Fireplace Chimneys - The Heart of Your Hearth

At the risk of stating the obvious, the woodburning fireplaces in your home cannot operate correctly without properly designed fireplace chimneys. In essence, it’s the chimney that keeps the fire burning, not the fireplace. Fireplace chimneys aren’t just the thing that vents the smoke away from the fireplace, they’re part of a cycle of events that contribute to efficient burning.

Firstly, even when there is no fire burning, if you opened the damper to the chimney (particularly in cold weather), it should have air rising up through it. Think about it - warm air rises, so if the weather is chilly outside, the warmer air in your house will begin to pass through the hearth and rise up the fireplace chimney. This is exactly what you want to have happen. Of course, if it’s cold outside, and you’re not using your fireplace, you should keep the damper closed to avoid losing all that expensively warm air!

Getting back to the explanation though, this process of having air enter the fireplace from the room is what feeds the fire (once it’s lit) the oxygen that it needs to continue burning. The air in the fireplace gets hotter and the draft up the chimney increases and sucks the smoke out with it. The air getting sucked up the chimney causes more air to be drawn in from the room and supplies the fire with more oxygen. If the fireplace chimney has been properly matched to the fireplace, the small amount of draft that is present even before the fire is lit, is what keeps the smoke from entering the room when you kindle, or first start, a fire. It is also this property that keeps smoke and ash odors from entering the room even when the fireplace is not being used.

The best way to ensure a good draft is to have the fireplace chimney installed (and insulated) inside the house rather than running up on the outside. This keeps the air in the chimney warmer than the outside air and keeps the draft moving upward. An outside chimney would be colder, and the air inside of it would be much cooler than the air inside the house. It would be much more susceptible to having the cold outside air forcing itself back down the chimney and pushing cold air and ash odor into the house.

Since fireplace chimneys are designed to draw air up through them, the air that is being drawn from the room (into them) has to come from someplace as well. With houses being more insulated for heating and cooling efficiency, the air inside the house has to be supplemented with additional outside air by means of a cold air intake vent, usually coming in to the basement. Otherwise, the chimney’s effect of sucking the air out of the house will start to decrease the inside air pressure causing a vacuum effect which will try to reverse the upward draft and start to pull smoke and fumes back into the house - not a good thing!