Worth sweeping an unused chimney?

I don't really know the answer to this, but a few years ago I had my chimney capped (full cover with a gap of about 1" for ventilation). On the assumption that soot might absorb moisture, I had the chimney swept just before the cap was fitted.

If there is a proper flu put up the chimney one would hope that it had been swept before that was put in for the gas fire. If not it cannot be swept. If there is no flue up the chimney, then I'd probably go for sweeping it simply to remove nasty old birds nests and anything that may be loose. It also of course depends on how you are going to terminate it in the house.. You may need it one day after all, but they can be draughty if not terminated correctly. Some also have branches for other rooms upstairs and down. Depends on the design of the house. I certainly do not think anything will change any discolouring, are you sure its what you think and not some kind of mould?

Brian

It should not be necessary to sweep a flue that belongs to a gas fire and it is too late now to sweep the old chimney before installing it. My installers for a wood burning stove were insistent that prior to fitting a flue the chimney was swept and certified by a sweep from a list that they provided. They reckoned to have had serious trouble with DIYers and dodgy sweeps leaving great chunks of rooks nests in old stacks.

More likely a lichen but it could also indicate water getting into the brickwork from inside and the damp making it look darker.