efficacy of firing doors

Watch (and listen) how the firing door improves combustion. How it works: the chimney determines how much air can get into the fire. Without the firing door, the air all goes in at once, and the fire burns primarily at the edges, where air meets flame. The firing door, however, limits airflow, forcing the fire to suck in a smaller "jet" of air. Because it's smaller, the jet has to move faster in order to get the same amount of air into the chimney. Increased air speed and pressure create more turbulence and better mixing in the fire -- you can hear it quite clearly, even in a low-quality video. Better combustion makes more heat, more heat heats up your oven faster. (Note that the opening is at the bottom of the door. Letting the air in at the top would interfere with the hot gases trying to get out.) So, to optimize combustion in your wood-fired oven:
1. use bone dry fuel (boiling away any water in wood takes over 500 times as much energy as just heating up the wood itself),
2. burn only a small amount of small pieces (more surface area means better mixing of fuel and oxygen, and more complete combustion; there should always be enough space around the wood for combustible gases to expand and burn completely. I would say roughly 1/2 to 2/3 of the volume of the oven chamber should be free of fuel),
3. maximize turbulence (a firing door: the openings should have an area approximately equal to the cross-sectional area of the chimney -- for most home ovens, that will be somewhere in the range of 20-30 square inches -- or about 4x5 to 5x6 inches.