As long as I can remember, we've used a polygon wall for a fireplace, with a 3D-only wall or slab above to take the chimney to the ceiling. This method has been developed pretty far.
This new method isn't going to give you the 1" chimney section for CDs, but for schematics, it feels a little simpler.
Draw a thick (24" at least) rubble wall from floor to ceiling. Use the layer A Fireplace.
Draw a slab with the shape and thickness of the firebox. Use the layer X SEO Show2. (If you don't have this layer, make it. It's a weird one. It should show in working plan, A1 plan, and working model and section/elevation. It should be hidden in A2 Section Elevation. I.e., It's visible in plan, visible when working on the model, and hidden in the output sections. It should always be set to wireframe.)
Bring the slab forward as necessary.
SEO-subtract the slab from the wall.
In the output section, the line at the base will be hidden.
