Using Your Wood Stove to Heat Water

March 1, 2013 · 2 comments

One of the most overlooked functions of a wood stove is heating water. A few decades ago many wood stoves (especially cook stoves) used what was a called a hot water front in conjunction with a tank (range boiler) beside/behind the wood stove to produce endless hot water.

Wood stove heating water

The hot water front (which is nothing more than a small tank with an inlet and an outlet) was placed in the fire-box of the stove and the tank was connected by copper, steel, stainless steel, brass or cast iron piping. None of these systems required pumps or fancy controls.

Instead of a circulating pump, the hot water would rise out of the water front into the top of the water tank. The colder water in the tank would fall out of the bottom of the tank and into the hot water front (small tank). In this system, the water circulates all by itself through the process known as thermosiphoning.  Years ago this type of water heater would have been the primary or only hot water heater in the home. Today these are more commonly used to preheat well or municipal water before it enters the main water heater although they can easily supply enough hot water for a typical family during the cold months your woodstove is required.

SAMPLE HOT WATER FRONTS

 

This hot water front is made from stainless plate and has 1 baffle to force water to travel more slowly and make surface contact with the hot surfaces.

This hot water front is made from stainless plate and has 1 baffle to force water to travel more slowly and make surface contact with the hot surfaces.

sample hot water front for heating water with your woodstove

This hot water front is made from stainless plate and has no baffles. Notice how the water travels from inlet to outlet making very little contact with the hot surfaces.

In the above illustrations is it obvious the more baffles you install in your hot water front the more hot water you can expect it to produce. However one is usually plenty. It gets very complicated to weld, the more baffles you install.

OTHER WOOD STOVE HOT WATER HEATING SYSTEMS

THERMA COIL (or homemade unit)

Heating water with your wood stove

There is a relatively new product available online called the “Therma Coil” which is a small u-shaped coil made to insert into your wood stove and attach to your range boiler. They can be installed at any time by drilling two holes in your wood stove and connecting the plumbing.

If you wish to make your own hot water coil only use rugged materials inside the firebox such as brass, steel, stainless steel with threaded fittings. Do not use copper pipe with soldered fittings or copper tubing with a 180 degree bend.

Copper is too soft and unforgiving if the pipe were empty of water with a fire going. Soldered joints will simply unsolder themselves immediately if overheated.

The same rules apply as when using a hot water front. You want the system to circulate without any pumps. Mount the U Shaped pipe horizantally as in the photo so there will be a top fitting and a bottom fitting.

The top fitting must be connected to brass, copper, steel , stainless steel or some other metal piping and go to the top fitting on your water tank (range boiler).

The bottom of the U-shaped pipe will connect with metal piping to the fitting closest to the bottom of your tank. The closer to the bottom of the tank the fitting is, the more storage of hot water you will have. If your lowest fitting is 8 inches from the bottom of the tank, the bottom 8 inches of your tank will store no hot water.

We have no experience with them but they look reasonably well made and the concept is proven.

BRASS FITTINGS PLACED ON TOP OF WOODSTOVE

Another slightly less efficient method of heating water with your wood stove is to make a maze of threaded brass or stainless steel pipe fittings and lay them on top of the wood stove if your wood stove has a flat top. You could also make a coil of soft copper tubing and set it on the top of the wood stove for the same effect.

Although this would not make as much hot water as a water front or loop of pipes inside the firebox, it will make a lot more than nothing and require no drilling or modification of your wood stove. This method of heating water would likely be one of the most insurance friendly systems as you are not modiying your wood stove and you could easily remove it if your insurance company decides to visit for a home inspection.

3. COPPER COIL INSIDE YOUR STOVE PIPE

Another method of capturing your wood stove’s heat is to make a coil of copper tubing that is just small enough to fit inside your stove-pipe. Usually you would use 1/2″ (13mm) soft copper tubing for this project and you would increase your stove-pipe diameter to make up for the space taken by the copper tubing. For example: If your wood stove’s manufacturer recommends 5″ stove-pipe. you would use 6″ as to keep the correct amount of draft for your wood stove.

The biggest issue we have found using this system is the creosite that forms on the copper tubing because of the large difference in temperature between the water and the super heated air going up your chimney. All of the systems mentioned in this article will form creosite to a certain extent but the copper tubing in the stove pipe seems to be the worst.

Once a thick layer of creosite has formed it reduces the size of the stove pipe. The other issue is the system loses it’s effectiveness as the creosite acts like an insulator between the hot air and cold water.

4. COPPER COIL WRAPPED AROUND YOUR STOVE PIPE

This is an expample of a nice looking installation of a coil of copper tubing wrapped around a 5 inch stove pipe and covered with a 6 inch stove pipe. The cold water enters the bottom and the hot water exits the top. The hot water tank is on the second floor.

This is an example of a nice looking installation of copper tubing wrapped around a 5 inch stove pipe and covered with a 6 inch stove pipe. The cold water enters the bottom and the hot water exits the top. The hot water tank is on the second floor.

The last option for making hot water with your stove is to capture the heat from the chimney by wrapping soft copper tubing around it or some other type of heat exchanger. For cosmetic reasons you could add a larger stove pipe to hide the copper tubing.

For example: If your stove pipe is 5″, you could install a 6″ over it. This will also act as a double wall stove pipe.

If you decide to capture the heat from the stove pipe and the range boiler needs to be above the copper coil, you will either need to install it (range boiler) on the floor above or use a circulating pump.

Like the brass fittings on top of your woodstove this stove pipe system would probably be fine for most insurance companies. You have not modified your stove in any way.

This system should not cause any creosite and should not ever get hot enough to melt the copper tubing if the system was to run dry (without water).

You should still use pressure relief valves on the inlet and/or outlet tubing.

DESIGN CONSIDERATIONS FOR CIRCULATION

As we are not using any pumps to circulate the water we must be very careful to ensure the following:

  • The tank must be located higher than the wood stove
  • The tank must be located very close to the wood stove to avoid long pipe runs
  • The drain of the water heater must connect to the input of the water front or coil and be higher than it
  • The hot water piping coming out of the front, coil or U shape heater must go into the top of the range boiler
  • All piping must be sloped at least 12″ for every 24″ of pipe run (45°)
  • The larger the piping the better (use 3/4″ or 1″)
  • Avoid 90 degree elbows or other restrictive fittings as much as possible

SAFETY CONSIDERATIONS

  • ONLY USE METAL PIPING. NEVER USE PEX, ABS, PVC from the stove to the tank.
  • Insurance companies will never like any of these ideas
  • Always use at least one pressure relief valve (preferably two) between the woodstove and the tank
  • Use temperature/pressure relief valves to make system safer.
  • NEVER INSTALL VALVES BETWEEN STOVE AND HOT WATER TANK

The best arrangement would be to have the range boiler on the floor above the floor the wood stove is on. That would allow for excellent circulation.

High efficiency circulating pumpIf your wood stove and water heater are a long distance apart or you are unable to make the system circulate by gravity, you can always use a circulating pump move the hot water back and forth. If you go the circulating pump route, resist the temptation to purchase a standard “off the shelf” pump. As you have to generate the electricity required for the pump you will need a super high efficiency pump.

Do not buy any circulating pump before reading this.

WARNING! PLEASE READ THIS FIRST!!!

When water is heated, it expands. The cold water that enters the system is heated to 120-180 degrees F and expands slightly. This expansion is usually handled well by the same expansion tank that keeps the pressure constant in your water system. This is how a “normal” hot water heater doesn’t always build pressure or blow the pressure valve.

However if that hot water turns into steam it will expand 1600 times. That will not be handled by the pressure tank. Instead, the range boiler, water front or piping will explode. Many people have been killed by this during the past century. Always have a pressure relief valve between the range boiler and the water front and preferably two of them. Steam is very dangerous.

Do not install valves on either pipe between the hot water front and the range boiler. You are just asking for trouble by doing  this. Someone (maybe a child) will close the valves turning your water heater into a bomb!

If your fire is lit and your hear bubbling in the system, it is likely the water is starting to boil. The correction is simple. Just open a hot water tap (or a few of them) and dump that excess hot water. If you don’t (or aren’t home), the pressure relief valve will do it for you. Our forefathers did not have pressure relief valves so they had to be very aware of steam production.

Try this method for making cheap hot water and let us know how it works for you!

Leave a Comment

{ 2 comments… read them below or add one }

John Messick October 7, 2013 at 12:55 am

I have converted my old AC unit to a wood stove by taking out the coils – compressor and placing two welded drums together for the stove. Inside insulated box house.
The existing duct works and blower will be used off the thermostat. When heat is called for the fan and small furnace blower = 2″= will come on.
I am going to use your system to heat hot water. Using my existing refrigerant heat tubing. “Heat Exchanger”.
Thank You, John Messick

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Loren Amelang May 30, 2013 at 12:19 am

I started my off-grid adventure with a water heating insert in my (Elmira Oval) cookstove. I suspect if you live in a climate with serious winter, where you keep the stove going constantly for months at a time, the concept works. In California it is just wrong – it works too well! When you build a nightly fire for a bit of warmth, the water heating load sucks all the heat and you literally can’t fry an egg on the stovetop until you’ve heated all the water. You can’t just turn off the flow, because as this article warns, you’ll get serious steam.

The next morning when the sun comes out and the solar panels take over, the storage tank is still hot and the free heat is wasted. Totally incompatible with solar. The insert is still stored in the attic, like new…

The other problem was having the storage tank above the woodstove, as required for convection. Putting 600 pounds on the second story is a bad idea for many reasons, not least of which is that when it eventually ruptures it makes an incredible mess. The tank was too far from the places the hot water was used, and hot water really hates to flow downhill, so not only did it take forever for the taps to get warm, but the hot water flow was always weak despite 3/4″ pipe everywhere.

My solar backup is now a serious outdoor water heating stove, that can take 30″ logs and only runs when there is no hope of solar heat. The cookstove is dedicated to cooking.

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