one caveat about the heat fin thing. If you put on too many heat fins, they will rob your smoke of the heat it needs to climb up your chimney. This is bad. OTOH heat fins *will* make your heating system more efficient, albeit only in the places that the chimney runs through. I don't know how many heat fins you can put on safely but hopefully this information exists somewhere out on the net and you will almost certainly have to be able to take the temperature of the smoke at the top of your stack to make use of this info.
Re: Heat Conservation