Somebody asked why the flue object can't show a thickness for the flue liner itself.
The other reason: The flue is designed with SEOs in mind. You have to use it as a subtraction operator, or there's no point. It is a simple solid tube (EXTRUDE, actually) which we use to make a simple void. If the flue wall has a thickness, it becomes a solid ring with nothing in the middle.
Well, I could wireframe the layer in building section combinations and make it solid in wall section, and I could make the thickness option scale sensitive in the flue and the smoke chamber, but we're into serious inelegance now. It's a void, except when it's not and it's simulating a void, and it's scale- and layer combination-dependent, and do I 'really' subtract it to 'simulate' a void, I can't remember.
Once again we have met a limit in GDL object technology where the simplest solution is for Graphisoft to give us more power. There is a directive, MODEL, which allows you to build shapes in wireframe or surfaces-only mode in addition to the default, normal, solid mode. MODEL WIRE makes a wireframe shape that looks exactly like switching a layer to wireframe. MODEL SURFACE makes a shape that looks normal from the outside but is hollow. This sounds promising for our problem until we realize that only MODEL SOLID shapes can act as SEO operators.
Therefore, I want a new MODEL option which will make shapes wireframe, but which will allow the shapes to act as operators. So the flue wall would subtract, and the void within the flue would subtract, but the void would still be a void. My first stab at a name is MODEL OPERATOR, but I'm open to suggestions.