“When you burn wood, soot builds up on the wall of the flue and, if enough soot builds up, under a certain set of conditions the soot fuses to become a highly flammable, oil based tar called creosote. Because, after all, wood alcohol and turpentine are wood based, flammable substances, that then becomes the chemical basis of creosote.
In the case of a classic flue fire: Flame and embers shooting from the chimney, inside, needing air to burn, sucking air into the fireplace and flue, the sound is that of a loud howling within your living room, and when the creosote ignites you now have a chemical fire that burns with such intensity that the mortar holding the chimney bricks melts and runs as lava causing the chimney to collapse setting fire to the ultra dry timber within the attic...
At this point all you can do is run.
So, if you note a shiny, black buildup on the upper portion of the firewall – the back wall of your fireplace – it would be a very good idea to call a chimney sweep.
A general rule of thumb is to have your chimney swept and inspected after every cord and a half of wood.
My suggestions to people considering using their fireplace is...
#1... Be sure your damper is open. The damper is the steel hatch that separates the firebox from the flue.
On most masonry fireplaces, the damper works like a drawer: pull the handle to open and push it to close.
When all ember are out, remember to close the damper because the nature of a chimney is to suck the hot air from the room below, and it doesn’t matter if the hot air is from the fireplace or your furnace.
#2... If you have a gas log lighter, after opening the damper, light the log lighter and let it run for a minute to heat the flue, thus dissipating any buildup of cold, heavy air that can fight the lighter, rising fumes and smoke.
When you pre-heat the flue, you fill the flue with hot air and hot air rises, thus creating a draw before wood is placed onto the grate.
My suggestion for lighting the log lighter is to use a section of tightly rolled paper towel by lighting it, laying it onto the log lighter, then... Then, when you are away from the front of the firebox, turn the gas on.
#3... Most important! As soon as you decide to make a fire, open a nearby window a bit to equalize the internal air pressure.
When your forced air furnace runs, it draws air into the cold air return and can cause a vacuum in the room that may make it hard to get a proper fire going and can, most certainly, form enough of a vacuum later in the evening that... Remember, the draw to the chimney is a vacuum, and if the vacuum in the room becomes stronger than the vacuum within the flue, smoke and fume will be drawn downward.
The open window will equalize the internal air pressure allowing the chimney to draw to it’s current maximum.
Remember also, that in hurricane or tornado country, people will open a window on opposite sides of their home to equalize the internal air pressure. So high winds can also cause a drop in internal air pressure.
If there are glowing embers when you retire for the night, leave the window cracked because as the fire cools, there are fewer units of energy (BTUs) taking the dust and fume up and if the internal air pressure is too great, you may wake the next morning having a room that looks as though spray painted gray.
