Log size
In this case, size really does matter. The optimum log diameter for efficient burning in most stoves is about 5 inches or 125mm, but why? Around 3/4 of the ‘fuel’ locked up in a piece of wood has to heat up and then ‘vapourise’ off the surface as flammable gasses. The rest of the ‘fuel’ is mainly ‘fixed carbon’ or charcoal. This carbon is what causes the surface of burning wood to glow and once the flames are gone, it becomes the glowing embers.
What’s that got to do with log size?
Big logs: Because most of the ‘fuel’ needs to heat up and ‘vapourise’ off the surface, log size makes a big difference. A large log has a small surface area relative to its volume, so the flammable gasses are given off slowly. As a result, the burning temperature falls, less of the gasses burn which causes inefficient burning and increased emissions. That’s no good for ‘Advanced Woodburning’.
Small logs / kindling: Small pieces of wood have a large surface area relative to their volume, so the gasses vapourise quickly. This is why small kindling burns furiously. If you use too much kindling or too much wood with a large surface area (e.g. pallets), your stove won’t be able to deliver air fast enough to burn all the gasses. If this happens, the fire box will be full of flames, the unburned fuel gasses are wasted to the chimney and there will be lots of smoke out the top. So, people who are ‘Advanced’ woodburners only use kindling to start a fire or to re-light a dying one.
