Flue fire

Few flues are stout enough to come through a severe flue fire unscathed. There is no such thing as a “friendly” flue fire that will safely clean out your flue. With a roar like a freight train, it can crack open your flue and spread to the structure of the house.

Flue fires are perennial. They happen every winter when creosote deposits flue walls are set ablaze by high heat or by flames licking up the flue.

A flue fire can be exciting and light up the night, but it is NEVER a laughing matter. Here’s how it happens and why it can threaten your life and your property.

Anatomy of a flue fire

Heated wood releases hydrocarbon gases. When these get hot enough (about 1100 degrees F) they mix with air and catch fire. When open or woodheater fires smolder, unburned gases condense and deposit on the inside of the flue as runny acids and liquid tars that harden into creosote. Both a cool flue and steam from green or wet wood encourage this condensation.

Creosote can appear as any of the following:

  • a sooty powder
  • a gummy mess
  • a hard glaze
  • or a deposit that looks like burnt marshmallows.

A creosote fire can burn with such blast-furnace intensity that it sets off this frightening chain of events: “Balls of flaming creosote shoot out of the flue top onto the roof; brick chimneys crack open; Stainless steel liners warp, buckle and separate at the seams; Masonry in chimneys expands with such force that sections of the chimney can blow out; flames can spread to the structure or roof of the house even explode into the room”.

Tar-glazed Creosote

This type of creosote makes for the hottest burning fuel for a flue fire. The thicker the layer of creosote, the hotter the fire. The heat generated by this inferno can raise to ignition point, the temperature of wood structures on the other side of a flue, so that it also starts to burn threatening the entire house.
Wood doesn’t necessarily need contact with fire in order to ignite. It just needs “air, oxygen and enough heat.”

A house may survive the first flue fire, but the intense heat starts pyrolysis*(decomposition of organic materials due to intense heat) to nearby combustibles, and thus lowering their ignition temperature. This makes the structure very vulnerable to a subsequent flue fire.

A damaged flue can no longer protect either the chimney or the house. And instead of being all burned out, creosote may instead be all puffed up to the point of partially or completely blocking the flue.

What is Pyrolysis?

Pyrolysis is chemical decomposition caused by heat. Severely pyrolyzed wood can ignite at only 2l2 degrees F, while it would normally have a catch-fire temperature of about 500 degrees F, before it had any exposure to intense heat.


If it happens to your flue, get everyone out of the house, because fire can flash right through the house with incredible speed. First shut up all air controls on the heater, then call the fire brigade from a safe distance such as a neighbours house or from your car. Do not try to put out the fire and do not hesitate to leave immediately.

Afterwards, if your house is still standing, call a chimney/flue service professional to clean and inspect the flue as well as assess the damage.

until you’ve had the flue professionally inspected and repaired!!

Excessive creosote build-up

Excessive creosote build-up is caused by one or more of the following:

  1. A flue too big for the appliance it serves. Most open fireplace chimneys are too big to be used to vent a wood burning stove or a fireplace insert with out a flue fitted. This causes a sluggish flue draft effect and gases expand to fill the space then quickly cool down, allowing creosote to deposit on the chimney walls, where it sticks like glue.
    Solution: Install an stainless steel fluing that is properly sized for the stove or heater insert.
  2. Poor wood burning habits, such as severely limiting the air supply in a heater to achieve an all-night burn. This causes a sluggish draft and a smoldering fire that doesn’t get hot enough to burn the volatile gases released by the wood.
    Solution: Burn smaller, hotter fires using seasoned firewood with a good draft never air-starve the fire. This way, the heat will quickly warm up the flue and increase the draft, while volatile gases burn up in the stove the way they should.
  3. An oversize or outdated heater; When a heater is too big for the space it heats, it’s likely to be burned in the closed down, creosote-producing mode. Also, many heaters sold in the 70’s and early 80’s are now obsolete. They’re not as cleanburning as EPA-certified models designed to meet new emissions standards.
    Solution: Replace your old heaters with a new high-tech unit correctly sized for the space you want to heat and matched with the proper venting system.
  4. A neglected flue. Many of us don’t give our chimneys or flue a second thought until something goes wrong.
    Solution: Find an experienced, professional chimney sweep right away. Put your whole heating system on a regular inspection and maintenance schedule.
  5. Do-lt-yourselfers. Heater installation and chimney service work are “NOT” home handyman chores. They call for professional know-how, special training and technical expertise.
    Solution: Always entrust installation and maintenance to a professional

If your house and flue were lucky enough to survive that first flue fire, don’t wait for a second or third event to do them in. Now that you’ve learned how a flue fire can take the whole house down, do use caution and take preventive action.