Use Your Wood Stove as a Water Heater

Your wood stove can heat more than your home. This hot water heating system uses extra heat to produce hot water that will stay warm up to 48 hours!
By Sundance & Louie
November/December 1976
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Types of wood stoves for use with Blazing Shower systems.
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Three years ago, we were sitting in our cabins wondering how we could satisfy our addictions for hot baths and showers, without paying ridiculous prices for disappearing reserves of fossil fuels. Putting our talents together (one of us is a mechanical wizard and the other a Ph.D. chemist), we devised a homestead alternate energy system — based on the use of otherwise-wasted stovepipe heat — that's allowed us to take those hot baths. We call our system the Blazing Showers Stovepipe Hot Water Heater.

Hot Water Basics

As you can see in Fig. 1 (see Image Gallery), an ordinary water heater is nothing more than a storage tank (located between a house's water source and its various hot water faucets) sitting over a gas or electric burner. Since water tends to rise as it's heated, cold water is piped in at the bottom of the tank, while hot water is drawn off from the top.

Fig. 2 compares such a conventional water heater with a Blazing Showers system. As you can see, our setup employs a coil of copper tubing — located inside a wood burning stove's stovepipe — to heat the water that's held in our storage tank. Hot smoke rising through the stovepipe warms the water in the copper coil, which causes it to rise (and thereby draw more cold water into the coil). Meanwhile, the stovepipe-heated water flows into the top of the storage tank, where it remains until someone decides to take a "blazing shower" and turns on a faucet.

Notice that there are no pumps in our system: Instead, plain ole thermal convection does all the work.

Hot Water... Overnight!

How long does it take to fill a tank with hot water this way? The answer depends on how cold the incoming cold water is, how many gallons your water heater holds, and how hot the flame is in your stove. We estimate that a blazing fire in an average-sized wood-burner can produce 20 gallons of hot water per hour. And—if you store that heated water in an insulated tank as we prescribe—it'll remain hot for up to 48 hours after the fire goes out. What this means in practical terms is that if you have a fire in your stove one evening, you'll still have all the hot water you want (for bathing, dishwashing, etc.) the following morning when you wake up. In fact, that water will actually remain warm for two full days ... even if you don't light the stove again at any time during that period.

First Things First: Storage Tank

The first thing you need before you can install a system of your own, of course, is a storage tank. If you already have a hot water heater, you can use it ... otherwise, look around for a "previously owned" unit.

Many water heaters—you'll soon discover—are discarded solely because of a broken thermostat or heating element. Such retired fuel-eaters—as long as they don't leak and aren't badly rusted—are perfectly suited to our purpose. To find one of these storage containers, search around at the local dump, the power company, or in abandoned houses (make sure, though, that a house is truly abandoned before you go rummaging through it). Or—if you don't have the time to scrounge up a water tank—see your local plumber. Chances are, he handles quite a few broken water heaters and can get you a good one for $5.00 or a basket of snow peas.

Naturally, as we've already pointed out, you want a tank that's watertight and at least relatively rust-free. We've found that the ease with which the various fittings (attached pipes and connectors) can be removed from an old water heater is—quite often—a good indication of the unit's all-around health. Or, to put it the other way 'round, if its fittings are rusted so badly that you can't get them off, the heater is probably not worth fooling with.

The Tank: Location

Once you've obtained a serviceable water tank, it's important that you install it correctly in relation to the stovepipe coil. Notice—in Fig. 2—that opening X is above opening Z . . . and that Y is above W. Obviously, Y must be higher than W because it's the rising column of hot water that forces the circulation of fluid through the system.

Note, too, that the vertical distance separating Y and W determines how far—horizontally—you can put the water tank from the stove: You can move the tank up to two feet away from the wood-burner for every foot that Y is above W.

The Tank: Insulation

Conventional water heaters lack adequate insulation, due (we believe) to the politics of consumerism and to the fact that each unit's storage drum is so close to its heating element. Because our goal is efficient heat storage (and since—in our system—the reservoir is somewhat farther from its source of heat), we can—and should—do a better insulation job.

One way to accomplish this is to [A] bundle the entire water heater—top, bottom, and sides—in four to six inches of fiberglass, [B] wrap a blanket (or sheet) around the fiberglass-clad tank, and [C] stitch the blanket's (or sheet's) edges together. Or you could build a box around your storage tank and fill the enclosure with natural materials—wood shavings, pieces of bark, sawdust, chicken feathers, rags, egg cartons, wool, etc.—that create insulating air traps.

In addition to protecting the tank from heat loss, we recommend that you also insulate all exposed pipes.

The Storage Tank Adapter

If you were to pipe the hot water coming from your Blazing Showers stovepipe coil directly into the top of your storage tank as shown in Fig. 4, any air bubbles in the pipes would soon become trapped at the system's highest point. This would impair the convection-driven circulation of liquid through the heating coil and, to prevent such an occurrence, we've designed a special adapter.

As shown in Fig. 5, our adapter assembly screws onto the hot water outflow pipe at the top of the storage tank and thereby makes it possible for newly heated water to get into the container via the same pathway by which it is drawn off to the faucets. Thus, any air in the system quickly exits to the hot water spigots and is eliminated. (Note, too, in Fig. 5, that the adapter assembly contains an anti-siphoning device to prevent cold water from being drawn from the bottom of the storage vessel when the hot water faucets are turned on.)

The Firebox Hot Water Heater

Our Firebox Hot Water Heater works in exactly the same manner as our stovepipe model, except that the heating coil is mounted in the stove's firebox instead of the flue. We designed this system specifically for use with highly efficient automatic drum-type stoves (such as the Ashley 25, King Automatic, Atlanta Automatic 2502, etc.) ... stoves that produce such cool stovepipe smoke that we're forced to put our heating coil right inside the firebox. (Fortunately, these wood-burners are spacious enough inside to accommodate both the copper tubing and a large quantity of fuel.) Fig. 6 shows how our Firebox Hot Water Heater coil is mounted in one of these stoves.

Solar Water Heating

With just a couple of modifications, our basic Blazing Showers system will work as both a stovepipe AND a solar water heating system. That is, a wood stove and a solar collector could be used either simultaneously or independently to produce hot water and feed it to the storage tank (see Fig. 7). Such a setup, of course, is ideal for folks who—as we do—live in the sunnier parts of the country where wood stoves aren't used all year round. And—since the sun-powered part of the system need work only during the hottest weather—the collector itself can be a rather simple, low-technology device. We intend to market our own super-simple "sunny day'' solar collector next summer.

It's a Natural

Piping-hot water—warmed by a wood stove's waste heat—is a natural. And it's economical! (Just think: With no more hot water bills to pay, you can pocket an extra $10 to $25 a month!) Of course we're prejudiced, but any way we look at it, the Blazing Showers system is a piece of cake . . . and added self-sufficiency is the frosting.


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Post a comment below.

 

Philb55
12/16/2013 2:41:51 PM
I know this is an old topic, but I'm looking for some feedback on something I'm wanting to try. Our home has a fuel oil boiler system with baseboard registers and hot water heat. Works great but who can afford to operate it at $4.00/gal for the fuel. We've been using wood heat with a Lopi wood stove in our living room, but our basement, bedrooms and bathroom get pretty cold comparatively. What I want to do is install a Blazing Showers Stovepipe Hot Water Heater and rather than hook up to water heater, hook it up to my boiler system so we can heat the house completely. Once tapped in, I think all I would need to do is have the existing circulating pump going continuously to keep a steady flow going. Everything needed is already incorporated into the lines, like the pressure relief valve, tempering valve, circulating pump, air bleeder valve. All there. I'd like to keep all intact so we can use the fuel oil boiler as backup. Maybe hook in our electric hot water heater as well. Does anyone see anything wrong with this picture, and is it possible to buy one of your heaters somewhere, or should I make my own?

Valerie Bronz
4/11/2012 3:04:56 PM
I saw this same type of heating about 20 yrs ago an thought itwas wonderful. My question is can this be easily adapted to flow threw floor piping to supliment your heat and keep those floors warm in winter?

Duane Weed
6/27/2010 7:25:05 PM
Where can I get the adapter assembly that screws onto the hot water outflow pipe at the top of the storage tank which contains the anti-siphoning device (at Y in figure 2)? Thanks

Roland Green
2/12/2010 2:27:07 AM
In Figure 2 the pipe work from the heating coil does not appear to return to the storage tank, but appears to feed directly to the fixtures. Surely the heated water should return to the storage tank otherwise the water in the heating coil, being static when no tap is being turned on, would soon overheat and boil. I would see the return from the heating coil entering the storage tank about the same level as 'W' allowing for convenction circulation of cooling water from above that level. Secondly, I assume that this system is a gravity fed system from a water storage tank in the attic and not pressurised because again, if the hot water is not used the storage tank could soon overheat and most likely burst as there is no way to control the heat. As a safety feature, rather than a safety valve the storage tank can be vented by a rising pipe, either directly to the open air, or venting into a header tank in the attic, which would be level with the general water stoage tank.

Gary Reysa
6/1/2009 8:03:45 PM
Hi, Good article -- thanks! One safety precaution I think should be emphasized is that these system should be protected with a pressure/temperature relief valve, and that no shutoff or bypass valves should be installed in the system that can isolate the stove from the PRT valve. Steam explosions that can result from water being heated in a stove coil can be extremely violent as this link shows: http://www.woodheat.org/dhw/dhw.htm Gary








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