There are a few things that wood boiler manufacturers "forget" to mention in their advertising. As a wood boiler owner (or potential owner) there are some things that you need to be aware of.
Cleaning: With hot-air wood furnaces or stoves, the heat exchanger surface can get very hot, well above the condensation point of creosote so they stay relatively clean. With boilers, there is fire on one side of the heat exchanger and water on the other side. This means that the heat exchanger surface can never get hotter than the boiling point of water under any circumstance and it usually runs much lower than that. It is well below the condensation point of creosote so there will be creosote build up. Wood boilers require weekly cleaning! If anyone claims otherwise, they haven't lived with a wood boiler! It is an easy job but you need to spend 1/2 hour cleaning your boiler every weekend.
Boiler Type: There are two types of boilers. With fire tube boilers, the fire goes through the heat exchanger tubes which are surrounded by water. On water tube boilers, the tubes are filled with water and the fire surrounds the tubes. Water tube boilers do NOT work with a wood fire because there is no way to clean the creosote off the outside of a cluster of tubes. A friend has a water tube wood boiler and the creosote builds up, insulating the heat exchanger which lowers its efficiency until finally the creosote catches fire. The resulting uncontrolled burn boils his furnace dry! Only use fire-tube boilers with wood fuel.
Gasification: Every manufacturer now claims that their boiler is a gasification boiler. Technically, that is true because wood does not "burn". The heat causes the wood to pyrolyze and the resulting gasses burn. A true gasification boiler will have two combustion chambers. The first (main) combustion chamber is where you load the fuel and set it on fire. The unburned gasses from the first combustion chamber enter the second combustion chamber where they are mixed with pre-heated air which causes them to burn more completely and at a much higher temperature. The gases then proceed through the fire tube heat exchanger to heat the water.
Clean Burn: When you first lite a wood fire, the moisture in the wood is evaporated and shows up as light grey or white "smoke". It is primarily water vapor plus some other combustion byproducts and particulates. After the water vapor has been driven off, the smoke from the boiler should be virtually invisible. On a high quality wood boiler, you will just see heat waves exiting the chimney. There should NEVER be any black smoke under any circumstances.
Installation: Bury the pipes below the frost line. The colder the surrounding soil, the more heat is lost from the pipes. Insulate the pipes well. Some of the very expensive twin pipe inside an outer shell is only rated at R4. Much less expensive commercial (not hardware store type) closed cell pipe insulation is available with 1 inch wall thickness and rated at R12. This type is rated for direct burial but not for submersion in water.
Sources: The above notes are a result of talking to the owners of several brands of wood boilers, personal observations, the research that I did before purchasing my boiler, and using my boiler over several winters. (My boiler is a Canadian built Empyre Elite by Profab. It is an indoor boiler which I installed in an out building.)