Creosote is a byproduct of wood combustion that accumulates on the interior surfaces of chimneys and flue pipes. Its formation is an ordinary part of burning wood. The variable is the rate at which it builds up: burning dry wood in a well-maintained appliance at appropriate temperatures produces minimal deposits that are easy to remove; burning wet or low-density wood at low flue temperatures produces heavy, glazed deposits that are difficult to remove and increase the risk of a chimney fire.
How Creosote Forms
When wood burns, combustion gases and unburned particles rise through the flue. If flue temperatures drop below approximately 150°C — which happens during smouldering fires, overnight burns on low settings, or when the flue is cold — these gases condense on the cooler liner surface. The condensate contains tar compounds, acids and particulate matter that together form creosote.
The primary conditions that accelerate creosote accumulation are:
- Wood with moisture content above 25%
- Restricted air supply that reduces combustion temperature
- Oversized flue relative to the appliance, which allows gases to cool before exiting
- Flue exterior exposed to cold air, particularly in uninsulated sections through unheated spaces
- Smouldering fires maintained for extended periods
Three Stages of Creosote Buildup
Chimney sweeps and inspection standards classify creosote deposits in three degrees of severity, based on the Chimney Safety Institute of America framework referenced in Canadian inspection training:
| Stage | Appearance | Removal | Risk Level |
|---|---|---|---|
| First Degree | Flaky, sooty deposits; easily brushed away | Standard chimney brush | Low — normal result of routine use |
| Second Degree | Crunchy, hardened tar; black or dark brown | Rotary cleaning system or chemical treatment | Moderate — annual professional cleaning warranted |
| Third Degree | Glazed, tar-like coating; may appear shiny | Specialist chemical treatment followed by mechanical removal | High — significant fire risk; appliance should not be operated |
Third-degree deposits are highly combustible and can sustain a chimney fire at temperatures that damage flue liners and adjoining structural materials. A chimney fire in a terra cotta tile-lined flue can crack the liner, allowing combustion gases and flames to reach the framing of the house.
WETT Inspections in Canada
The Wood Energy Technology Transfer (WETT) program is the Canadian national standard for training and certifying technicians who inspect and service wood-burning systems. A WETT-certified inspection is recognised by most home insurance underwriters operating in Canada and is commonly required when a property with a wood-burning appliance is sold or refinanced.
What a WETT Inspection Covers
A standard WETT inspection — formally a Basic Inspection — includes visual assessment of:
- The wood-burning appliance itself: firebox condition, door seals, grates and ash management
- Connector pipe: the section between the appliance and the chimney base, including clearances to combustibles
- The chimney system: flue liner integrity (visual, not structural), exterior masonry or metal condition, cap and termination screening
- Clearances: distances between the appliance, connectors and combustible materials relative to the manufacturer specifications and applicable codes
A Basic Inspection does not involve dismantling the appliance or using camera equipment inside the flue. A Level 2 inspection — triggered by a change of fuel, appliance replacement, visible damage or a suspected chimney fire — includes video inspection of the flue liner.
Insurance requirement: Many Canadian home insurance policies require confirmation that a wood-burning appliance is WETT-certified before providing coverage. Contact your insurer before operating a newly purchased or renovated system to confirm specific requirements in your province.
Cleaning Frequency
The WETT program and the NFPA 211 standard — which is referenced in Canadian building code contexts — both recommend annual inspection of wood-burning systems, regardless of how frequently the appliance was used during the preceding season. Cleaning frequency depends on the amount and type of use:
- Light use (fewer than two cords per season) with dry hardwood: annual inspection, cleaning if deposits exceed 3 mm
- Regular use (two or more cords per season): annual inspection and cleaning
- Heavy use, softwood fuel or older uncertified appliances: inspection and cleaning twice per season
Preventing Creosote Practically
The most direct method of limiting creosote is consistent use of dry wood — below 20% moisture — burned at temperatures that maintain a hot, clean fire rather than a smouldering one. Practical steps that contribute to lower accumulation rates:
- Season firewood for the recommended time before burning (see the stacking and seasoning article for timelines by species)
- Avoid operating the appliance on its lowest air setting for extended periods overnight
- Open the damper fully when starting a fire and during the first 20 minutes of a burn
- Avoid loading the firebox to capacity with large pieces that restrict airflow
- Inspect the flue at the beginning of each heating season before first use
Chimney Fires: What to Do
A chimney fire produces a loud roaring sound, visible flames or sparks from the chimney top, and dense black smoke from the appliance or connector pipe. If a chimney fire is suspected:
- Close the appliance air inlets and damper to reduce oxygen supply to the fire
- Call 911 — do not attempt to manage a chimney fire without fire department involvement
- Evacuate the building if there is any sign of the fire spreading to building materials
- Do not use the appliance again until a Level 2 WETT inspection has assessed the flue liner for damage
Canada's National Fire Protection Association data indicates that chimney fires account for a meaningful proportion of residential heating-related fires. The majority involve accumulations that a routine annual inspection and cleaning would have identified and removed before they became a hazard.
Finding a Certified Technician in Canada
The WETT Inc. website maintains a directory of certified technicians searchable by province and postal code. Technicians hold different certification levels; a Basic Inspector can conduct standard annual inspections, while a Technician or Senior Technician certification covers installation and more complex assessments. The WETT Inc. directory identifies each technician's certification level.