f^cP / £/£/?* & /S£.? <5 Heating Household Water 50C With A Wood Stove Water heating is the second stovepipe, or on the outside of the Savings from wood 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 heat 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 heat exchanger 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 heating system 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 thermostat 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 Flue-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 storage tank 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 heat pump 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 ISPressure/temperature relief valve Thermometer Tempering information: valve Hot PUBLICATIONcurrent most THIS For http://extension.oregonstate.edu/catalog Drain Figure 2. Thermosiphon single-tank system Air pockets Bottom of tank below stove DATE. Figure 3. Thermosiphon PROBLEM AREAS OF OUT Normally open Normally closed Pressure/temperature Thermometer relief valve. IS Cold information: Hot Tempering valve Electric heating elements PUBLICATIONcurrent most THIS For http://extension.oregonstate.edu/catalogDrain 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, fan 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.