Smokeless firepits aren’t smokeless, exactly. Fire makes smoke, and smoke generally means fire.
But here’s the thing: Smoke, especially black or gray smoke, is a sign of the inefficiency of a fire. Much of smoke is uncombusted material, carried aloft into the air. If you burn more efficently, you make a lot less uncombusted material and therefore less smoke. Certainly, you make smoke that’s less dirty and less full of particulate matter that’s both unpleasant and unhealthy to inhale.
Most modern smokeless firepits achieve this with a quite similar design. The firepit is double-walled design, with a gap in between the walls and vents on the top and bottom of the pit. The base of the pit is elevated, so that oxygen can reach the flames at all times.
Air gets sucked into the bottom vents and gets heated while traveling up in the space between the double walls. When it leaves the top vents, it’s hot enough to reignite and burn the unexpended particles in the smoke, in a process called secondary combustion. This secondary combuston makes the fire hotter, and more efficient, and less smoky in general.
What’s fun is that this is a self-reinforcing cycle: If the fire burns hotter, the circuit of air through the double walls also gets hotter, and accelerates the cycle. And so the firepit will get hotter, and more efficient, and less smoky over time. Typically, as you first start the fire, you’ll still get quite a bit of smoke leaving the firepit, until the fire gets hot enough to begin secondary combustion.
This increased heat will also end up meaning you burn fuel faster, even as you’re burning it more efficiently. Anecdotally, burn through fuel about twice as fast with an efficient smokeless pit—I’ve also achieved coal temperatures well in excess of a thousand degrees. Needless to say, this makes a smokeless firepit kind of terrific for cooking.
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