This page will include how to make your firebox, waterjacket, and heat exchangers. This is the core of the outdoor wood boiler, and is designed to be very efficient. Before I get into the actual design, I want to expalin the theory of how it will work. It is pretty simple really. The idea is to heat water by using a wood fire. The trick is to get it done efficently, and without producing a lot of smoke. If you have ever had a wood burning stove in your house, you will know it is difficult to maintain a good temperature, and slowly burn your wood without producing a lot of smoke, which will eventually plug your chimney.
In able to produce a clean burn you need high temperature, and plenty of oxygen. The only way to do this with a conventional wood burning stove is to burn very hot (which also goes through wood very fast, and usually produces more heat than you want). The reason for this is because when wood is heated it releases gasses (smoke). This smoke is what actually burns because it is full of hydrogen, carbon monoxide, and many other flamable gasses. However if there is not enough oxygen to burn it all, it will only be able to do a partial burn. Also tars will not break down and burn until they are heated to over 1,000 deg F (to fully remove all tar, it has to reach 2,200 deg F). The process of slowing your fire down by reducing oxygen works, but it also sends all the tar, and other flamable gasses out your chimney.
The way to get around this, and still be able to slowly burn your wood is to have all the gass forced back down through the flame, where there is more oxygen injected in to fully burn the gas, and tars. This also should be done in an insulated high temperature area (a wood flame can reach nearly 3,600 deg F if no heat is lost) .
There are two locations where air is injected into the burn chamber. The first is for the primary burn. We need enough air in the primary chamber to get the fire started, and to heat up the chamber, but since we have a secondary burn, we don't need this primary chamber to get so much air that it burn all the wood too quickly. The secondary air injection will be just behind the exhaust port (the hole at the back of the burn chanber). This secondary air injection will be injected in the secondary chamber to keep the burn going, and complete the burn. Here is a video where you can see how the secondary burn works.
Once the burn is complete, You now have very hot air, and need to transfer all that heat into the water. For this reason I have the chimney, and the entire firebox surrounded by the water, and I am also using two chimneys to give more surface area for better heat exchange (these pipes also connect at the bottom of the stove, so they travel from the bottom of the water jacket all the way to the top). As you can see by the picture on the left, there is a lot of surface area to heat up the water.
The outter water jacket is pretty simple. Basically you need to weld a box around the fireplace. I used 1/4" steal for my firebox, and 10 gauge steel for my water jacket. I found the thinner water jacket was a little more difficult to make a nice water proof weld. I didn't have any leaks welding my firebox, but had several pin hole leaks with my water jacket. This was not a big deal, I just needed to grind down the welds, and weld them again (just where there were leaks). I made my water jacket big enough to have at least 6 inches of water on every side of the firebox, and at least 12 inches on top. This ends up giving me about 350 gallons of water.
Ok, so now we have a Nice tank of hot water we can use to circulate to our house to heat our house using in floor heat, or radiators (with fans, without fans, or in your central air). However we still don't have a way to heat our hot water, and this can be a large source of energy. If you purchase an out-door wood boiler, they usually don't come with a way to heat your hot water without purchasing a special heat exchanger that you basically pump water from the boiler thorugh, and water from your hot water heater through, and it heats the water. I really don't like this method, because it takes two pumps, and is not nearly as efficient as placing the heat exchanger right in the water jacket. You can see the heat exchanger in the earlier picture, and here is another view after the water jacket is welded together.
As you can see it is just a bunch of coper pipes mounted direcly in the water tank. You can now just use one pump to circulate water from the hot water tank through these pipes to heat your hot water. I will make the top of the tank removable so I can get in and access these if I need to.
Click here for actual dimentions to the firebox, and welding tips