George,
Sorry for the late reply. Some start the chimney directly on top of the arch. Or, at least with what few courses it takes to level off on top of the arch. However, it is a bonus for draw, if there is a funnel shaped smoke chamber to gradually transition from the outer vent arch to the 6" round ID of your chimney. The more gradual this transition the better. For an oven the size of yours, I would guess that 5 to 6 courses of brick above your vent arch should be about right to accomplish this.
Sorry for the late reply. Some start the chimney directly on top of the arch. Or, at least with what few courses it takes to level off on top of the arch. However, it is a bonus for draw, if there is a funnel shaped smoke chamber to gradually transition from the outer vent arch to the 6" round ID of your chimney. The more gradual this transition the better. For an oven the size of yours, I would guess that 5 to 6 courses of brick above your vent arch should be about right to accomplish this.
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