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Found this sketch that explains why the chimney is at the front. Also when you first light the fire you start it at the front directly under the flue so get a good draw going and the fire starts quickly. You then move it back as you add more fuel. Also I've made a door that fits inside the arch beyond the chimney. When this is in place you have an "airtight" oven, which is needed for baking bread.
strange place for a chimney - would have thought it would be better placed at the rear. then again, i've never so much as made a bed, never mind an outdoors oven. you posh git.