Why Does Smoke Rise in a Fireplace Flue? It's Not Just Heat!

Sitting on the runway and cleared for takeoff I look out onto the wing and pray that the construction of the very thin-looking metal will, with all those exposed wires and tubes, structurally hold all of these people and all of their bags and all of their dreams knowing that the plane was built by the lowest bidder. It's an unsettling feeling after being hit by that puff of jet fuel exhaust. Then I remember that demonstration in school that showed me that the shape of the wing doesn't actually lift the airplane off the ground it pushes it off the ground. As a fireplace designer this makes me think of fireplace flues and how instead of warm air simply rising up through them they actually suck the warm air up and out of the fireplace. This sucking is called draft.

Draft in fireplace flues are a combination of HEAT in the firebox, PRESSURE in the flue and a TEMPERATURE DIFFERENTIAL with the outside air. I'm currently working on a custom masonry fireplace off the coast of Florida, and one obstacle for us is the temperature differential. The hot temperatures outside will create a smaller difference between the flue temperatures and the outside air and limit the draft needed to suck the smoke from the flue, so we have to compensate in different ways for the fireplace to draft well.

The fuel load in the fireplace is important as well. While a big fire that fills the inside of the firebox can overcome a fireplace and its flue the opposite is true as well. Problems can come when the fuel load is too small for the fireplace and its mechanical flue system. Having a very large fireplace with a small fire and fuel load may not be big enough to generate the heat needed for the large flue to develop the draft, so sizing your fuel load and your wood-burning grating with the size of your fireplace is an important part of the overall fireplace design. There are both upper and lower limits to the size of the fire in any fireplace.

As the airplane is coming down toward the ground, in a controlled way of course, it again uses the same aerodynamics as when it took off. It lands and pressures decrease against the wings. Similarly, when the fire is going out in a fireplace, the pressures decrease, the temperature reduces and heat is going away. This is arguably the most vulnerable time in the life of a fire in a fireplace. When the draft is diminished, but there are still heat and coals smoldering giving off smoke, it's hard to make sure the smoke, heat and smell are sucked up the flue. This is where we have to compensate for the draft so we can eliminate the smoke coming into the room. A properly designed fireplace will take off, fly and land without problems, makes you feel safe (cause you're literally playing with fire!) and gives you the warmth experience with your family. We like safe landings.