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 EARNS ITS RECORD. 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, published here as a boundary rather than a method. NO ACCUMULATION THRESHOLD IS PUBLISHED, and the recurring eighth-of-an-inch figure is not carried here.
Where the sources disagree
How often should a wood-burning chimney be swept — twice a year, or quarterly?
There is no scope difference to hide behind. 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.
Both positions are recorded · safety-critical
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)
Verification: a person has read the passage relied on.
Read on — The whole technical update: the classification principle, the per-fuel deposit descriptions, the creosote types, and the cleaning methods. Supports the claim as stated. Closes the Sprint 32 evidence gap on the identification and treatment halves and splits creosote out of the soot record, where it had been sitting as an ALIAS since Sprint 1 — a modelling error this evidence exposes, because an alias asserts the two are the same thing and the removal instruction is different. The accumulation-threshold half of the gap stays open and is narrowed to it.
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)
Verification: a person has read the passage relied on.
Read on — The whole technical update: the classification principle, the per-fuel deposit descriptions, the creosote types, and the cleaning methods. Supports the claim as stated. Closes the Sprint 32 evidence gap on the identification and treatment halves and splits creosote out of the soot record, where it had been sitting as an ALIAS since Sprint 1 — a modelling error this evidence exposes, because an alias asserts the two are the same thing and the removal instruction is different. The accumulation-threshold half of the gap stays open and is narrowed to it.
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)
Verification: a person has read the passage relied on.
Read on — Sprints 5-10 read fifteen entries stain by stain. Sprint 30 enumerated the database from the publisher's own spot-solver sitemap — 296 entries — and read 134 more, chosen by open demand and by soil class rather than alphabetically, plus the generic 'Unknown' entry. 135 of 296 entries now read. Supports the claim as stated.
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)
Verification: a person has read the passage relied on.
Read on — The advice page's question list, principally 'Why do I need to have my chimney / flue swept?', 'How often does my chimney need sweeping?', 'What is the best fuel to use?' and 'Why do I need my chimney swept before it is closed off?'. Supports the claim as stated. Supplies what sweeping removes, corroborates moisture content as a formation factor from a second body, and creates the wood-frequency conflict with HETAS. The masonry sentence is the first evidence in this register that bears on soot ON BRICK rather than in a flue, and it moves that row off zero without pretending to be a method.
What CleaningHQ cannot answer yet
These are recorded questions about this subject that we have not yet researched to a standard we would publish.
- The accumulation at which a creosote deposit becomes a fire risk
Last reviewed by a person on .