Creosote
Not a kind of soot. A flammable tar condensed from damp, oxygen-starved wood burning — and the hardest forms do not answer to a brush at all.
In detail
Creosote is what a wood fire deposits when combustion goes wrong in a particular way, and the trade association states the conditions as a conjunction rather than one cause: burning wood which is DAMP, in a LIMITED OXYGEN environment, with INSUFFICIENT HEAT or INADEQUATE TURBULENCE. That is why it is not simply heavy soot — soot is unburnt carbon, creosote is condensed tar. IT COMES IN FOUR NAMED FORMS, listed as types rather than stages: liquid tar; rock-hard black deposits likened to a toffee apple, called GLAZED; hard flaky deposits; and expanded creosote structured like a honeycomb. TWO PROPERTIES DECIDE WHY IT MATTERS: it expands up to six times its thickness and can easily block flues, and it is the main cause of chimney fire. THE REMOVAL ANSWER IS WHERE THE DISTINCTION MATTERS MOST. Fine soot answers to a medium or firm brush. Creosote does not — some forms cannot be removed by abrasion alone, and the very hard glass-like deposits must be modified with chemical treatment and heat first. That is professional work rather than a domestic method. NO AUTHORITATIVE SOURCE PUBLISHES AN ACCUMULATION THRESHOLD, and the recurring eighth-of-an-inch figure has no authority behind it.
What works
Chimney sweeping is effective against Creosote Supported
Why
- Creosote is explained by Why soot is tenacious
- Creosote is explained by Adhesion and cohesion
Where the sources disagree
How often should a wood-burning chimney be swept — twice a year, or quarterly?
The two positions have the same scope. Both are UK bodies, both answer the same question for the same fuel, and HETAS names NACS among its own associated associations — so this is not an outsider disagreeing with an authority. What separates them is what each figure is FOR. HETAS states a floor expressed as a minimum: 'at least' twice a year. NACS states a cadence tied to use: quarterly WHILE THE APPLIANCE IS IN USE, which for a chimney used only in winter may be two or three sweeps rather than four. Read that way the two are closer than the numbers suggest — but they are not the same instruction, and averaging them to 'about three' would invent a figure neither body publishes. Both make the same point about what actually governs: NACS lists moisture content of the wood fuel among its factors, and the deposit evidence says damp wood is what produces creosote rather than soot.
What to do: Take quarterly-while-in-use as the working figure for wood and twice a year as the floor beneath it, and treat neither as a substitute for what the flue looks like. The two bodies agree on the thing that matters more than the number: wood is the fuel that needs the most frequent attention, and how wet the wood is changes the answer. A chimney burning damp wood is making creosote rather than soot, which is a different deposit, a worse one, and the one the sweeps' association names as the main cause of chimney fire. Burning dry, seasoned fuel does more than any interval.
The sources genuinely disagree
Sources for this page
Sources
Soot classification, soot cleaning — Technical Update: Soot (Part 2)
Sweep Safe, United Kingdom · Trade or professional body
Relied on for: That burning damp wood in a limited-oxygen environment with insufficient heat or inadequate turbulence produces flammable creosote, and that it occurs as liquid tar, rock-hard glazed deposits, hard flaky deposits and expanded honeycomb-structured creosote. (states this directly)
Soot classification, soot cleaning — Technical Update: Soot (Part 2)
Sweep Safe, United Kingdom · Trade or professional body
Relied on for: That creosote can expand up to six times its thickness and easily block flues, that it is the main cause of chimney fire, and that some forms cannot be removed by abrasion alone — the very hard glass-like deposits requiring chemical treatment and heat before removal. (states this directly)
Spot Solver: spot and spill removal procedures
The Carpet and Rug Institute (CRI) · Trade or professional body
Relied on for: That creosote is treated on the solvent branch — a non-flammable spot removal solution or dry-cleaning type solvent is preferred — and that solvent must never be applied directly to the carpet or allowed to reach the backing because it can damage the latex holding the backings together. (states this directly)
The National Association of Chimney Sweeps (NACS), United Kingdom · Trade or professional body
Relied on for: That regular sweeping removes creosote and that doing so helps prevent dangerous chimney fires. (states this directly)