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How to Build One Mean, Clean Fire
If you've ever built a campfire that was a little smokier than you wished it was, you'll want to read this. If you have a fireplace or a woodstove in your house, you'll want to read this. If you've never built a fire for any purpose and never plan to, a) I pity you, and b) don't read this article.
Dear fire user: Here's what you surely want to try next time you build a wood fire: light it from the top and let it burn down rather than the other way around. Say what!? You heard me. Make your fire lay with the big pieces on the bottom, then the smaller chunks, followed by kindling and tinder at the very top- and light the top.
If you're building this fire in a stove or boiler, make sure that the very top of your lay is at least four inches from the top of the firebox. If you get too close to the top of the firebox, you'll have all kinds of trouble getting the thing to light. This top-down approach to fire lays is nothing short of magical in the way it lessens smoke from a fire. And, as we all know, less smoke equals better efficiency.
Before I explain why lighting fires from the top creates the magic that it does, let me address a few quick thoughts that may be going through your head (and if not, someone will eventually make similar comments when they see you building your fires this way).
Comment: But fires burn from the bottom up! Hello! Heat rises! Your little fire on top will just go out in ten seconds!
Response: It's true that most fires people see burn from the bottom up, but that's because most people build them that way, not because of some law of physics.
Speaking of laws of physics, heat does not rise. There, I've said it. Heat does not rise - heated fluids (like air) become less dense than the surrounding cooler fluid, and are thus buoyed up by displacement. Gravity is the operating force. That's not only technically different, but conceptually different, from saying something so simplified (and incorrect) as "heat rises".
What this means for your fire is that although the hot gases do rise away from the fire and will not help ignite the rest of the fire, the radiative heat from the fire goes in all directions, including down into the wood below. It's the radiative heat that will warm the wood below and get it burning. For the real naysayers who maintain that fires will only burn from the bottom up, go light a candle. I'm willing to bet that they don't burn their candles from the bottom up.
Now that we've satisfied those pesky naysayers, it's time to ask: What exactly is going on that makes top-down fires less smoky? The short answer is that building a fire this way facilitates complete combustion, so you get less smoke. There are three main ways this is accomplished: hotter gases, controlled pre-heating of wood, and rudimentary gasification.
The first is pretty easy to grasp. Complete combustion (no smoke) requires a pretty high temperature, much higher than the ignition temperature of wood. When you first light a fire, a lot of the energy released in combustion gets used up heating the rest of the wood in the fire-lay - energy that could otherwise be raising the temperature of the gases released in combustion and fully burning them off. In a bottom-up fire, the hot gases from the wood at the bottom of the lay move up through the stack and lose energy to the other pieces of wood. This cools off gases that would otherwise burn. That's smoke - unburned gases. In the top-lit fire, those gases go straight up (just like in the bottom-lit fire) but now they don't get cooled off by the other wood and are able to finish burning.
The second main advantage, controlled pre-heating of wood, is pretty simple too. It's related to the first one, in a way that's probably obvious: since the gases aren't giving up energy to heat the rest of the wood, the radiant energy heats the wood. So everything heats up more slowly, with fewer pockets of high temperature that could get combustion going in places we don't want it starting yet, like at the bottom of the lay. Moisture is driven off in a more orderly way, the gases come off more slowly and encounter higher temperatures along the way, and the oxygen-to-fuel ratio stays much better than in a bottom-up scenario.
The third advantage of the top-lit fire has to do with the ways wood burns at different temperatures. It's a more complex overall concept, but here's the essential point: having the coals form at the top of the fire ensures that they stay hot enough to release and burn all of their gases.
Now that you have some idea of what's going on, make sure to try this top-down method for your next fire. I think you'll be pleasantly surprised. One final note: don't use this technique in a furnace that's designed as a gasifying furnace, since those generally rely on bottom-up fires for their magic. If you don't know whether your furnace is a gasifying model, chances are that it isn't. Gasifying furnaces are expensive and very particular about how their fires are built and managed.