Extension Bulletin 0947 Heating Domestic Water with Wood

Steve Denner and Edwin Valbert-Washington State University

If you use a considerable amount of wood to heat hot water they will produce. This will help in sizing your home, it may be possible to use your wood the heat exchanger. heating appliance for domestic water heating. Under the proper conditions, a wood water heating system Between the heat exchanger and the storage tank can be used with a new wood stove, as an add-on to an there is a system of piping and valves to transport the existing stove, or in a fireplace. A wood water heating water. Either copper or galvanized steel pipe is com­ system will not provide 100% of your annual hot monly used. It is best to try to avoid mixing types of water needs, but can provide a significant reduction pipe since different metals increase the likelihood of in your water heating costs. In a wood water heating corrosion. A number of valves should be added as system, water is circulated in or around the wood ap­ safety precautions. A temperature and pressure relief pliance to absorb the heat from the fire. The water is valve should be located at the top of the storage tank. then transported from the appliance to a water tank This valve will release when there is either excessive where it is stored until it is needed for household use. temperature or pressure in the tank. A pressure only At times the wood water heating system can heat relief valve should be located at the hot water outlet water to temperatures hot enough for household use. of the heat exchanger as a similar precaution. A mix­ In other situations, the system will only function to ing valve may also be necessary to insure against preheat cold water before it is transported to a tradi­ scalding from high water temperatures. The mixing tional hot water tank. For example, water main valve functions to moderate excessively hot water by temperatures of 45-50°F may be heated to 100°F by automatically mixing in cold water. the wood appliance. System Types: Open Loop or Closed Loop Heat Exchangers: Coil or Tank There are two common system types used for heating The heat exchanger is the component through which water with wood; open loop and closed loop systems. water passes to absorb the heat from the wood burn­ The open loop system actually heats the potable ing appliance. The heat exchanger for a fireplace household water itself and then transports it to a tank system is usually a specially designed circulating grate for storage. The term "open" refers to the fact that made of either stainless steel, threaded steel, or black fresh water is always being circulated through the pipe. For wood stoves the heat exchanger can be wood stove. The closed loop system circulates a fixed either a coil or tank type. Coil exchangers are placed amount of water through the system continuously. In inside the wood stove and are generally made of cop­ a closed loop, there are two heat exchangers. The first per or stainless steel. If copper pipe is used it must be heat exchanger is at the heat source. The second heat of sufficient wall thickness to withstand high exchanger is located in the storage tank and transfers pressure. Type K or L rigid copper pipe is sufficient. the heat to the potable water in the tank. In a closed In addition, copper coil heat exchangers should not loop system an expansion tank should be used to al­ have any soldered joints within the wood stove itself. low for the expansion of hot water without triggering Soldered joints could break down when exposed to a pressure relief valve. The choice of using an open or high firebox temperatures. Tank heat exchangers are closed loop system depends on cost, space availabili­ usually small, one-fourth inch plate steel tanks that ty, frequency of burning, and amount of hot water can be located either inside the wood stove or on an needed. The open loop, having only one heat ex­ outside wall. Tank heat exchangers should be baffled changer, is very fast at delivering hot water and is so that the tank is divided into a top and bottom sec­ very efficient. The closed system, having two heat ex­ tion. This forces the water to circulate around the baf­ changers, is slower to deliver hot water and slightly fle , allowing it to absorb more heat. Sizing either a coil less efficient. or tank heat exchanger depends on a variety of fac­ tors: 1) size of the wood stove, 2) amount of hot water Water Circulation Options: Natural needed, 3) storage capacity. Most commercially made Convection or Pumps heat exchangers come with estimates as to how much Two options are available for circulating water in a College of Agriculture Washington State University Pullman CooperativeExtension In cooperation with the United States Department of Agriculture. @ Issued in furtherance of the Acts of May 8 and June 30, 1914, by Washington State Cooperative Extension, J. 0. Young, Director wood water heating system: natural convection or them. Areas that experience periodic power outages pumps. Natural convection, or thermosiphon, works will probably want a system that is capable of func­ on the principle that water, as it is heated, becomes tioning without the aid of a pump. less dense and rises. In order for natural convection to be effective, the system must be properly designed. Safety Considerations The hot water storage must be located above the Working with hot water raises special precautions wood stove so that water heated in the wood stove that must be observed to insure safe performance of a can rise naturally. A rule of thumb is that for every 2 wood-heated hot water system. It is dangerous to horizontal feet the storage tank is placed away from have a dry heat exchanger with no water flowing the wood stove, the tank should be elevated 1 foot through it to carry away heat. Firebox temperatures above the stove. Pipe of 1 inch in diameter is recom­ can damage the heat exchanger in this condition. Fur­ mended for natural convection systems to minimize thermore, introducing cold water into a hot heat ex­ pipe friction. Minimizing turns in the pipe will also changer can create high steam pressures and the reduce flow restrictions in the system. In a natural possibility of explosion. The heat exchanger should convection system, the pipes leading from the wood be cold before water is introduced. Also, pressure stove heat exchanger to the storage tank should not problems can occur if water remains in the heat ex­ contain any downhill sections. The absence of down­ changer too long due to blockage or inadequate circu­ hill sections prevents the development of air pockets lation. If the water returning to the heat exchanger is which could block the flow of water through the too hot, pressure problems can develop. This prob­ system. lem can be reduced by installing a smaller heat ex­ changer or adding more storage capacity. Safety The second option is the use of an electric pump to valves may prevent serious problems but proper move water around the system. When using a pump, system design is the key to safe and successful wood the locations of the wood burning appliance and the water heating systems. To insure a safe operation, storage tank are not critical. Since the pump provides most localities will require you to obtain a plumbing more force than does natural convection, air blockage permit. is not as much of a concern. Smaller pipe diameters of 1/2 to 3/4 inch are commonly used. The pump must Suggested Reading be designed for the specific system. Size is determined Wood Heat Safety. Jay Shelton. Garden Way Publ., by the amount of water to be moved, the distance it Charlotte, VT. has to travel, and whether it is pumped uphill or Home Energy Digest. Winter 1978. "A Primer on down. These factors make up the head pressure Wood-Fired Water Heating," Frederic Langa. requirements of the pumps. Pumps are rated either by Solar Age. Sept. 1979. "The Solar-Wood Connection, horsepower or flow rate. The pump should be de­ Part I, " Joe Carter. signed for use with hot water. Some cold water Solar Age. Feb. 1980. "The Solar-Wood Connection, pumps can be damaged if hot water is run though Part II, " Larry Gay.

Published August 1981. Extension programs are available to all persons without discrimination.