The drain valve and cold-water outlet to the stove
For years, we’ve been experimenting with different ways of heating water using our wood-burning stove. Initially, we had a small woodstove into which I plumbed a copper pipe from an old metal mortar box I bought at an army surplus store. It held about 8 gallons of water and was great as a standalone system for baths for our little children and a container to pour over ourselves for a shower.
After we built our mini masonry heater (see my article for MOTHER EARTH NEWS here) we just switched to heating water in big pots on the very large cooktop, which we then put into a watering can mounted in our shower. For about 1.5 gallons, we had a nice shower. But, like so many things that change as your children become teenagers, we needed an upgrade to keep hygiene and morale up in our wacky urban homestead.
Enter the plumbed woodstove water heating thermosiphon system.
While visiting some older friends of ours who’ve been living off-grid for decades, I noticed their woodstove hot water system. It was something I’d learned about several years ago but had never seen one firsthand. For me, 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.
The Water Heating Design
Like our outdoor solar shower (read another MOTHER EARTH NEWS article I wrote for this project here), this system takes advantage of the thermosiphon effect, whereby cold water starts lower, gets heated, and rises, creating a circular flow without any pumps or pressurized water.
I started by getting an old, 30-gallon, leak-free water heater from my neighbor. There are a lot of these around that still hold water but may have a burned-out heating element — so see what can be salvaged before buying a new one.
I then built a raised platform in our water heater closet to get the tank higher than our stove. This is essential: It won’t work (or work well) if the tank is not higher than the heat source. Fortunately, that closet is just a couple feet from our stove (you can see in the photo). From there, it’s just a matter of plumbing the tank.
Cold water in at the bottom and hot water out the top
Normally, with a typical water heater that’s hooked to electric or gas, there are four ports: cold water in, hot water out, pressure-relief valve, and drain. The cold and hot water lines go in at the top. Cold water enters (it goes in the top but actually gets brought to the bottom of the tank where the heating element is), gets heated, and rises in the tank toward the hot water outlet where it goes off to your sinks and shower or cycles back down to heat up to whatever temp the thermostat is set.
On the upper side of the tank is a pressure-relief valve in case the temperature inside the tank gets way too high and needs to be released before something blows. From this valve there’s usually CPVC pipe that runs to a drainage area — under or away from of your house. On the bottom, there’s also a drainage valve so the tank can be emptied. All of these are sized at ¾-inch.
The whole setup at a glance
In the woodstove system, the cold and hot are in the same spots at the top. The drainage port, however, is given a “T”: one direction for the drainage valve and another for a pipe to take cold water into the woodstove. The relief valve also gets a “T”: one direction keeps the relief valve and the other is the hot water return from the woodstove.
My friend’s system uses ¾-inch pipes throughout, because he was able to buy a threaded, stainless-steel insert that goes into the burn chamber made by a guy on the East Coast. Drill a couple holes in the woodstove, plop the pipe in along the inside wall with the ends going out the holes, and then hook up to your tank. Easy.
Cold water into the tank and hot water out to the house
Unfortunately for me, that guy has since retired (there may be other products online, so check before you build). So, not knowing how to braze metal and knowing that a soldered joint would not hold inside the hot firebox, I chose to buy ½-inch flexible copper tubing ($35 for 10 feet) for my pipes and just used reducers to attach them to the ¾-inch threaded ports on the hot water tank.
Flexible tubing allowed me to make a curved, C-shaped loop that would fit inside one of our masonry heaters chambers. I drilled a hole in a firebrick in the back of the stove — the low, cold water entry — for the looped pipe to exit and then shaped it to fit under the cooktop (hot water exit) at the top. From there I connected both ends to the pipes running to and from the tank.
In my first design, I used all copper tubing with sharkbite couplings. After I redesigned the system (see below), I realized I could have used pex here and there (outside the stove, obviously) and did so. Pex is so easy to work with and the fittings are much cheaper than sharkbites.
The pressure relief valve and the hot water being returned to the tank
Results Using Our System
This worked well but not great. We got hot-ish water in our sink and shower but it wasn’t quite hot enough to really have an enjoyable shower. The reason was twofold.
First, because of the design and location of our masonry heater, I put the tubing in a secondary chamber outside the burn chamber of our stove. This meant lower temperatures. Second, the ½-inch pipe is narrow with less surface area to heat the water sufficiently.
I imagine I could have made the pipe curvier inside our stove, thereby exposing more pipe to heat, but I struggled with bending the copper that tightly without kinking it. Also, because of the stove design, we don’t have a lot of wiggle room in that secondary chamber. You might have more luck with a regular woodstove that has easier access inside the burn chamber.
Making Improvements After Use
After a couple months of this setup and thinking about how to improve the system, I decided to add another loop inside our masonry heater. I drilled a second hole in the back and now the cold-water pipe splits into two loops and rejoins the hot-water pipe after exiting the stove.
The results are awesome! We have water so hot that it steams when it comes out of the sink if not tempered with cold water. In about 5 hours of wood burning, we had enough hot water to take two very long, luxurious hot showers. During the winter, when we burn wood for at least part of each day, the water stays consistently hot. It is very satisfying to heat our house and get our hot water at the same time all by using wood — an OG of renewable energy.
Kyle Chandler-Isacksen is a tinkerer, natural builder, and community organizer in Reno, Nevada. He and his family run the Be the Change Project, a fossil fuel-, car-, and electricity-free urban homestead and learning space dedicated to service and simplicity and inspired by the principles of Gandhian Integral Nonviolence. They were honored as one of MOTHER’s Homesteaders of the Year in 2013. Read all of his MOTHER EARTH NEWS posts here.
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