For those of us with access to low-cost firewood, it often makes sense to heat our homes with an outdoor wood furnace. When we built our house in 2001, we included a Security Chimney BIS Ultra Fireplace in our main floor living room to minimize the fuel consumed by our forced air system's natural gas furnace. After having used this indoor fireplace for many years, I found that it was a lot of work to keep the fireplace continuously stoked. Our Security Chimney BIS Ultra Fireplace is rated to produce up to 60,000 BTU/hr for up to 8 hours with a load of wood. In practice, with the wood we have in our woodlot (mainly ash, silver maple, and elm) a full load of wood in the fireplace doesn't last much more than 2 hours, 3 if we're lucky. Waking up at 2 am to every night reload the fireplace gets old after a few winters.
With Federal Government's 2009 Home Renovation Tax Credit, in 2010 we decided to add an outdoor wood-fired hydronic boiler to our heating system. The payback of the wood furnace wasn't great because our heating costs were low for two reasons: 1) our house is well insulated (meets the R-2000 standard) and 2) we were already burning wood. The deciding factor was the potential cost of heating the in-ground pool planned for later that year. We weren't sure how much energy the pool would require but we figured it wouldn't be cheap to heat it with gas and, by using wood, we could extend the swimming season.
Because cutting wood is time-consuming and the amount of wood on our woodlot isn't infinite, we decided to go with a high-efficiency EPA-certified hydronic furnace. There are several such outdoor furnaces commercially available and, upon much deliberation, we decided to get Central Boiler's E-Classic 1400.