Convert a Hot Water Heater Into a Wood Stove

About 6 years ago, with the cost of heating my house rising, I began looking at designs for wood stoves. In The Mother Earth News (Issue # 49- January/February 1978) I found a promising article about using discarded hot water heaters. Using that article as a guide, I constructed a wood stove that I have been using for about 6 years.

The stove is used in the first floor of a 20 foot by 36 foot uninsulated brick house. When the temperature is 20 degrees outside, the basement is 80 degrees and the floor above is 55 degrees. The temperature difference would be reduced by connecting the floors with ducting.

Here in the city, it takes about 16 square blocks of scavenging territory to scrounge up enough discarded wood to supply one house with one years worth of fuel. It helps to have a car or truck to transport the wood that people throw in the alleys. Discarded furniture made from particle board is great because it is easy to break into small pieces. Keep an eye out for folks with dead trees that you can offer to remove.

The Mother Earth plan calls for electric water heaters. My variation uses a gas heater, which is much more common in cities. For this instructable, I made a second stove and documented what I did. Since there are so many metal scavengers in my area, it is very difficult to find a heater in the alley, so I traded an old car battery for a water heater from a nearby scavenger.

Note, before starting this project, check with your homeowners insurance to make sure there won’t be a problem.

Photo A shows the heater with the covering and insulation removed.

Step 1: Materials Needed.

Materials.
1. A 40 or 50 gallon gas water heater. The 40 gallon heaters are about 4 feet long and 16 inches in diameter (with the insulation, base, and outer covering removed).
2. A large helium gas container of the type used for balloons.
3. A saber saw and plenty of metal cutting blades.
4. A Dremel or Rotozip tool fitted with a metal cutting saw.
5. An angle grinder with a metal cutting saw.
6. A 3/8 inch drill.
7. An assortment of bolts and two hinges.
8. Some fiberglass cloth or ceiling tile.
9. A knob for the door.
10. A latch for the door.
11. Scissors to cut the fiberglass and stapler to secure the pieces.
12. Measuring tape and marker.
13. Enough 5 inch stove pipe to reach to the chimney.
14. A wrench to remove the gas valve and remaining pipes

Step 2: Cut Out the Door.

Remove all the outer coverings, insulation, base coverings, and pipes from the heater. There will be a sacrificial anode inside that also needs to be removed. I discovered that in 3 out of 4 heaters, this anode is made of magnesium. If scrapings from the anode can be set afire with a match, you have magnesium.

Get some ear plugs and goggles, along with an angle grinder or saw with a metal cutting blade. Put the heater on it’s side and mark out the door opening at 9 inches wide by 18 inches long. See photo B for details. Cut the door opening at the domed (top) end of the heater and about 4 inches from the end. You will get longer life from your blade if the rotation of the blade and direction of the cut are as illustrated.

Save the cutout for later where it will be used as part of the door.

Step 3: Cut Out the Stovepipe Hole.

Use a piece of stovepipe as a template and center the stovepipe hole about 9 inches from the bottom end of the heater. Drill a starter hole and use the saber saw to cut the opening for the 5 inch stovepipe. I found that the blades dulled quickly and broke, probably because of the glass lining of the heater. Keep the cutout piece for use later on. Photo C illustrates this.

Try to keep the fit as tight as possible to minimize smoke leakage.

Step 4: Cut the Vent Tube Out.

The water heater will have a 3 inch diameter vent tube in the center, running from top to bottom. This tube will be cut out and used as the draft tube of the stove. The tool I used to cut the tube is illustrated in photo D .

I used the Rotozip tool to cut the tube from the inside, and close to the ends so as to keep the tube as long as possible. After both ends of the tube are cut, you will have to use the saber saw to cut the hole for the draft tube and remove the tube through that hole. This is illustrated in photos E and F . To recap, the 3 inch tube that used to be the vent for the water heater is now being used as the draft tube for the wood stove you are making.

You will probably need two cutting disks because they wear out quickly.

Step 5: Drill Vent Holes.

After you get the vent tube (now the draft tube) out, get out your drill with the biggest bit you have. I used a ½ inch drill bit. Now, drill a line of holes along the tube at about 2 inch intervals. These holes will allow air to enter the stove along the length of the tube. The holes don’t have to be accurate or neat.

Use a center punch to mark the places to drill, otherwise the bit will be hard to start. The punch will also break any glass coatings. You will need lots of pressure to get through both sides of the tube. I found that I had to resharpen the bit every 2 or 3 holes. Photo G shows this.

Step 6: Install the Draft Tube.

Put the finished draft tube back into the hole that you cut in photo E , leaving a few inches of it poking out the front (like in the insert of photo H). Then, at the back, drill an anchor hole through the tube and the bottom of the stove. You will use a 4 inch bolt in this hole to fasten the tube to the stove.

I used the 5 inch stovepipe hole as an access for the drill. This was cut in photo C . Photo H illustrates all this.

Step 7: Cut the Door.

Find a 12 to14 inch diameter helium container. Use the angle grinder to cut a door approximately 9 inches by 27 inches. Measure to make sure it’s right for your stove. You will find the curve is close to the amount that you need. One of these containers is depicted in photo I .

Step 8: Assemble the Rest of the Door.

Center the piece from step 2 into the piece you just cut from the helium jug and drill holes for 5 mounting screws along each side. Like before, I had to resharpen the drill bit often while doing this. See photo J .

After the two pieces are bolted together, fit the door on the stove and when you have the best fit, mark the position for the two hinges. When you mount the door, put a light and mirror inside to check the alignment as you progress. Photo K is a closeup of the hinges. You can use hinges of steel, aluminum, or brass. The alignment of the door was troublesome and took time to get right.

You will want to be able to replace the hinges and fiberglass gasket, so have the nuts on the outside for easier removal.

Step 9: Install the Gasket Seal.

Photo L shows the fiberglass cloth that I used for the gasket. I cut it into strips 10 inches wide and 22 inches long. This came from an automotive supply shop.

Wind the fiberglass on a smooth rod as shown in photo M . Actually, the rod was half an inch in diameter.

Use a stapler to flatten the rolls into strips as in photo N . Put a staple about every two inches.

After you have finished, you should have 4 fiberglass seals, as in photo O . The two shorter pieces were made from fiberglass curtain cloth. The curtains came from the alley.

Use the inner plate, of the door, to hold the pieces of gasket in place. Photo P shows the whole door and Photo Q is a detail view.

There are other alternatives for gasket material. One possibility is the gasket from the oven door of a discarded gas range. Another idea is to cut strips from a piece of fiberglass ceiling tile. These sources are illustrated in photo R .

The latch for the door can be anything available. I found a glass knob to use as a door handle, but you could use an old sash lift instead. Photo S shows what I did.

Step 10: Finishing Up.

The 3 inch opening at the rear of the stove (what used to be the bottom of the hot water heater) can be used with three inch stove pipe to divert hot combustion gases to other purposes like heating water, but I just plugged it up using the piece I cut from the stove pipe hole in step 3. Photo T shows this.

When you are finished, your stove will look something like photo U . I stuck it on a small refrigerator for the photo, but you will need a base made of bricks or perhaps made from scavenged water pipe. A base or stand made of pipe is preferable as it allows more heat into the room. There will be holes where the plumbing used to be. These can be plugged with 3/4 inch plugs.

Step 11: Final Thoughts.

You can use a torch to light the stove using the front opening. After lighting, I cover the opening with an empty 5oz catfood can. I have three cans, two having different size holes in them for differing air allowance. I typically use the torch on short lengths of wood lath to start the fire going. When the supply of lath runs out, I use an ax to split up small boards.
The torch lighting is shown in photo V .

Put a grill over the draft tube. This will keep the coals from burning through the bottom of the stove and allow air to come in through the draft tube. You will probably have to replace the grill each year. See photo W .

The stove will create about 1 to 2 pounds of ash each day. It is best to remove the ash after each day because a buildup will reduce the heat output. Photo X shows the tools I use to remove ash. I use a magnet to remove nails and screws from the ash because I put the ashes on the lawn in the spring.

You will have to deal with a buildup of soot (creosote) in your chimney. I tied a rope to a brick, and wrapped an old t-shirt around it, and pulled it up and down the chimney every year.

I have looked into “soot-destroyer” chemicals and found them wanting. Tests of these chemicals show they barely work if at all. One popular class is a mixture of 90% potassium nitrate and 10% charcoal, which actually promotes chimney fires. The idea being to have frequent small chimney fires instead of one big one. These materials have ignited the soot in the stove pipe, causing it to glow a dull red.

A second type of soot destroyer is a fire retarding chemical that vaporizes in the flames and then condenses on the soot deposits. It acts to make the soot more difficult to ignite. You still have the problem of buildup in your chimney.

Another possibility is to put small pieces of broken plasterboard in with the wood. The idea is that hot sulfur dioxide is formed and it reacts with the soot in the chimney. As I recall, plasterboard is calcium sulfate and it decomposes into calcium oxide and sulfur dioxide when heated.

The first use of this stove should probably be outside so as to burn off any combustibles on the outer surface. Like wise, don’t paint the outside of the stove because it will smoke and burn off.

If you have welding equipment, you should add a lower chamber for collecting and removing ashes. This could be made from another helium container.

I found that keeping flammable materials at least 3 feet from the stove was adequate for fire safety. This is especially true of the area directly over the stove, where heat is concentrated. Any smoke alarms should be positioned away from the stove because some smoke is often emitted when you refuel the stove.

As I used the stove, sometimes the cap on the front opening would pop off after a new charge of wood was added. So, I added a bolt to hold the cap in place. This is illustrated in photo Y .

The valve in the photo was something that I added on the stove I’ve been using. It has proven to be completely unnecessary and always open.

If you are using this stove in the city, start with small amounts of wood. Start in early fall with small loads so as to get the neighbors used to seeing smoke come from your chimney. City people, seeing that, OMG!!!, there is smoke coming from a chimney, will call the fire department to investigate. That, in turn, could lead to confiscation, condemnation, or regulation. In my case, the fire department was fascinated and found nothing to criticize.

As much as possible, keep your wood supply dry and out of sight. You don’t want neighbors claiming that a woodpile is an “eyesore” or a “rat haven”. Keep some wild cats around to control any rodents.

You can also burn small amounts of various plastics. Some plastics will produce quite a lot of quick heat.

I found that, on really cold days, the stove can be used to melt aluminum for casting. My cast iron crucible holds about 18 oz of water and that amount of aluminum requires a lot of heat to melt. Even on cold days, melting aluminum can easily raise the interior temperature to 100 degrees. It takes about an hour and a half of stoking the fire to completely melt it.

Photo Z is an illustration of charging the stove with wood. I have to split most logs to get them in. Much of my fuel is broken furniture. The fire in this photo is simulated.

The main design principle here is the use of a tube or pipe to admit air the fire within. This tube serves two purposes: One, to preheat the air entering the stove and two, to dampen oscillations in the air column through the stove and up the chimney. These principles can be used in masonry stoves, barrel stoves, and cast iron stoves.

The thing about this project that gave me the most trouble and frustration was compiling the text and pictures on a Windows computer. Windows programs seem designed to impede and infuriate the unfortunate user. I think we will never have good software until Microsoft is out of business.

Next time, I’ll use numbers for the pictures. I almost ran out of letters on this one.