We’ve been experimenting with different ways to heat water with our wood-burning stove over the years.Originally we had a small wood stove and I inserted a copper pipe from an old metal mortar box that I bought at the army surplus store.It holds about 8 gallons of water and works great as a stand-alone system for our young kids to bathe, it provides enough water to pour over us in the shower.After we built our mini masonry heater, we switched to heating water in a large pot on our large cooktop, and then we put the hot water in a watering can installed in the shower.This setup provides approximately 11⁄2 gallons of hot water.It worked fine for a while, but, like many things that happen when your child becomes a teenager, we need an upgrade to maintain the hygiene and morale of our urban homes.
While visiting some friends who have been living off-grid for decades, I noticed their wood stove thermosiphon water heating system.This is something I learned about years ago, but I’ve never seen it with my own eyes.Being able to see a system and discuss its capabilities with its users makes a big difference to whether I’ll be working on a project – especially one involving plumbing and heating.After discussing project details with friends, I was confident to try it myself.
Similar to our outdoor solar showers, this system utilizes the thermosiphon effect, where cold water starts at a low point and is heated up, causing it to rise, creating a circulating flow without any pumps or pressurized water.
I bought a used 30 gallon water heater from a neighbor.It’s old but not leaking.Used water heaters for projects like this are usually easy to find.It doesn’t matter if the heating element goes out or not, as long as they don’t leak.The one I found happened to be propane, but I’ve used old electric and natural gas water heaters before too.Then I built a raised platform in our water heater closet so the tank is higher than our stove.Having it above the stove is essential as it won’t work very well if the tank is not above the heat source.Luckily, that closet was only a few feet away from our stove.From there, it’s just a matter of plumbing the tank.
A typical water heater has four ports: one for cold water inlet, one for hot water outlet, a pressure relief valve, and a drain.Hot and cold water lines are located on top of the heater.Cold water enters from the top; moves to the bottom of the tank, where it is heated by heating elements; then rises to the hot water outlet, where it flows to the home’s sink and shower, or circulates back into the tank.A pressure relief valve located on the upper side of the heater will relieve pressure if the tank temperature is too high.From this relief valve, there is usually a CPVC pipe leading to the drain area under or away from the house.At the bottom of the heater, a drain valve allows the tank to be emptied if necessary.All of these ports are typically ¾ inch in size.
In our woodstove system, I left the hot and cold water ports in their original location on top of the water heater, and they perform their original function: delivering cold and hot water to and from the tank.I then added a T-connector to the drain so there is one outlet for the drain valve to function properly and another outlet for piping to bring cold water into the wood stove.I also added a T-connector to the relief valve, so one outlet keeps the relief valve working and the other outlet serves as the hot water returning from the wood stove.
I ended up reducing the ¾” fitting on the tank to ½” so I could use off-the-shelf flexible copper tubing to carry the water from the tank through our bookshelf wall to our wood stove.The first water heating system we built was for our small masonry heater, I used copper pipes all the way through the brick wall of the furnace into the secondary combustion chamber, the water was heated in the pipes and flowed out of the masonry The heater is in a big cycle.We’ve converted to a standard wood stove, so I purchased a ¾” Thermo-Bilt stainless steel coil insert instead of using copper tubing in the burner.I chose steel because I don’t think copper will hold up in the main combustion chamber of a wood stove.Thermo-Bilt manufactures coils of various sizes.Ours is the smallest – an 18″ U-shaped curve that mounts to the inside sidewall of our stove.The coil ends are threaded, and the Thermo-Bilt includes all the hardware needed for installation, even a drill bit for cutting two holes in the furnace wall and a new relief valve.
Coils are easy to install.I drilled two holes in the back of our stove (you can do the sides if your orientation is different), passed the coil through the holes, attached it with the nut and washer provided, and attached it to the tank.I ended up switching to PEX piping for some of the piping for the system, so I added two 6″ metal fittings to the ends of the coils to keep the plastic PEX away from the heat of the furnace.
We love this system!Just burn for half an hour and we have enough hot water for a luxurious shower.When the weather is colder and our fires burn longer, we have hot water throughout the day.On days when we had a fire for a few hours in the morning, we found the water was still hot enough for a late afternoon shower or two.For our simple lifestyle – including two teenage boys – this is a huge improvement in our quality of life.And, of course, it is satisfying to heat our house and get hot water at the same time, all through the use of wood – a pristine renewable energy source.Learn more about our urban homestead.
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