Introduction: Convert a Electric Water Tank to a Outdoor Wood Heater

I recently developed an interest in welding and so began looking for a welding project. With winter around the corner I began scanning Instructables for outdoor wood heater inspiration. Lots of great ideas and creative approaches. Many use gas cylinders which are a perfect size but has an element of risk. By chance I came across a dumped electric hot water unit, and even though a bit on the larger size, I thought why not.

Materials & Tools
1x expired electric water heater of any size.
1x 2M length of 100mm round tubing or equivalent for the flue
Reinforcement steal rod
Steal RHS
Steal angle
Angle grinder with scourer pad and cut off discs
Multi Tool
Jig Saw
Welder (Mig)

Step 1: Heater Strip Down

To be honest this step started out as a bit of fun but I was over it by the end. The dome top and bottom of the cylinder being the trickiest and therefore most time consuming. The body was relatively easy, just score a grid pattern around the circumference. The horizontal scores were about 30cm apart, the vertical score was the depth of my multi tool blade – about 5cm. Then basically shear the thing like a sheep. As you can see it came off pretty cleanly in rectangular blocks, created a huge mess in the shed, but cleaned up very well. Note that I actually left the bottom section on right up to Step 6, this allowed me to use a spirit level to mark out in the next step.

Step 2: Cut Out

The first thing here is to point out that the bottom of the tank is the top of the heater. This was not a deliberate decision but one that worked out very well when I got to the flue stage. Once clean I noticed several horizontal tooling marks on the body and decided to use these as reference. Using the spirit level I marked down a vertical reference line. Using two of the lines (approx 41cm apart) I marked out a 40cm wide door. Using two more of the tooling marks I nominated a section to remove, shortening the height of the unit. First the door and then the mid-section was cut out using an angle grinder with cut off disks (plural), and finishing the door cut with a jig saw. The inner plastic tubes and anode were then cut out.

Step 3: Fire Grill

From the removed mid-section a 7.5cm ring was cut – diameter of the tank is 60cm. This ring was shortened to 125.66 cm and re-joined. This was the first weld to the inside of main tank and where I discovered that it is coated in glass. All prospective weld points required grinding down to bare metal. Closing the ring manually was a bit of a task that means the new ring is “circle-ish” in shape. New diameter is now 40cm. Two 40cm pieces of reinforcement rod were cut and used as bracing inside the ring. Using a thin piece of scrap timber as a guide each rod was marked, cut, and welded in place to complete the task. The fire grill was installed through the open top of the unit. The grill itself is too large to fit through the door area. It is very solid and easily takes my body weight so I do not see any need to be able to completely remove. The ring can be comfortable lifted through the doorway to access the bottom of the unit for things like cleaning.

Step 4: Ash Collection

The original thought was easy enough – create a door of some kind to remove ash from the unit. I looked at the scrap pile and using an off cut of 9cm square tubing a solution (maybe not the solution, but a solution none the less) came together.
The ash collector uses two pieces of the tubing. The longer of the two – approx. 15cm makes the main chute. The second was cut into four sections - one retaining the corners for use as the bottom, another retaining a section of angle to use as a sliding door, and two thin sections to control the sliding bottom door.
The thin sections were clamped and welded to the main chute first. The bottom piece was next after being trimmed to allow clearance of the sliding door. A square hole was then cut into the bottom using the angle grinder, finished with the jig saw. The door slide itself has a bold welded to it for use as a small handle.

Step 5: Air Intake

The idea for air under the fire was to help promote a good draft and so a cleaner burn. Unfortunately I found out after that wood fires work best with air at the same level as the flame. So this was one of those learning exercises that I did this time and will change in the next.
Anyway what I did was take a 15cm x 5cm off-cut, which was 30cm in length, and cut a square hole in it for my ash collector. I then cut out a 15cm square from the bottom of the tank. To have this tube sit flush I needed to cut an arc in the front of the hole. This arc is horizontal across the curve of the bottom dome. Having access to a laser level would have make this very easy. No laser meant I simply gave it my best guess. As it turned out it was not perfect but reasonably close. Once happy with the cut I placed the tube in, used two magnets to hold it horizontal while I marked the arc on the tube from the inside. I didn't cut this arc exactly, I wanted to retain a small lip that I could weld to from the inside of the tank.
From here I welded the ash collector and a back piece to the tube and, after confirming the fit on the main unit, welded it in place. The outside weld was very straight forward, the inside weld was a bit awkward. A bit of grinding was required to expose the bare metal before I was set to continue. The depth of the unit made getting the mig hand piece close enough quite tricky. In the end though I did succeed in completing the joint.

Step 6: Top Section

As I mentioned earlier I did not clean off the insulation from the bottom of the heater until now. When I did I discovered an access port that I was not aware of. The fortuitous part of the discovery is that with the access door removed the hole is a perfect fit for my flue tubing.
The door I put back to use as a damper. For this I welded 3 bolts to the dome, after grinding off enough glass, and simply bolted the door down. Using the bolts means the damper can be adjusted or removed any time.
Hint: The flue was not installed at this time. At about this stage in the project I started thinking about evolving this concept into something more.….

Step 7: Base

Another ring was cut from the removed mid-section. To get an even cut I marked around the ring using two off-cuts of timber, ensuring that the line at least was parallel with the floor. Again the angle grinder with a cut-off disc(s) made the cut. Steel RHS was cut and joined to make a cross. The ring was placed over the top, positioned/centered, and marked for cutting. Welding this back in completed the metal work of the base.
Before continuing to the legs I used the ring to mark out a round cut on a sheet of MDF. At this point I am uncertain of whether I will use or not. My preference is to use some cement sheeting in which case I will use the MDF as a template.

Step 8: On Its Feet

Four 75cm legs were cut in 5x5cm angle iron, and once squared up were welded to the base ring.
With the unit upside down I cleaned up the welding areas, positioned four wooden sections, and sat them on timber wedges around the unit. The base/legs were rested on the blocks and using the wedges adjusted until level. Once I was happy the levels I gave each leg a small tack weld and confirmed levels again. After completing the tack welds all-round, the unit was laid horizontal and the leg welds completed.

Step 9: Securing the Top

The body of the tube is cylindrical but not perfectly round. The lid will sit on the top, it does closely line up, but not exactly. Plus I was also considering the evolution of this project and so was not keen to just weld it back the best I could. What I decided was to clamp the top on.
Another ring was cut from the mid-section and cut on the original joining weld. Two pieces of angle iron were prepared and tacked to the ends. To get the ring to sit in place while the top is positioned I drilled a couple of small holes and slipped in a nail. This is enough to hold the ring in the correct position while the lid was positioned and then tightened. In hindsight I should have used this method to create the fire grill.

Step 10: Test Drive

Take one sunset, a bucket timber, and some paper – light it up. The first test fire worked very well. There was a little bit of cross wind that seemed to draw smoke through the door but when that died back the flame/smoke was straight up. The little ash that was left was basically just a powder. The second fire was a good bit more intense. A larger fire running for several hours that at one stage started to appear through the open top of the unit. I am very happy with the heat retention and operation of the unit. The large doorway does however let out more smoke that I would prefer. I am going to lower the damper before the next test to compare.

Step 11: Final Tidy … Before Phase 2!

The body of this unit has five water input/output ports that will need to be trimmed and sealed. There are also the two element ports, one of which I have already done. Final task for this phase will be to remove the remaining paint from the exterior and cover with Pot Belly Black paint.

Step 12: Post Mortem

All in all a great project, and one I have thoroughly enjoyed. My gasless Mig welder has had a really good workout and I must say that I am much more comfortable with it now. Not that you can see my welds very closely but I describe them as functional, certainly not pretty.
About half way through the project I realized that this thing was going to be huge. Perhaps too big really for what I had originally intended. Rather that cut it down further I chose to press on. The original plan was simply to create a wood heater from an electric hot water service, which has been achieved. Now I want to evolve this into an outdoor wood pizza oven. Fix a door to the unit, work on a better air flow system, fit a shelf with fire bricks in the top section, create a second “oven” door, properly flue the top, and permanently secure the top section.
Defiantly another Instructable in the Pizza Oven conversion!

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