Gas condensing boiler has vertical flue through roof and ends 600mm above slates, I want to extend it to 2.5m so it clears newly installed dormer window. There is a 70mm grey plastic pipe inside a 110mm black plastic pipe. Would this be possible?
The overall flue length may be an issue here. You would be advised to check the boiler manufacturers installation manual where the Maximum permitted flue length will be stated. Remember to allow for reductions for bends when measuring the overall length.