Outdoor wood furnaces require a lot of tending. You need to go out to the furnace and add wood daily for about half the year, depending on your climate. You may need to load wood as often as three tines a day when it's very cold.
If the outdoor furnace is your only source of heat or it's for heating water in your home, you won't be able to leave in the winter for more than a day or two without having someone else tend it to prevent freezing pipes.
Outdoor furnaces need monitoring and regular adjustment, including adding water to the boiler. You don't simply add wood. This requires some mechanical ability.
Despite manufacturers' claims, outdoor furnaces are inefficient and will burn a lot of wood over the course of a year. You may need as much as 10 cords of wood and ideally a large wood shed to store it in. If you don't have access to wood on your property to harvest and burn, buying wood can be a considerable investment. Factor in the cost of wood before you buy one.
Almost all furnaces will need a source of electricity to operate the fan, open and close the air vent, and run the pump. If you lose power, you'll need an alternate source of heat.
Smoke can be a big problem with outdoor wood furnaces. Depending on the weather conditions, and how well seasoned your wood is, expect the furnace to create lots of smoke. Furnaces smoke least in cold temperatures when burning seasoned hardwood. For this reason, outdoor wood furnaces shouldn't be used in dense residential areas.
Furnaces need to be placed downwind from your home and away from neighbors.
Some manufacturers say the units can be placed as far as 500 feet away from the house, but so much heat is lost in the pipes after such a long run that the unit is extremely inefficient.
Claims that some units don't smoke are exaggerated. How much smoke the furnace produces is determined by variables like the weather and the wood you're burning. The manufacturer has no control over this.
Some makers claim that the furnaces can operate for up to 96 hours without being loaded. This may be the case in the summertime when you're only using the furnace to heat water. During the heating season, the furnace needs to be loaded daily.
Sparks may be a problem despite what the manufacturer says. Make sure the unit you purchase has a spark arrester.