Twin wall flue refers to the silver pipe you can see in the picture on the left. It is well insulated to retain the heat in the rising smoke and gases (heat rises so keeping it hot is good), thus allowing it to be used instead of a stone or brick chimney and outside of a property.
In this situation where there is no chimney, the stove fitter will be looking at the proposed location of the stove and evaluating if a route for the new twin wall flue is possible. He might decide a different stove location is more suitable.
He will look at whether the twin wall flue should leave the room and proceed outside of the building, or whether an internal route through the structure of the building might be the better choice.
He will be thinking about distances to combustible materials and legal regulations on chimney height and performance. He will be bearing in mind the maximum amount of bends he is allowed in the chimney (it’s four by the way – all detailed in the relevant section).
He will take notes on what flue materials will be necessary; what size stove for the room; is an air vent required; is there an extractor fan in this or an adjoining room that might cause problems. He will speak with the householder about stove choice, hearth choice and the process of making good following any holes in walls and ceilings.
With twin wall flue he will also be readying himself to give a price to the customer. Twin wall flue (materials cost, not fitting) is approx. £75 a metre with supports, cowl and other auxillaries swiftly adding up. This is a chimney being built and it helps to bear this in mind when looking at the flue materials costs. A brick or stone chimney would hurt the wallet a lot more.