Curves inside ovens

[Geezer]

Hi all,
im building an arch oven with a flat front and rear face. Do i need to create a downward curve from the top of the arch to the door or is it ok to leave it vertically flat, which would be about 20cm(3 bricks approx). A bit like attached with external chimney.

[david s]

With the flue at the top of the chamber you have an updraft system which is not as efficient, meaning greater fuel consumption as a cross draft design which pulls the flame down a little before exiting outside the oven chamber itself. while this is a disadvantage it does increase the oven floor area and chamber size as well as giving better access to the oven chamber, making working the oven easier. However an updraft system also produces poor flame circulation with the flame wanting to jump straight to the flue entry pipe and sometimes flames in the flue pipe itself. This makes light up far less smoky. but for retained heat cooking, door in place, also leaves heat to escape from the oven chamber far more easily. Updraft ovens require a shut off valve at the base of the flue for this reason which if perfectly sealed, which they rarely do, eliminates heat loss by convection, but still allows heat loss by conduction into the flue pipe.
(The pic you posted appears to have such a valve control on the outer face above the oven mouth).

On half barrel ovens, many new builders are attracted to this design because it appears to be simpler to build. Unfortunately it presents more problems than the hemisphere when considering thermal expansion. Because the half barrel form is inherently less stable than a hemisphere and because of the joints at the ends (make sure the end walls are underneath the vault, not beside it, otherwise the expanding vault wants to push out the end walls). Steel bracing or wall buttressing is usually required to improve stability. The circulation in half barrel ovens is also significantly poorer than that of a hemisphere. If you have your heart set on a half barrel search this forum for "half barrel ovens". A normal Google search will produce better results than the internal forum function. Try "Forno Bravo barrel ovens".

[david s]

I was having difficulty sourcing this drawing, but it explains better than words how a cross draft for a half barrel oven is superior to an updraft system. Hope this helps.