When we had our fireplace built we had to extend the height of the chimney to get it to draw well. I’ve read here of issues of smoke not going up the chimney properly for some builders. In addition, about half of my lower row of chimney bricks was removed to get them to fit. So my question is, would it be ok or even prudent to add a fourth layer of bricks to the chimney?
You’ve run into the mechanics of chimney draw before, so it’s good to be thinking about it now. There are discussions elsewhere on the forums about the load that even a single extra course of brick will add, and as you point out, you had to cut some of the base course to get it to fit. (It’s not quite like cutting through a rafter or a joist, but there is less structure to support your chimney at the bottom.)
If you are concerned, I would first see how the chimney performs as built. You can add an extension to the chimney pipe if it is needed. The pipe is sealed to the top of the chimney, and so an extension would give you draw without adding significant weight (and load) to that point on the barrel structure.
Definitely try it out first, though. The issues with your fireplace were unique to that structure, and you might find there is no issue at all with your oven. If there is, you’ve got a plug-and-play fix available for it.
Hopefully we’re not in trouble but we did add one extra row of bricks to the chimney prior to reading your response. So there is now the base layer plus three rows including the cut bricks ready for the cap base plate.
I don’t think it’s serious trouble at all. If you’d added several courses I’d be concerned.