Homebuilt outdoor wood furnace (boiler)

[lapeer20m]

I've decided to build an outdoor wood boiler and have it installed before fall arrives, and i want to build an efficient means of heating water for as little money as possible. I live on 60 acres, mostly wooded. Trees are falling and rotting faster than i can currently use them.

In my opinion, traditional outdoor wood boilers tend to smoke a lot, primarily because the fire box is surrounded by jacket filled with relatively cold water making the wood burn less efficiently than it does in a wood stove with a nice hot firebox.

My solution: Take a traditional cast iron boiler designed for fuel oil, remove the burner, and marry on top of an existing woodstove. I would rotate the boiler 90 degrees so the hole where the burner was traditionally located will be facing the top of the woodstove. The heat and flames can directly impinge on the first boiler plate, while the smoke and hot gasses will have to weave through the nearly infinite number of cast iron fingers inside the boiler, eventually escaping where the traditional chimney would mount to the boiler. Below are photos that may help explain this better.

To further improve efficiency, I plan to line the stove with fire brick, and build 2 fire brick lined baffles into the design between the stove and the boiler.


Traditional boiler plate:

wood stove:

wood stove with baffles and boiler on top

Same photo with fire brick illistration:

Thoughts?

 

[stihly dan]

Seems to me you will create more creosote. The cooler flue gas condensing on the water jacket of the boiler. I would think you want to use an injection loop, or close boiler piping like they do on condensing gas boilers. That way you never have cold water hitting the fire box, the water is pre heated.

 

[lapeer20m]

and i neglected to mention that i already have the boiler and the wood stove. I still need to buy some material to build the baffles, and most of the plumbing items. plan to put the entire contraption in a well insulated shed. plus i plan to insulate the boiler itself, although i don't think i can feasibly insulate the wood stove or baffles.

Creosote buildup on the inside of the boiler is a concern, although as long as the creosote only needs to get cleaned once or twice a season that would be acceptable. If it builds up so fast that it would require weekly cleaning, this would not be acceptable.

 

[iowa]

My first wood furnace was home aid and was the basic same design. It was a creasote making machine! Build one like the traditional way that most manufactured boilers are made.

 

[H-Ranch]

I believe the smoke is generally related to the quality of the wood - too many believe that they can throw any green wood (or non-wood products) in and burn it. While it may burn, it does not have the heating value of wood with a proper moisture content that has been properly split and stacked. It also tends to smoke more.

The firebox in mine gets hot enough to make the steel smoke baffle red hot so I'm not sure how much hotter I would want it. They do idle with little air when the water temperature does not call for heat which may mean it is relatively cool much of the time, especially during moderate weather. I believe this will be true even with your set-up. I would be concerned about being able to clean out the baffles and cast iron fingers in the traditional cast iron boiler since they will likely have creosote form on them. I do like the baffle concept and that is how I have the exhaust configured on mine.

With 50+ acres of woods you should never run out. Various sources say 1/2 cord of new wood or more is grown each year for each acre of woods. So you may have 25 cord to burn each year (not counting what's already down and I'm guessing you have a lot of Ash to clean up.) Even the least efficient design is not likely to burn that much.

I'll be interested to hear how it works if you decide to tackle it. Good luck.

 

[lumberjackchef]

or you could build a downdraft gassifier similar in design to these that are highly (90/95) efficient and never have any creosote build up!