Bought a house with a two flue brick chimney. The flue for the wood fired boiler and the basement wood stove share a clean-out but has a separate thimble for each. I don't know that it needs to be re-lined (and will know Tuesday when the Sweep comes) but say it does, what is involved? Specifically, how do you know what size you need, what holds it in place, how does is attach at the top and how do the thimbles connect?