[fifilura]

I'd say that is a symtom of bad combustion (too little air). Not what kind of wood you use. Sap burns just fine.

[alrlroipsp]

It's both too low temperature burning leaving resiude in the chimney that can catch fire later, AND pine/spruce having way too much sap to burn alone. You should absolutley mix it with other wood when you burn it, same as oak.

[fifilura]

Fair enough. Feels like you have more (possibly hard-earned) experience than me on this issue.