The entire burn tunnel is buried under the floor. It took a little while to get the drafting to start. It didnt help that the internal chimney wasnt insulated yet (missing some parts for it), but once the system got warmed up, it turned into a raging inferno.
A picture without the flash shows how well the fuel is burning in the tunnel.
I ran out of sand so I wasnt able to completely bury the horizontal exhaust. It was quite hot to the touch, but as you moved down the pipe, I could hold my hand against the duct and it was slightly warm. It was quite funny to see the sand around the duct drying out so quickly. It will be interesting to study the temperatures along the system in the future!