Introduction: Outdoor Wood Boiler From Junk

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

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.

Step 2: The Tank

Picture of 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

Picture of 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

Picture of 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

Picture of 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.


bh456 (author)2017-10-28

if you added a fan to draft the fire would you still need the damper and if so should a second aquastat be added and set to shut off fan a little lower than the damper?

Garlandr (author)bh4562017-11-12

I added a blower fan that is hooked up to a thermometer it turns on when the water is below 160. Works great

heathbar64 (author)bh4562017-10-28

In my application a blower is not needed because I have a chimney to create the draft. Most commercially made boilers do need a blower because they don't use a chimney ( or a super short one).

If you chose to go the blower route, I think you would still need a damper to keep the fire under control. I see no reason though the blower would need a separate aquastat. The damper could just close at the same time the blower shut off.

bh456 (author)heathbar642017-10-29

that sounds reasonable, i will try that. and thank you for answering back. also i found a 20 inch hot water tank and it is only a little over three feet long is that smaller than the one you used?

LarryB157 (author)2017-05-09

Would this work with a 55 gallon drum as the firebox?

heathbar64 (author)LarryB1572017-05-09

I dont recommend a drum for the firebox. they are really quite thin and would be difficult to weld, and would probably not last very long. I don't use any kind of grate, the fire is just built on the bottom. I let the ash build up quite a bit and then scoop some of them out when the fire is low. Fire never goes out completely until spring.

PeterB62 (author)2017-01-13

OK oil tank found $100, old cast iron radiator had for $50, and massive fan for $25.. time to design the firebox..

techmang (author)2016-09-29

Is the firebox still holding up? How many Winter before you think it will burn through?

heathbar64 (author)techmang2016-09-30

Yes, the firebox is still holding just fine. I see no indication of trouble, so I guess it will last for a long time yet.

Rexanvil76 (author)2016-08-31

thank you I have wanted to build one forever nows the time

heathbar64 (author)Rexanvil762016-08-31

Great! Show us how it turns out.

DaveF41 (author)2016-03-04

What about using a 100lb propane cylinder instead of the water heater ?

heathbar64 (author)DaveF412016-03-05

Any tank will work as long as it is big enough. I think the 100# propane tank might be a bit small in diameter.

userdk2 (author)2016-02-18

Heatmaster SS also has a how it's made video on their website.

AdamC99 (author)2016-01-22

I am about to try this, over the upcoming weeks. so we will see how it goes, i will add pictures as i go

Garlandr made it! (author)2016-01-21

I have to commend the creator of this junk boiler. I just finished building one pretty close to his designs. And it WORKS GREAT!!! thanks again for the idea / post. I had less than $350.00 in the project.

UAfabricator1984 (author)2016-01-16

Will this design work with anthracite coal? Besides wood, cheapest fuel in my area

I just don't know. I've never burned coal, and have never seen a stove designed for coal.

JeffC81 (author)2015-12-21

i made it and it works great when it works. Dont get me wrong, i no its me somewhere? it keeps getting air locked. I have 180 degree at stove but cold water at duct coil.. ive purged it, put a auto air vent at coil.. ive tried to pressurize it, all with the same results.. Im at my last straw not to mention with the better half also,,shes bout to explode unlike any boiler explosion ever.. lol I need your help please! any advice will be greatly honored. ill answer any questions you may have to the best of my ability... thanks Jeff

heathbar64 (author)JeffC812015-12-21

Ok, Jeff this is how I work with that. I don't use an auto vent. When I hooked it up initially I fixed it so I could shut a valve and open another to feed water under pressure directly from the garden hose through the coil and then into the tank.

What I do now is just unhook the coil, and lay it down flat with the hoses still connected, the kick on the circulator pump and it flows the air right on out quick and easy. Hope this helps.

JohannD4 (author)2015-12-20

I'm considering modifying this plan to burn wood gas from a DIY gasifier instead of just plain old wood. I'll be sure to post the results of my project after it gets underway.

heathbar64 (author)JohannD42015-12-20

I'll look forward to seeing your project.

TylerG13 (author)2015-12-10

how big is the building your heating, mine is over 2200 sq ft.

what size pex did you use
what size is your heat exchanger
how far is the boiler from your house
how deep was your trench
how big a fuel tank did you use?

heathbar64 (author)TylerG132015-12-11

I'm heating a little less than 1000 square feet. My friend is heating much more, something like3000 sf I think. Obviously he burns a lot more wood than I do Mine idles most of the time.

We both used 3/4 " pex.

My heat exchanger is 18" x 20" three layer.

My boiler is about 30' from the house, my friends is 75'

trench is 42" deep as that is the frost level here.

the outer tank is a standard 275 gallon (US) fuel oil tank.

krid51890 (author)2014-12-17

You should definitely post some recent pics of it now after its all insulated and finished!

krid51890 (author)2014-12-17

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!

krid51890 (author)2014-12-16

Thank you so much man! Is it still running strong?...if you could make any mods to it what would they be?

heathbar64 (author)krid518902014-12-16

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.

krid51890 (author)2014-12-15

What size pump and brand did you use?

welder41 (author)2014-11-01

What aquastat are you using ?

welder41 (author)welder412014-11-02

Ok thanks!!!

joseph.bunkercass (author)2014-10-18

What size hot water heater did you use

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.

backcreekwv (author)2014-10-07

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)backcreekwv2014-10-07

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

backcreekwv (author)heathbar642014-10-10

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

GYD535473 (author)2014-08-28

Nice build. Wondering if the damper closes off completely or allows some combustion air through?

heathbar64 (author)GYD5354732014-10-07

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.

GYD535473 (author)heathbar642014-10-07

that makes sense. thanks

carey.whitford.5 (author)2014-09-23

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


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.

roberto.torres.969 (author)2014-09-12

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

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_Dan (author)2014-04-27

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_Dan2014-04-27

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.

dtrembly (author)2014-02-09

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)dtrembly2014-02-10

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

dtrembly (author)heathbar642014-02-10

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

About This Instructable




Bio: I'm just an all around tinkerer inventor. I work with just about any type of material.
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