I will try to flesh this out when I have time. It's a solid method, and touches on a lot of useful techniques.
• 3 coincident walls for fire box: Stone veneer, block core, fire brick. All go on A Fireplace.
• Use roof for back of fire box; SEO add to firebrick wall. Roof goes on X SEO General.
• SEO Subtract firebrick assembly from core. Subtract FB assembly and core from veneer.
• On top of firebox, 2 slabs or walls, one inside the other, veneer and core. Both go on A Chimney3. Subtract core from veneer.
• Smoke chamber: Mesh. connect firebox void to smoke chamber with slab. Both go on X Flue. Use print-white pens. Subtract both from the core Chimney3 element.
• Flue: Object, Flue JM9. Can be plumb or offset. Build the flue out of segments, attach to smoke chamber, work your way up. Some flues will show in plan, check the "Show Cut" box in settings.
• Subtract the flues from everything they pass through.
• Use slabs for hearths, adjust Chimney3 elements as needed. Fill in with Chimney3 walls or slabs as needed.