Chimney safety
How do chimney fires start?
Safe to use for now, and it gets more expensive every month you leave it.
The short answer
A chimney fire is creosote burning. No creosote, no fire - it really is that direct.
How do chimney fires start is a question with an unusually simple answer for a safety question. Wood smoke carries unburnt volatiles; when they meet a flue wall cooler than about 250F they condense as creosote. Creosote is fuel. Once enough of it lines the flue, a hot enough fire - or a burning ember carried up - ignites it, and a flue full of fuel with a strong draft through it burns extremely hot. So everything that prevents chimney fires works on one of two levers: burn in a way that deposits less, and remove what has deposited.
This is what burns
Flaky stage one brushes off. Shiny stage three glaze is the one that sustains a fire, and no brush touches it.
Before we get there
What to do right now
None of this fixes it. All of it stops the problem getting more expensive between now and the visit.
If it is happening now: get everyone out, call 911, and close the damper and air supply only if you can do it safely on the way.
Never throw water up the flue. The steam expands violently and can crack the liner and split the masonry.
Afterwards, do not light another fire until a Level 2 inspection has been done. The standard requires it and insurers ask for it.
Stop damping the fire down overnight. A smouldering fire is the single largest creosote producer.
Check your wood. Split and covered for six to twelve months, or it is depositing far faster than it needs to.
What actually matters
How do chimney fires start, and what actually prevents them
What to do, why it matters, and the part people most often get wrong.
Unseasoned wood cools the flue
Water in the log takes energy to boil off, so the fire burns cooler and the smoke condenses instead of leaving. Wood needs six to twelve months split and covered before it belongs in a fireplace.
Damping down overnight is the worst habit
Choking the air feels efficient and produces exactly the cool, smoky, incomplete burn creosote forms in. A hot short fire deposits less than a smouldering long one.
A cold flue condenses more
An oversized flue, or one running up an exterior wall, keeps gases below the temperature at which they stay gases. This is an installation fault presenting as a maintenance problem.
Stage three is the dangerous one
Flaky stage one brushes off. Hard shiny glaze does not, and it is what sustains a fire once lit. Knowing which you have is the difference between an hour and a job.
Softwood and treated timber deposit fastest
Resinous softwood loads a flue far quicker than hardwood. Painted, treated or manufactured board should never go in a fireplace for reasons well beyond creosote.
Sweeping is the whole prevention strategy
Remove the fuel and there is nothing to catch. An annual sweep on a correctly burned fire is not housekeeping, it is the fire prevention.
Where to go next
Who handles this
The VIO Standard
- A written estimate first
- License and insurance checked
- Nothing starts without your OK
Good to know
Common questions
What temperature does creosote ignite at?
Can a chimney fire happen without you noticing?
Do creosote logs prevent chimney fires?
How fast does creosote build to a dangerous level?
Is a chimney fire covered by insurance?
The people behind every visit
Founded by Ryan Carter in 2019. Every page you read here is reviewed by Ethan Miller, our chimney specialist. A written estimate before any work, and the findings in writing whether or not anything needs doing.
More about who we are →