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Water heating is the second stovepipe, or on the outside of the Savings from stove largest energy drain for most stove. Water circulates through the water heating households. About half the exchanger whenever a fire is burning, energy consumed for everything other either naturally (thermosiphon), which The typical Pacific Northwest family than space heating goes to water. relies on water rising as it's heated, or of four uses 4,800 to 7,200 kilowatt- If you have a wood stove, however, by means of a pump. hours (kWh) of electricity each year to you may be able to use it to heat Heat exchangers come in three heat water. Annual water heating costs household water and thus trim your forms: a serpentine coil of tubing, a average aboutDATE. $270, but may range energy bill. boxlike minitank, or a small solar from $75 to $400. collector-type absorber plate (see fig- Savings from a wood stove water ure 1). Most heat exchangers are coils heater depend on how much the family How does a wood stove or minitanks mounted inside the stove usesOF its stove, the size water heater work? body. and location, and how much hot water Wood stove water heaters use heat the family uses. Most families heat exchangers to transfer heat from the water with wood only during the winter stove to the water. Exchangers can be months. Savings vary from about $25 to $200 per year. mounted inside the stove, in the OUT These savings, however, are offset by the cost of the wood stove water and the value of the IS additional wood burned. A wood stove consumes about 10 percent more fuel when a water heating device is attached. Wood stove water heating systems vary widely in cost, depending on the components used. Heat exchanger prices range from $50 for a simple information:black iron coil to $150 for a stainless steel unit. If you need a pump and controller, add at least $200 more. A commercially installed system with pump usually costs about $700. Be sure to consider other factors, including safety requirements and the environmental effects of wood burning current (air pollution), when deciding on PUBLICATION whether to install a wood stove water heater. most THIS EC 1164 • January 1984 For http://extension.oregonstate.edu/catalog

OREGON STATE UNIVERSITY EXTENSION SERVICE effective when mounted outside the You can lower your water heating bills, whether or not you use your stove. External heat exchangers have wood stove to heat water, by doing the following: large, flat panels similar to solar absorber plates. • Install low flow showerheads. Since the amount of water heated is • Add an insulating wrap to your water tank. determined by use of the stove, the • Set the on your water heater to 120oF or lower if family optimum heat exchanger size depends use allows. (Some dishwashers require 140oF minimum.) on three things: • Fire size Safety precautions -type (stovepipe) exchangers are • Fire frequency Many people have been injured by not used in air-tight stoves because • Household water use. poorly installed wood stove water they depend on high exhaust gas heaters in recent years. Any pressur- temperatures to be effective. For Typical installations use coils of Yi ized water heating device can explode if Franklin-type stoves which are not inch or V* inch diameter, 8 to 12 feet in a control failure results in high air-tight, stovepipe exchangers may be total length, mounted in the firebox (50 temperatures and excessive steam a good choice. The exchanger is percent longer if located in the pressure. Houses have been destroyed composed of an oversized piece of stovepipe).DATE. Minitanks usually have 70 when conventional electric or gas stovepipe surrounding a cylindrical coil to 200 square inches of surface area heaters exploded. through which water circulates. To exposed to the fire. External absorber Like all water heaters, the wood- make your own, bend soft copper plate exchangers are often 250 square fired water heater requires safety tubing to the desired coil diameter by OFinches or larger and are sized to cover precautions to prevent dangerous filling it with dry sand to prevent the back of the stove. pressure buildup. A properly installed kinking, and then winding it around an • Interpretation of the Building wood water heater can be operated appropriate size cylinder. Creosote Code and Mechanical Code by officials with no danger to the household. buildup can be a problem with in some areas does not permit inter- Hundreds are now in routine use in all flue-type exchangers, so check yours nally mounted exchangers, so check to parts of Oregon. frequently. OUT see if they're allowed in your area. Before you begin working on your Externally mounted heat exchangers Remember that holes cut in the stove wood stove, obtain the appropriate eliminate the need to make holes in the may void the manufacturer's warranty permits from your Building Depart- stove body. The exchangerIS intercepts and laboratory listing. Check with ment. Follow the instructions given radiant energy from the stove body, so your insurance company to see if with plumbing details in this publica- its surface area is very important. modifying the stove affects your tion to ensure safe installation; then Normal coils and minitanks do not homeowner's coverage. have a qualified inspector check for receive enough radiant energy to be safety hazards. information: Materials and types of exchangers Exchangers are commercially made, built in local shops, or homemade. They can be made from copper, galvanized iron, or stainless steel. However, copper is not a good choice if there is a chance it will be jarred orcurrent damaged by hastilyPUBLICATION loaded firewood; your water may look and taste rusty if you use steel or black iron; and galvanized metals release a toxic gas at high temperatures. Therefore,most even though it's the most expensive, stain- lessTHIS steel is the best material for internally mounted heat exchangers. Because of the extremely high temperaturesFor possible inside the stove, 1 be sure your heat exchanger has no T^^ mechanical jointshttp://extension.oregonstate.edu/catalog inside the stove body unless they are welded—not soldered or silver soldered. Figure 1. Woodstove heat exchangers Circulation pockets. An air vent like that shown in need to install a valve in the hot water To prevent boiling, water must figure 2 will eliminate air pockets, but line (see Plumbing for Safety). circulate through the heat exchanger must be located where any dripping Thermosiphon systems normally use whenever the stove is used. Use from the vent will not cause problems. V* inch or larger water line to minimize thermosiphon circulation if your water You could connect the heat ex- flow resistance. As a general rule, the is located near the stove changer return line with a tee (T) to the horizontal distance from the stove to and is higher than the heat exchanger. hot water supply line to the house, but the tank should be no more than twice You will need to purchase a circulating cool water would then be drawn the vertical distance from the for all other cases. Thermosi- through the stove whenever a tap was exchanger outlet to the top of the tank. phon systems are better than pumped turned on, even when no fire was Check valves will prevent flow and systems because a power outage which burning. It is better to connect the hot should not be used in a thermosiphon stops pump circulation could result in water from the stove through a circulation loop. Make sure the bottom overheating of the water in the heat different tank opening than that used of the tank is above the heat exchanger exchanger. to supply the house. so that no "back-siphoning" can occur Plan to direct the exchanger outlet when there is no fire, and so that all of Thermosiphon system line upwards immediately behind the the water in the tank can be heated. Figures 2 and 4 show typical stove. This helps initiate water flow DATE. thermosiphon plumbing details. As when you start a new fire. water is heated, air dissolved in it is If you ever need to remove the stove, released as bubbles. Pockets of air in you'll find it easier if you have a valve the piping prevent circulation and must on the cold circulation line. Before OF be avoided. Figure 3 shows common stove removal, drain water to the level problem areas. A water tank with an of the hot return connection on the opening in the side, as shown in figure tank to prevent flow back through the 4, is the best way to prevent air line. By doing this, you'll eliminateOUT the

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Figure 4. Thermosiphon double-tank system Pumped system Figure S shows plumbing details for The pump must have a control to You'll need a pump to circulate a typical pumped system. A check switch it off when there is no fire. Snap water through the stove loop if you valve prevents hot water from rising to disc switches, controls, and can't position the tank where required the stove from the tank when there is aquastats have been used with only for thermosiphon flow. A pump will no fire and the pump is off. limited success. A better, but more push water past air pockets and If a power or pump failure occurs, expensive choice is a solar propor- through any length of pipe. water in the heat exchanger will quickly tional differential controller. It electroni- Pumped systems require little power boil. A pressure/temperature relief cally compares the temperatures of to maintain flow. Circulation pumps valve at the heat exchanger outlet will sensors at the tank and exchanger are usually smaller than 1/20 horse- vent off steam as long as there is water outlet and adjusts pump speed accord- power and draw little electrical current, pressure to continue filling the ex- ingly. usually less than 150 watts. In open changer with water. You can also use a An on-off differential controller can systems where fresh water is routinely water level limit control with manual be used but will result in pump cycling introduced, submerged pump compo- reset to prevent pump restart until the during medium and low fires. Intermit- nents must be stainless steel or stove cools, thus eliminating any tent boiling noises sometimes occur bronze. Don't use cast iron pumps; they chance of heat exchanger damage or a because of system cycling. won't be guaranteed by the manufac- steam flash when the heat exchanger turer for this use. refills. DATE. OF

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most Drain Back-up tank THIS Sensor Figures. PumpedFor double-tank system http://extension.oregonstate.edu/catalog Single or double tank? tank has reached a certain temperature. can signal it to operate as a backup at You might want to add an extra tank The valve allows the wood-heated times when household hot water use is to store wood-heated water (figures 4 water to offset heat losses from the heavy. The tank at other times can backup tank. Heat losses from the store lower temperature water that is and 5). A conventional water heater water tank to its surroundings often being heated by the wood stove. It is will heat the water from the storage account for 20 percent of the energy a better, however, to use a larger tank tank to final desired temperature. water heater uses. Temperature in the storage tank can which can store a full day's supply of o o water. vary from 50 F to 180 F. A timer attached to a small single tank In some cases a single tank ac- complishes both functions. Don't use your wood stove to heat water for an electric tank with both elements operating (or a gas water heater) because you can only overheat the Plumbing for safety extra long stem which can reach water to a temperature higher than the through the tee into the tank (see figure thermostat set point. Instead, you A standard pressure/temperature 2). should disconnect the lower element of relief valve on each tank releases water PrepareDATE. for the possibility of the an electric water heater to allow the or steam if for any reason the pressure tank overheating. Direct all relief valve bottom half of the tank to store or temperature exceeds preset limits, outlets to a safe area of good drainage wood-heated water at intermediate, usually 125 pounds per square inch or where the line cannot freeze, such as a fluctuating temperatures.When you 210oF. A pressure relief valve at the OFfloor drain away from people or a use the upper element only, however, stove will provide protection against crawl space. the amount of electrically heated water high pressures which could result from Use a tempering or mixing valve on on standby will be limited to that obstructions in the lines between stove all systems. The valve mixes hot and quantity stored above the upper cold water when necessary to insure and tank. This relief valve must be that water comes out of the faucet no element; therefore, you may want to installed on the stove side of any use a larger tank. You should find OUT hotter than a chosen temperature. In shut-off valves if such valves are greater energy savings with a single some plumbing arrangements, a check tank system, but you may also have to installed on both circulation lines. valve may be required on the cold water line which supplies the tempering cope with occasional delays while the The stem on theIS pressure/temperature backup recovers. valve so that no backflow of hot water valve senses water temperature. It can occur. As shown in the illustrations, In a single-tank system, it's impor- should extend into the storage tank. If tant that water from the wood stove the tempering valve should be mounted not return to the top of the tank at a a tee (T) is used at the tank outlet, below the top of the tank to prolong lower temperature than the backup purchase a special relief valve with an thermostat life. thermostat setting because that would turn on the backup power. The flow information: control valve shown in figure 2 can reduce flow in a thermosiphon system so that the heat exchanger will raise the Safety checklist water temperature more. In a pumped • Provide complete water circulation loops in all system with a single tank, the water installations from the stove should be returned • Install pressure/temperature relief valves on all through a dip tube to slightly below the tanks. upper electric element. current • Install at least one pressure relief valve no further You can purchasePUBLICATION a thermostatic than 6 inches from the heat exchanger at the stove. valve for two-tank systems which will direct water from the stove to the • If shut-off valves are installed in the circulation backup tank after water in the storage loop, remove the handles to prevent inadvertent shut- off. most • Install a mixing valve between the hot water tank THIS and house faucets. • Never fill a hot heat exchanger with cold water. For http://extension.oregonstate.edu/catalog General tips • Use only threaded fittings within • Solar and wood water heaters • Insulate all hot water lines which 18 inches of the stove. If the rest of the work well together, each producing at plumbing in the system is copper, use are farther than three feet from the its best when the other is idle. A stove. You can insulate lines inside threaded brass pipe adjacent to the separate pump and controller should house walls economically by using stove. Copper used with iron will cause be used for each, because much of batt-type insulation when the the iron to corrode. spring and fall weather will require house is under construction. Elsewhere, • Use gate valves or full port ball operation of the two systems to use slip-on foam insulation designed valves instead of globe valves in overlap. for temperatures up to 250oF. thermosiphon circulation loops to • Plan to flush sediment out of the reduce flow resistance. heat exchanger every three to four • Thermometers cost $10 to $20 and • Although adding a heat exchanger months of operation. If you have hard are well worth the investment. They to your stove could increase the rate of water, flush it monthly. eliminate the guesswork of determining creosote formation in the chimney, the water temperature and let you watch opposite is more likely to occur. If a the system operate. Friends will want stove is regulated to maintain desired to see it work, too. Good thermometer , the heat taken by locations are shown in the illustrations. the exchanger will require the operator SourcesDATE. of equipment • Copper is usually the easiest pipe to run the stove with a hotter fire. This, Many of the items you'll need for material to work with. However, do of course, will actually reduce creosote building a wood stove water heater are not use soldered joints near the stove formation. available through plumbing supply where a heat exchanger filled with air • If you fully your loaded shopsOF and hardware stores. Specialty or steam could heat the joints above stove at bedtime, the addition of a heat items such as the heat exchanger, the solder softening temperature. exchanger will result in more creosote pump, and differential controller are Table 1 shows the relative melting accumulation. Plan to clean the available from wood stove stores and points of common soldering materials. chimney more frequently. solar dealers. OUT IS Table 1.—Melting points of common solder and brazing metals

1 Soft solders Hard (silver) solders Melting Completely Melting Completely Composition begins molten begins molten (% Tin/%Lead) (0F) (0F) information:Composition (0F) CF) 40/60 362 460 7% Phosphorous/93% Copper 1310 1460 50/50 362 418 6% Phosphorous/5% Silver/ 89% Copper 1190 1485 95/5 362 434 5% Phosphorous/15% Silver/ 80% Copper 1190 1475 95% Tin/5% Antimony 450 464 PUBLICATIONcurrent most THIS For http://extension.oregonstate.edu/catalog DATE. OF

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The OSU Extension energy program offers education, training, and technical assistance on conservation and renewable resources to help Oregonians solve energy-related problems. This publication was prepared by David McCowen, Extension energy agent, Descbutes County, Oregon State University, in cooperation with the State of Washington Energy Extension Service. It was prepared with the support of the U.S. Department of Energy (DOE) Grant No. currentDE-FG51-80CS69120. However, any opinions, findings, conclusions, or recommendations expressed PUBLICATIONherein are those of the author and do not necessarily reflect the views of DOE. Extension Service, Oregon State University, Corvallis, O. E. Smith, director. This publication was produced and distributed in furtherance of the Acts of Congress of May 8 and June 30,1914. Extension work is a cooperative program of Oregon State University, the U.S. Department of Agriculture, and Oregon counties. most Oregon State University Extension Service offers educational programs, activities, and materials without regard to race, color, national origin, or sex as required by Title VI of the Civil Rights Act of THIS 1964 and Title IX of the Education Amendments of 1972. Oregon State University Extension Service For is an Equal Opportunity Employer. http://extension.oregonstate.edu/catalog