I have a renovated farm house, heated solely with wood, inside furnace forced air system. I burn 5 full cords on an average winter. I am heating roughly 2,000 sq. feet. Outdoor boilers burn more wood in my anecdotal experience. My engineering opinion is the heat loss is far greater, snow doesn't accumulate on an outside boiler....

I would use natural gas but alas, all my neighbors got wells the last time thru, not me, so I bought a new chainsaw not a new furnace. I am cutting next year's wood already. My furnace is a Charmaster Brand, built in Minnesota.

Burn full cords and heat 4000 SF,burn 12 months out of the year with outdoor boiler.