01What a sweep actually clears
Every fire leaves behind creosote — a tar-like residue that condenses on the inside of the flue as smoke cools on its way up.
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A little of it is normal. Enough of it, built up over a season of regular fires, is fuel sitting inside a chimney that occasionally sees temperatures hot enough to ignite it. A chimney sweep is the mechanical removal of that buildup: a brush sized to the exact diameter of the flue, run its full length from the firebox to the top of the liner, breaking the creosote and soot loose so it can be vacuumed out rather than left to accumulate season after season.
In a house that burns wood regularly through a Seattle winter, that is not an occasional chore. It is annual maintenance, the same as changing a furnace filter, and it is the single most effective thing an owner can do to keep a working fireplace safe to use.

