I'm installing a radiant gas fire. On the back of the fire is an exhaust outlet, 95mm (3 3/4") dia. It projects about 3". The bottom of my chimney is currently sealed off with a piece of board. I'd like to cut a 4" dia hole in that board and use a short length of flexible flue liner going from the hole, to the outlet on the back of the fire.
Is this OK to do? As the outlet is 95mm dia and the flexible flue liner would be 100mm dia (I presume), what should I use to pad out the 5mm space, such that the flexible liner can be pushed onto the exhaust oulet, and be reasonably gas tight.
Where can I obtain a short length of 4" flexible flue liner? Can it be bought by the metre anywhere? What materials are suitable?
Thanks...
Al