Based on my propane consumption last winter I'm guessing 2-3 cords of hardwood or 4 cords of pine (I live in a red pine forest) if I wanted to heat entirely with wood. This is based on BTU content of propane vs. wood and the best efficiency information I could find on each kind of heater.
The furnace is over 90% efficiency, including both unburned fuel and heat lost out the exhaust vent, while the numbers I could find for the heater indicated 94% burn efficiency (ie 6% of the burnable gases get away) and 65% heat transfer efficiency (35% of the heat goes up the chimney) for a total of about 60%.
I'll let you know for sure in April
This winter might be a bit of a challenge since I need to find a few cords of well seasoned firewood and by September the piles of seasoned wood are getting low. The local firewood vendors are starting to sell green wood and fake a coughing fit when you ask how long ago it was cut.
