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Wood Fired Hot Tub

Fire and Water 2 - Building it

The Cost of Heat

Fire and Water

Domestic water heating from your woodburner

by Art Sussman and Richard Frazier

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Steve's Note: Click the illustrations to see a larger version. You will want a tempering valve as wood fired hot water can burn you due to high temperatures. Contact me for sources.

Anyone already persuaded to go with wood heat in his home ought to be a soft touch for this next idea. Youíve got the heat, now add a pipe or two and rid yourself of the second most gluttonous energy consumer in your home, your water heater.

The average family in 1977 spent about $250 For water heating, which should tell you something about payback time. After a year or two your wood water-heating system will be paid for and more or less self-sufficient, the only cost being wood for your stove.

If you burn wood all year you can take care of all your water needs, but if youíre like most of us you wonít want to stoke a fire during the warm months, and a back-up system of gas or electricity will probably be necessary. But the best method for obtaining year-round hot water is to use a simple, low-technology solar water heater during the summer. In spring and fall both the solar collector and the wood stove would heat your water. In the winter, stove heat is all youíll need.

There is a reason for using a dual system, and not just the solar water heater all year. It turns out that using solar power alone to heat water in the cold, often cloudy wintertime requires a more complicated, costly system. Cheaper simpler low technology devices can provide year round hot water if your winters are exceptionally mild; in colder climates, these sun-powered heat exchangers nicely complement the wood systems or even fossil fuels. In reality, there is very little added complexity involved in using more than one energy source to heat your water.

Two complementary systems often provide more benefits than a single uniform system. The optimum solution to the energy shortage involves people in different locations using the power sources most available and appropriate to their situations. Thus, people in the southwest can use passive solar heating while New Englanders can rely on wood burning for warmth. Ignoring peopleís capacity for the common sense to use locally appropriate energy sources, energy experts argue that solar heating isnít practical since it wonít heat homes everywhere, that therefore we need nuclear power stations. These arguments fly in the face of reason as well as natureís law that the ability to diversify and adapt to local conditions is the key to biological well-being and longevity. In contrast, the concentration of efforts and abilities in only one direction yields only temporary advantages which are doomed to extinction when inevitable changes occur (such as depletion of energy sources or accumulation of toxic wastes).

Controlling convection

The basic principle underlying the operation of both conventional and alternate energy water heaters is that hot water rises. An ordinary water heater is nothing more than an insulated storage tank sitting over a gas or electric burner (see figure 1). Since hot water rises as it is heated, cold water is piped in at the bottom of the tank while hot water is drawn off the top.

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Now letís look at a typical alternate energy water heater. Figure 2 shows an alternate energy heat exchanger connected to a storage tank. It doesnít matter if the heat exchanger is a solar collector or a grid of pipes in a fireplace or a stovepipe coil on top of a wood stove; the fundamental principles are the same for all three. Cold water from your water supply enters the bottom of the tank at "A." As in conventional heaters, hot water leaves the top of the tank at "B." The water heating circuit takes the cold water from the bottom of the tank, passes it through the heat exchanger and discharges the newly heated water at the top of the tank. Many systems use thermal convection Ė the process of hot water rising and cold falling in a container Ė to move the water in this circuit through the heat exchanger. In thermal convection or thermosiphon systems, you can have a complete hot water system without any electricity since the power from the sun or the burning wood provides the energy for both heating the water and circulating it between the heat exchanger and storage tank.

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In Figure 2, cold water leaves the tank at "C," travels upward through the coil as it is heated and continues into the storage tank at "D." Hot water rising through the coil draws cold water from the bottom of the tank (which is in turn heated in the coil), it rises, drawing more cold water behind it. This circulating motion Ė taking cold water from the bottom of the tank and discharging hot water at the top of the tank Ė fills the storage tank with hot water.

By properly insulating your storage tank, the water remains hot long after the fire has died out. In our experience, the water will stay hot in a well-insulated tank for 48 hours or longer.

Finding a tank

There are a variety of sources for an appropriate storage tank. But the most obvious candidate is your present water heater. In fact, you can use your operating water heater retaining its power supply as a backup system or you can totally unplug or de-gas the tank and use your water heater simply for storage.

If you donít have a water heater, itís easy to obtain a storage tank. Most discarded water heaters are abandoned because of breakdowns in the thermostat or heating element. These heaters are 

perfect for your purposes. Youíll want a tank that has no leaks and isnít too badly rusted. You can find one of these storage tanks at the local dump, the power company or in abandoned houses. Local plumbers handle quite a few broken water heaters and can probably get you a good one for $5 or a basket of snow peas.

For the thermosiphon to work circulating the water from the heating device to the storage tank, the tank must be located above the heat exchanger. Remember that we are depending on the tendency of hot water to rise to provide the power for circulating the water through the water heating circuit. In Figure 3, note that opening "X" is above or level with opening "2"; opening "Y" is above opening "W". Opening "Y" must be above "W" because it is the rising column of hot water in the pipe that forces the circulation of water through the system and replaces cold water in the tank with hot water. The higher "Y" is above "W," the better.

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In deciding where to place the storage tank horizontally relative to the heat exchanger, remember that the higher the rank is above the heat exchanger, the further you can move the tank horizontally away from the heat exchanger. A convenient (but not entirely accurate) formula to use is that you can move the tank two feet horizontally for every foot that "Y" is above "W." Figure 4 shows various ways that you can locate your storage tank relative to a fireplace or woodstove heating coil. The tank can be placed in a loft, in an adjoining room or even outside the house; Each gallon of water weighs about eight pounds. Thus, the stand for a 30 gallon tank should be able to support at least the weight of two people.

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Some people already have water heaters in their basement and do not find it either psychologically or physically possible to change their plumbing so the water heater or storage tank is above their alternate energy heat exchanger. The easiest way to surmount this difficulty is to use a pump to circulate the water from the water heater through the heat exchanger.

The second way to keep an operating water heater below the heat exchanger and still enjoy many of the financial and energy savings of alternate energy is to use the heat exchanger as a preheater for the current water heater. This method, shown in Figure 5, involves setting up a thermosiphon storage tank and feeding the hot water from it into the cold water inlet of your water heater. A single tank system is probably better in general, but the preheating method might be the better system if you want to keep your water heater operating in the basement and donít want to use a pump to circulate the water between the heat exchanger and the water heater.

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Piping hot

Letís next examine the water heating circuit connecting the heat exchanger to the storage tank. Use the former drain opening for piping cold water from the bottom of the tank into the heat exchanger. This is quite simple. A more complicated method is piping the hot water from the heat exchanger into the top of the tank. In the good old days, when many people used pipes in the firebox of their wood cookstoves for heating water, they had tanks with upper side openings (see Figure 6). These tanks are still readily available, but if you canít find one and are a competent welder (or have access to the services of one), you can burn a hole in your storage tank and weld in a 3/4" iron pipe coupling along the side about 3" down from the top and in a straight line above the drain opening. If your tank is galvanized, have good ventilation when welding and avoid breathing the poisonous gas formed by heating galvanized metal to a high temperature.

 

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There is another way to pipe the hot water from the heat exchanger into the top of the tank. Notice that your tank has several threaded openings in the top, through which you may be tempted to pipe the hot water from the heat exchanger directly into the top of the tank. As shown in Figure 7, this method wonít work, because any air in the system will collect and get trapped in the area of pipe indicated by cross-hatching. This trapped air will impair the heat driven circulation of water through the heat exchanger.

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Antisiphon devices are available to prevent cold water from the bottom of the tank from bypassing the tank through the heating coil and diluting the hot water at the pipe above the tank. To keep air from building up in the pipes during long intervals between draws at the tap, itís best to install an automatic air vent at the top of the thermosiphon loop (Figure 8).

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The drawings show only one heat exchanger connected to the storage tank, though a combination of wood and solar water heating provides a more complete alternate energy water heating system for most people. All you need to do is connect the second heat exchanger via two teeís into the pipes connecting the first heat exchanger to the storage tank (Figure 9). Be sure to install a pressure relief valve, as shown, for safety when the collector heats with other valves closed. Figure 10 shows. a cutaway drawing of a house with a rooftop solar collector and a stovepipe coil feeding into a storage tank in the loft.

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Wrap it up

If youíre using both wood and sun, you can expect to meet all your hot water needs without a gas or electric backup. Still, youíll probably want to have the gas or electric available if you use very large amounts of hot water or if you can take advantage of only the sun or only wood water heating, so donít disconnect them altogether.

No matter how you heat your water, be sure to insulate the tank well. The entire tank can be encased in 4" or 6" thick fiberglass insulation. This insulation comes in 16" wide rolls and can be bought at your local building supply or lumber yard. To hold the fiberglass in place, wrap the whole insulated tank in non-combustible material and stitch up the seams. If your tank is going to be exposed to the weather, an additional covering of plastic (keep it away from the gas flue) is needed. If you use gas for heating your water, you must leave adequate space for air to reach the gas burner at the bottom and for the hot gases to escape through the vented flue opening at the top.

Instead of using fiberglass, you can insulate your tank by building a box around it and filling this box with natural materials that will create air traps. Some suggested materials are wood bark, sawdust, chicken feathers, wool, rags, egg cartons, wood shavings, etc. But if youíre using gas, avoid flammable materials.

Insulation will also help maintain the temperature of the water. If you want hotter water, insulate the pipes between the storage tank arid the heating element (you should do this for most solar collectors though it is often not necessary for wood fire heat exchangers). If the distance from your storage tank to the hot water faucet is long, you may wish to insulate those pipes.

These alternate energy-water heaters, like conventional water heaters, use a temperature and pressure relief valve as a principal safety device. This valve opens when the temperature or pressure inside the tank exceeds a preset level, and releases the overheated water. The released water should be piped to a safe place. Itís wise to limit the size of the heat exchanger to minimize overheating the water while producing an adequate supply of hot water. You can program the amount of hot water produced in your system by varying the size of the heat exchanger, the size of the storage tank and the amount of insulation.

Whatís best?

Letís look at wood fire heat exchangers in more detail. People burn wood in many different ways. For example, you can use fireplaces, cast iron box stoves, cook-stoves, airtight stoves with internal baffles, fireplace stoves, barrel conversion stoves, potbelly stoves, thermostat controlled air-tight stoves with sheet metal walls, and thick steel airtight stoves with internal firebricks. These woodburners have a variety of relative advantages and disadvantages that determine which would be most ideal for your situation. A variety of heat exchangers is available, and the one to choose depends primarily on the kind of wood heater you have.

A pretty but inefficient place to burn wood is a fireplace. The inefficiency results from the fact that most of the heat escapes up the chimney. In addition, warm air from the room is sucked by the fire through the chimney and is replaced by colder air from the outside. Still, the radiant heat from the fire warms the immediately adjacent area sufficiently for some situations.

The heat exchanger that works best with fireplaces is a grid of black iron pipe, arranged to get maximum exposure of the pipes to the flame. Iron pipes should be used here; theyíre sturdier than copper pipes and therefore more resistant to the rough handling that a fireplace heat exchanger will receive.

The woodstove attains a higher wood burning efficiency than the fireplace because of its ability to control the amount of air that enters the firebox. Many different airtight stoves are available that enable one to totally shut off the air supply of the fire. In these stoves, wood can burn over a much longer period of time and overnight fires can be the rule rather than the exception. These stoves generally use the wood more efficiently because of this enhanced control of the air supply and due to internal baffles, secondary combustion chambers or other design features that aim to burn all the combustible materials in the smoke before it can exit from the stovepipe.

Watch your stovepipe

Airtight stoves have cooler stovepipe smoke because they often burn wood more slowly, as when you load your stove and close the air inlet. When the smoke going up the chimney is cool, creosote condenses on the interior of the pipe. At higher temperatures, the creosote-causing compounds are burned or remain as a gas and escape up the chimney. But the reality is that many airtight stoves, when operated in a long-burn mode with limited combustion air, tend to accumulate creosote.

The cure for creosote accumulation with all stoves is to regularly inspect the stovepipe. You will soon learn how often you have to check its condition. For masonry chimneys, the art of the chimney sweep is reviving. The chimney or stove-pipe can be cleaned manually with brushes or with a piece of cable lowered through the stovepipe. Or, you can use commercially available powdered salts which are sprinkled on hot roaring fires in weekly treatments. These salts help burn up the accumulated creosote (by lowering its ignition temperature) to prevent it from reaching a critical mass where it could burn out of control in your stovepipe.

We have explored the world of the stovepipe in detail because one of the simplest woodstove heat exchangers involves placing a coil of copper pipe within the first section of stovepipe above the stove. Since this heat exchanger extracts heat from the flue gases for water heating purposes, it cools the stovepipe smoke. If you use too large a heat exchanger or if your stovepipe smoke already tends to be cool, this cooling of the smoke will cause unacceptable accumulation of creosote. That is why stovepipe water heating coils are not used with airtight stoves.

Figure 11 shows different woodburning stoves and the heat exchangers recommended for various situations. Basically, the stovepipe coil is recommended for non-airtight stoves like the cast iron box stove and the potbelly stove. It is not recommended for cookstoves since these have cool stovepipe smoke due to their small fireboxes and the dissipation of most of the flue heat to the oven and cooking surfaces.

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With airtight stoves and cookstoves, place the heat exchanger directly in the firebox since the stovepipe smoke is often too cool. You can use either the flat firebox grate of iron pipe (as in a fireplace) or a coil of copper pipe shaped to fit your stave. In either case, the heat exchanger should be exposed to a maximum amount of direct flame but not interfere with putting wood inside the stove. This usually involves placing the heat exchanger in the upper back of the stove or along a side wall.

The previous was an excerpt from the book,  Handmade Hot Water Systems

Home Energy Digest & Wood Burning Quarterly, Summer 1980, Pages 161-176

This material provided under "Fair Use" guidelines.

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