Outdoor wood boilers have become a popular option in the cold country for heating the home.
The advantage is that you keep all the smoke, mess, and fire danger outside and away from the house.
The down side is the cost. A factory made one costs several thousand dollars. We burn wood to save money, thus the spending of several thousand doesn't support that goal.
I will show you how I built mine for almost no cost.

I ran the first one of these all last winter with no problems at all. I didnt buy a single whiff of gas for the whole winter. now I am making this one for a friend

Step 1: The Fire Box

we start with the construction of the fire box. I aquired from a dumpster a large water heater that I am using for my firebox. about 20" to 24" diameter is good. I've found the round firebox works great because the flames play all around the top for good heat transfer.
* First we start with removing the outer cover, insulation, burner assembly from the water heater. all thats left now is the tank.
*Next, I cut out the flue running up the center, and all the pipe fittings, and welded patches over the holes.
* Then, I reused the flue from the center and welded it to the outside of the tank. I welded a plate on the end for pressure testing.
* Now we have to pressure test the tank for leaks. For this, I welded a small pipe fitting in a convenient spot and installed an air valve. this is like a tire valve stem with pipe theads on it. they are used in campers to pressurize the water tank. we only need about 5 psi. with the tank pressurized, I used a thick solution of dishsoap and water in a spray bottle to find the leaks.

Step 2: The Tank

For the tank part I used an old fuel oil tank. this one we removed from a house we were remodeling. we will need to add some pipe fittings, and of course the fire box goes inside. I saved the fittings I removed from the water heater and will weld them into the fuel oil tank in the proper places these are 3/4" pipe fittings. we need 5 fittings. they will be for
* aquastat           3/4of the way up the tank
* thermometer     3/4 of the way up
* overflow          very top of side
* water inlet          bottom near front end
* water outlet         on bottom of tank use a standpipe inside about 2/3 way up tank near back
We will also need 2 larger fitting on the top of the tank. the existing 2" pipe fittings will be used. they are for,
* vent pipe
* Float valve
First step is to cut holes in the tank. I used my oxy acetylene torch, but you use what you have. during the cutting process, you will probably ignite the remnants of fuel oil in the tank. don't worry, fuel oil burns slow. Just stand back and let it burn out. we will need a large hole for the firebox tank. This will be cut in one end of the oil tank.On the other end of the oil tank you  will need to cut a hole for the flue to exit. this will be a close fit for the flue pipe. now we need the holes for the pipe fittings located as stated before.
Next weld in all the fittings, and when they are done slide in the firebox. leave about 8" sticking out the front of the oil tank. Leave the end on the firebox  until it is firmly welded in place in the tank.Other wise it will get pulled out of round.
once everything is welded into the tank, you can cut the protruding end off the firebox. 6" of it still sticking out of the tank. save the cut off end for the door.

Step 3: The Damper

Next we will make the damper. It needs to be operated by a solonoid. I made it like a throttle plate. Take a short piece of pipe. I used a piece from a basketball pole. it was 3 1/2" diameter. drill two 1/2" holes exactly opposite each other for the throttle shaft. Now find a piece of 1/2" rod about 16" long. this will be your throttle shaft. we want it long so it will extend all the way over to the door hinge where our solonoid is located. slip the shaft into the pipe and mark the shaft inside of the pipe on both sides. Now pull out the shaft and grind it flat between the marks. Next, cut a piece of steel plate to an elipse that will fit at an angle inside the pipe. Drill two holes on it's centerline about 3/4" from each edge. Put the shaft in place and put the throttle plate up to it and mark the holes on the shaft. Now drill and tap the shaft for machine screws. put it together with the screws and see that it fits perfectly. adjust as neccesary. we need to add a weight to shut the damper. I used a small piece of 1/2" steel with a hole drilled near one end. slide it onto the shaft and weld in position where it will hold damper closed
that's it for the damper, just set it aside for now.

Step 4: The Door

We will be flipping the tank end around so the convex. is out.
* First you will need to create a channel around the perimeter of the door to contain the gasket rope. I did this by taking a piece of 1" angle iron and cutting slots in one leg of it, every 1/2" or so. that let me bend it around to the contour of the door. I welded this onto the perimeter of the door so that about 3/8" of it protrudes beyond the doors edge. Next, I cut a flat strip of metal about 3/4" wide, and curl it around inside the angle iron forming a groove 1/2" wide for the gasket. a few blocks of wood cut 1/2" thick will help get a uniform channel. weld it in place there.
Now cut a hole for the damper assembly, slide it in and weld.

Step 5: Door Hinge

we need a hinge for our door. I welded on a short bit of channel iron to give me a flat surface for the hinges. one on the firebox, one on the door. On my first boiler I had some ready made hinges in the junk box. that is what the pictures are showing. for this one, I will need to cut something similar from heavy angle iron. You want to bolt the hinges on so you can adjust them. while we are at it, drill a 1/4" hole in the bottom of the channel on the door. this will hold the solonoid.
We will also need a latch. on the other side of the door, I welded a bolt on which the latch will pivot. then a piece of strap with a notch is put on. a short bit of angle iron is welded on the firebox to catch the latch.

Step 6: Hook It Up.

If you are putting this in an outbuilding as I did, you can wrap it with fiberglass insulation and you are basically done. At this point I have not spent a dime on this. That is about to change. You now will have to purchase the periferals to hook it up. This is what you will need.

* A duct coil, / water to air heat exchanger. this will need to fit into your furnace or ducts.
* A circulator pump 1/25th horsepower plus connection flanges.
* An aquastat, contact open on heat rise. also an immersion well to go with it.
* A 115 volt pull solenoid. about 1" travel
* A 24 volt to 115 volt relay
* an extra thermostat. Heat only
* thermostat wire, from boiler to house.
* UF romex wire (underground) from boiler to house
* Pex tubing. enough to go from boiler to furnace twice.
* Chimney pipe, and roof boot etc.
* piece of 2" pipe. long enough to go through roof. with roof boot.

Let's hook it up!
I guess the first thing we will do is install the coil heat exchanger in the furnace. I installed mine right in the furnace where the cold air return comes in. You will want to mount it so that is pretty much sealed to the blower intake so all the air will be drawn through it.

Next attach the circulator pump to the standpipe fitting on the bottom of the tank.

Now dig a trench from boiler to house and bury two pieces of pex tubing, one thermostat wire and one romex wire. insulate the pipes really good and make sure the insulation will stay dry.

The water hoses will attach from the output of the circulator pump to the inlet of the heat exchanger, then from the other side of heat exchanger back to the return water fitting on the bottom of the tank. You may want to include a Tee fitting and a valvesomewhere in the loop to add water from your household water supply.

the thermostat circuit will draw it's power from the existing transformer in your furnace. You need to mount your auxilliary thermostat on the wall in your house. take power from your furnace transformer (Red Wire) to the thermostat, then from the thermostat (White terminal) to the relay near your boiler. then from the other 24v terminal on the relay, back to the negative on your furnace transformer. You will also need to connect the white wire from your new thermostat to the fan relay in the furnace.
Now pick up some house current somewhere in the house and connect that through the relay to the circulator pump.
so when the new thermostat contacts, it will turn on the pump and the furnace blower.

Now for the damper control. install the immersion well into the appropriate fitting on the tank, and mount your aquastat on it. set the aquastat at 180 degree's F
Now we will attach the pull solenoid to the door hinge. bolt a small piece of wood to the hinge as a heat isolator, also make a heat shield from sheet metal to surround the solenoid. now attach the solenoid to the block of wood with screws. we will need to attach a lever to the damper shaft. Mine had threads on the shaft so I used two nuts to attach it. You need to be able to adjust it, so don't weld it on. a stiff wire linkage from this lever to the solenoid finishes the mechanics. we will take our house current and connect the black wire through the aquastat to the solenoid, and of course the white wire goes directly to the solenoid.

Install the chimney pipe through the roof, install boot etc.

screw a piece of 2" pipe into the top of the tank running up through the roof. this is our vent to let any steam out.

And we are done!!!

Just fill with water, build a fire and be warm this winter. If you set your regular furnace thermostat a bit lower than our auxillary themostat, you will have that as a backup.