I'm just getting started on a large, low-budget rehab project on a farmhouse that was built before they knew what modern heating was. For the past thirty or forty years it's been heated (??) by two propane wall furnaces and a bathroom space heater. Less than half the house is heated directly, and while tearing out ugly plaster in three rooms, I plan to install 3x12 ducts between the joists for the installation of a new heating/cooling system. So far I can't find much info on the volume of ductwork needed to insure adequate airflow. Anything would be better than what it has now, but I don't want to pay for energy that isn't being used efficiently. Any suggestions?