Picture of outdoor wood boiler from junk
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
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Step 1: The fire box

Picture of 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.
krid518903 months ago
You should definitely post some recent pics of it now after its all insulated and finished!
krid518903 months ago
Very smart man!...I'm about to start my project...with a custom built firebox. Can't seem to find a water heater any bigger than 16 inches in diameter. I will try to keep pics posted. Thank you again you've been such a big help!
krid518903 months ago
Thank you so much man! Is it still running strong?...if you could make any mods to it what would they be?
heathbar64 (author)  krid518903 months ago

Yes, still going strong. I had to patch a leak this year, but otherwise no problems.

At some point I added an automatic filler for the water. Just a simple float switch connected to a solenoid valve from a dishwasher.

I really wouldn't change anything. it works great just as is.

krid518903 months ago
What size pump and brand did you use?
heathbar64 (author)  krid518903 months ago
welder415 months ago
What aquastat are you using ?
welder41 welder414 months ago
Ok thanks!!!
heathbar64 (author)  welder415 months ago
What size hot water heater did you use
heathbar64 (author)  joseph.bunkercass5 months ago

It was either 80 or 100 gallon water heater. One that large is hard to find normally, but any tank will do this is about that size.

backcreekwv5 months ago

Thank you for this plan. I made this and so far it works great. One thing about the air tight welding, it can be a pain.Do your best to make it air tight by welding but in the end dont worry about the pin holes. I kept chasing pin holes time after time, I would fix one and another would pop up. I got tired of this and thought of giving up.I did some research and found some boiler sealer, I think it is made by Gunk. The sealer sealed everything up in no time with no leaks. Welding air tight is better Im sure but dont give up if you have pin holes you cant get there is relief. Thanks again for the plan.

heathbar64 (author)  backcreekwv5 months ago

Thanks for the suggestion, backcreek. I'm a fairly experienced welder, but I still had to chase a few leaks. It's a drag.

Yeah I figured you were experienced. I was just puttin that out there for others who might find it overwhelming. Thanks again.

GYD5354737 months ago
Nice build. Wondering if the damper closes off completely or allows some combustion air through?
heathbar64 (author)  GYD5354735 months ago

the damper pretty much closes completely, but being hand made it leaks a little. probably the door seal leaks a little too. It would be easy to add an adjustable stop screw if you found it necessary.

that makes sense. thanks

Hi I have been doing alot of research on wood furnaces approx how many cords of wood would your design use on an average winter , looks great and shows good hoping to hear from you


heathbar64 (author)  carey.whitford.55 months ago

Of course it depends on the winter, and how much house you are heating, how well insulated etc. I think last winter I used about 5 cords. It was a long cold winter and my place is not especially well insulated.

Dude this is awesome, you think if I tackle some like this and get stuck in the process I could possibly get some assistance from you either by e-mail or phone I have been wanting one of these but are extremely expensive, and thank's for posting this bright design

heathbar64 (author)  roberto.torres.9696 months ago
Yes, I'm always happy to help. the hardest part of this project was welding everything water tight. If you can do that, the rest is easy.
Toga_Dan11 months ago

re: torch + fuel oil tank.

I've heard of a couple cases of oil tanks (used motor oil?) blowing up when they were welded on. Perhaps there was a wee bit o' gasoline in the oil? Anywho, when i've cut steel barrels, I've filled em with h2o first. Inert gas is another option. Nitrogen, co2, etc. A hunk o' dry ice would work. Let it turn into gas.

heathbar64 (author)  Toga_Dan11 months ago

That's always worth caution. I knew that these tanks contained only fuel oil since I personally removed them from the basement and drain the fuel oil out of them.

With all the plugs removed and knowing that the fuel oil is slow burning I felt safe in just letting it burn out.

You of course should use your own best judgement to secure your own safety.

dtrembly1 year ago

Love this post! Have welding skills and a $750, bill for 200 gallons of propane as motivation to start this project. Was wondering if you only support the firebox by welding it to your fuel tank on the end that sticks out? Was also wondering if you'd be willing to give me some email support if i needed it?

heathbar64 (author)  dtrembly1 year ago

Yes, the only support for the fire box is around the front where it's welded in, and the smoke pipe in the rear also supports it. It might be a good idea to add legs under the back end of it, but I couldn't figure an easy way to put them on with no access.

I'm not any expert on this, but would be happy to help you with my experiences.

e mail

outstanding! How has it done this winter? Do you get 12 hours out of a load of wood in the box? searching around for "junk" to build mine!

Just sent you an email to touch base

rimar20002 years ago
Wow, what a great job, congratulations.
And thank for the captions in video!
Love it, plan on doing my own similar project someday, have a seen a few similar projects on youtube, you should do a quick video presentation and throw it up there, yours looks a lot nicer than most of them on youtube, seems to better utilize the existing heating cooling system parts!
heathbar64 (author)  digitalenigma2 years ago
Ok, so I've made a couple video slide shows. Hope they are ok.
heathbar64 (author)  digitalenigma2 years ago
Thanks, I just might do the video. I haven't posted anything to youtube yet, but was thinking about trying it out. I have to give a lot of credit to my neighbor who built one a few years before i did. I utilized a lot of his experience to make mine better.
Always gotta give credit to our predecessors! When I make mine I will utilize concepts from all those who made them before me, gotta love the internet allowing us to share all the knowledge!
bernie3082 years ago
This is great. Gives me lots of ideas for building one.

Thanks for sharing.
Best Regards,
heathbar64 (author)  bernie3082 years ago
I hope you do build one. If so, show us some pictures. I think you would find the welding a lot easier if you used heavier tanks like old propane tanks.
Navarone2 years ago
Looks like a giant flowmaster muffler!

Mount that on a honda for moar powa!
This is butt a$$ ugly, I LOVE IT!!!

How long a burn do you get out of a load of wood? How much wood does it hold at a time?
heathbar64 (author)  RedneckEngineer2 years ago
well the firebox is almost 4' long so you can get a lot in there. I usually topped it up each morning and evening, but in all but the coldest weather there was still half a load left. Even if you let the fire go out, the water will hold enough heat to go most of the day,but then you have to start a new fire from cold.