I've really been enjoying the game so far, but I do have a small suggestion. It would be nice if there was some way to set a "fuel" inventory for the furnace or something like that. Keeping items intended as fuel out of the furnace's processing inventory can get tedious at times.
Edit: I just thought of this after making the post! There could be a new Funnel structure that only sends fuel into the furnace when it needs it but doesn't send fuel into the processing inventory.