has purchased a property and it has an open fire but the previous owners had an electric fire sat in the opening and she would like the opening bricked up and a much larger fire fitted flush on the wall. I have looked up the flue and there looks like a pillow up there now if I brick the opening up with an air vent fitted do I remove the pillow to allow air flow in case of condensation.
Could be a chimney balloon (yes, that’s an actual ‘thing’). You’d be best removing this and fitting a vented cover on the chimney pot / stack to allow air to circulate and to stop rain / snow going down the flue.