For years, we’ve been experimenting with different ways to heat water using our woodburning stove. Initially, we had a small woodstove into which I plumbed a copper pipe from an old metal mortar box that I bought at an Army surplus store. It held about 8 gallons of water and worked great as a stand-alone system for baths for our young children, and it provided enough water to pour over ourselves during a shower. After we built our mini-masonry heater, we switched to heating water in big pots on our large cooktop, and we put the hot water into a watering can mounted in our shower. This setup provided about 11⁄2 gallons of hot water. It worked fine for a while, but, like so many things that change as your children become teenagers, we needed an upgrade to keep hygiene and morale up on our urban homestead.
While visiting some friends who’ve been living off-grid for decades, I noticed their woodstove thermosiphon water-heating system. It was something I’d learned about several years ago, but I’d never seen one firsthand. Being able to see a system and talk about its functionality with its users makes a huge difference in whether or not I’ll tackle a project — especially one that involves plumbing and heat. After I’d discussed the project details with my friends, I felt confident trying it myself.
Assembling the System
Similar to our outdoor solar shower, this system takes advantage of the thermosiphon effect, in which cold water starts at a low point and then gets heated, which causes it to rise, creating a circular flow without any pumps or pressurized water.
I started by getting a used 30-gallon water heater from my neighbor. It’s old but leak-free. Used water heaters for this type of project are typically easy enough to find. As long as they don’t leak, it doesn’t matter if the heating element has gone out. The one I found happened to be for propane, but I’ve used old electric and natural gas water heaters before too. I then built a raised platform in our water heater closet so the tank sits higher than our stove. Placing it above the stove was essential, because it won’t work well if the tank isn’t higher than the heat source. Fortunately, that closet is just a couple of feet from our stove. From there, it was just a matter of plumbing the tank.
A typical water heater has four ports: one for cold water coming in, one for hot water going out, a pressure-relief valve, and a drain. The cold and hot water lines are located at the top of the heater. Cold water enters at the top; moves to the bottom of the tank, where it’s warmed by the heating element; and then rises toward the hot water outlet, where it goes off to a home’s sinks and showers or cycles back down into the tank. The pressure-relief valve, which sits on the upper side of the heater, releases pressure in the event that the temperature inside the tank gets too high. From this pressure-relief valve, there’s usually CPVC pipe that runs to a drainage area under or away from a house. On the bottom of the heater, the drainage valve allows the tank to be emptied if necessary. All of these ports are typically sized at ¾ inch.
In our woodstove system, I left the cold and hot water ports in the original locations at the top of the water heater, and they serve their original functions: carrying cold and hot water to and from the tank. I then added a T-connector to the drainage port so there’s one outlet for the drainage valve to operate as normal and another for a pipe to take cold water into the woodstove. I also added a T-connector to the relief valve so one outlet keeps the relief valve functional and the other operates as the hot water return from the woodstove.
I wound up reducing the ¾-inch fittings on the tank to ½ inch so I could use off-the-shelf flexible copper tubing to carry water from the tank through our bookshelf wall and into our woodstove. The first water-heating system we built was for our mini-masonry heater, and I used copper tubing all the way through the stove’s brick wall into the secondary burn chamber, where the water was heated in the tubing, and back out of the masonry heater in a big loop. We’ve since converted to a standard woodstove, so I purchased a ¾-inch Thermo-Bilt stainless steel coil insert instead of using copper tubing in the burn chamber. I chose a steel product because I don’t think the copper would’ve held up in the woodstove’s main burn chamber. Thermo-Bilt makes a variety of coil sizes. Ours is the smallest — a single 18-inch U-curve that mounts along the interior side wall of our stove. The coil ends come threaded, and Thermo-Bilt included all the hardware needed for mounting, even the drill bit for cutting two holes in the stove walls and a new pressure-relief valve.
The coil was easy to install. I drilled two holes in the back of our stove (you can do the side if your orientation is different), set the coil through the holes, attached it with the nuts and washers provided, and then plumbed it to the water tank. I wound up switching to PEX pipes for some of the system’s tubing, so I added two 6-inch metal nipples to the ends of the coil to keep the plastic PEX farther away from the heat of the stove.
A Hot Water Home Run
We love this system! With just a half-hour burn, we have more than enough hot water for a luxurious shower. When it’s colder and our fire is burning longer, we have hot water all day long. On days when we have a fire for a couple of hours in the morning, we’ve found that the water is still hot enough to have a shower or two in the late afternoon. For our simple lifestyle — which includes two teenage boys — this is a fantastic improvement to our quality of life. And, of course, it’s satisfying to heat our house and get our hot water at the same time, all by using wood — an original source of renewable energy. Learn more about our urban homestead.