Thats where a newer furnace comes into play. You can get 8 hour burns with your furnace, but you will fill your chimney with cresote. A furnace like mine combines preheated air into an insulated firebox. Once up to temps, I can load and damper down fully and achieve a long clean burn. I'll get almost all of my heat from under the baffle with secondary combustion, along with a cleaner chimney. If you modify the furnace, then sell it and something happens it could be a bad thing. You may or may not ruin your firebox, but will destroy your baffle and very possibly overheat the furnace. If you do it, do at your own risk. The clean burning epa stoves have preheated air, fully insulated fireboxes, insulated baffles and burn from the top down due to being on firebrick. You would get more heat, but also send more heat up the chimney without a heat exchanger. Being you just bought it, I would opt out for modifying the furnace. Take care of it, save your money and when it comes time considering purchasing a better unit and sell the old. Also we never loaded our old furnace all the way each time. I didn't mind throwing a few pieces on the fire every 3 or 4 hours until bedtime, then we would load it up. That said, we heated an old victorian home 2400 sq ft with a 1500 series hotblast furnace. Last year we used around 6 1/2 cords of wood. Take advantage of the tax credit in the form of insulation for the home. It will be the best money spent, instead of heating the outdoors.
