Analyze the performance of non-catalytic secondary burn systems. Optimize air settings for maximum heat and minimum smoke emissions. Understanding the reburn process allows you to extract more energy from every log while keeping your chimney cleaner and safer.
Set the primary air opening as a percentage, the temperature of the air reaching the secondary tubes, and the wood load in kilograms. The model starts every burn from a 70% combustion baseline and stacks two bonuses on top: up to 20 points as primary air is throttled down, which pushes flue gases through the reburn path, and up to 15 points from secondary air temperature.
The temperature bonus scales linearly and saturates at 600°C — secondary air at 300°C earns half the bonus, anything at or above 600°C earns all of it. A worked case: 50% primary air with 600°C secondary air gives 70 + 10 + 15 = 95% combustion efficiency, leaving 5% unburned; the same damper setting with 300°C air manages 87.5%. Results cap at 99%, since no real firebox burns absolutely everything.
The CO reduction readout tracks how complete the secondary burn is on its own scale, climbing in proportion to the temperature bonus until it tops out at 85% once secondary air reaches 600°C; at 300°C it reads 42.5%. Fully developed secondary combustion is the visible curtain of flame that re-burns smoke above the wood load in modern reburn-tube stoves.
In this model the primary-air term contributes (100% − opening) × 20 points, so a wide-open damper adds nothing and a nearly closed one adds close to the full 20. The physical picture: restricting primary air slows the fire at the grate and routes more unburned gases past the secondary air supply, where they get a second chance to ignite. At a 50% opening the term is worth exactly 10 points.
That is the saturation point of the temperature bonus: the model scales the 15-point contribution by min(1, T ÷ 600), so 150°C earns a quarter of it, 300°C half, and anything from 600°C upward the full amount with no further gain. Smoke needs hot oxygen to re-ignite, and pre-heating the secondary supply toward that mark is what turns passive air tubes into an active afterburner.
It is the complement of combustion efficiency: the share of combustible gases that escaped both the primary fire and the reburn stage, leaving as smoke, carbon monoxide, and condensable creosote precursors. Because efficiency is capped at 99%, the unburned figure never reads below 1%. The separate CO-reduction number isolates how much of the cleanup came specifically from hot secondary air.