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 piping. All of these systems required no 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 to water front. 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.

Other Wood Stove Water Heating Options

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.

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

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. 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.

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.

Some Hot Water Front Design Considerations:

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 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

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!

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{ 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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