In a perfectly cylindrical 42" oven, a 2" lower door translates into 264 square inches of additional brick surface area contacting the hot air
of additional brick surface area and who knows how much additional mass that must be heated.
and 2772 cubic inches (19 cubic feet) of additional hot air volume. That triples the volume of the 1" head space created by the original door.
Triple sounds impressive, but consider the numbers. (N.B. 2,772 in3
Let’s say the superheated air is 1500F/1089K
At 1089K, the density of air is .3243kg/m3
, so you have 0.0147kg of air in the extra 2”
At 1089K, the specific enthalpy of air is 1,149kJ/kg, so the energy of the extra 2” of air is 16.9kJ or 0.017MJ
There is about 10.4MJ/kg recoverable energy in oak firewood, so the energy in the air in the extra 2” is about the equivalent of 0.0016kg or 0.06oz of oak.
Here is what that looks like (paper clip for scale):