[Dutchoven]

Okay guys! I have a bit of experience not with building these but having seen photos of and CAD drawings of designs. As Uno will I am sure relate to they are a very challenging process from initial conception to final build. A true "white oven" the exhaust gases from the burning fire NEVER enter the oven chamber itself. They get redirected through a system around the bake chamber heating the firebricks from the opposite side. The system can work very well for bread because as the temperature curve drops you can refire without having to clean the chamber once again. Many French bread ovens are called "guellard" ovens and vent the exhaust gases from a separate firebox into the oven chamber through the use of a removable, usually cast iron, elbow called the "guellard." This also allows the possibility of refiring in between bakes as the temperature curve begins to fall. So this in truth would be a hybrid between the "white" and "black" ovens. The flue systems of both of those are tricky. For the "white oven" you have to slow the gases down enough to allow them to heat the masonry but, and it is a big one, not slow them down too much to create the possibility for condensation and creosote issues/fire hazard. The "guellard" has to have the system to move the gases into the oven chamber and then also one to remove them from the oven chamber plus some sort of "diverter" to close off the oven from outside air coming in and going out and yet another flue to exit that external firebox's exhaust. The masonry heaters you spoke of use the same process as the "white oven"of slowing down the gases through a system of either horizontal or vertical flue tubes(which is why some designs include a "white oven") but they have the same challenge. That is the reason why most firecodes for stove pipe require very specific relationships between the length of horizontal runs and vertical runs...and also the restriction of angles to connect them. Uno would know more about this being in the trade I think.
I hope this shed some light on this topic. I can probably find a way to dig up some more information on this topic but, I will attach a CAD design I have and maybe you can see some of what I spoke of. The other is a design for a German bakeoven.
Best Dutch