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A wood- or coal-burning furnace can be grafted onto the ducts of an existing forced-air system in one of three different configurations. For the sake of brevity, only the addition of a wood-burning unit will be described on these and the following pages. But the procedures for adding on a coal-burning furnace would be identical. In a downstream installation, cool air returning from the rooms of a house (re turn air) passes through the old furnace before entering the wood furnace. In a parallel installation, return air is diverted from its course to the existing furnace and routed to the wood burner. An up stream installation has return air passing through the wood furnace before it reaches the old heating plant. The upstream installation is not recommended since air heated by the wood tire can cause the blower motor of the old furnace to overheat. The parallel installation also has a drawback: It requires an additional blower for the wood furnace and, thus, an additional expense. But the shape and area around the existing heating plant may prevent a successful down stream installation and the parallel con figuration will be your only choice. Also, some downstream installations need an additional blower themselves. Enlist the aid of a professional forced- air heating engineer in planning the sys tem for your house. Unfortunately, an add-on furnace increases air resistance through the entire system. This puts an extra burden on the blower of the existing furnace and changes the rate at which air moves past the fireboxes to be warmed. Only a qualified technician can properly assess your existing system to determine if its ducts and blower are adequate for an add-on wood furnace. The heating engineer can also tell you what complications your air-conditioning equipment may pose to an expanded heating system. Determining the size of the add-on furnace is also an important step. Even though on some days the wood burner may heat the house by itself, it does not require as high a BTU rating as the old furnace does. For one thing, the wood- burning furnace will always have a dependable backup; for another, most oil and gas furnaces have greater heating capacity than is actually necessary, as a measure of insurance against severe cold spells. Ideally, the wood-burning furnace should have a BTU rating slightly less than the BTU loss from the house on the coldest days of the year—a larger furnace would run at less than capacity on most days and permit too much creosote build-up in the flue. In selecting a wood burner to suit your needs, be wary of salesmen’s claims. The salesman’s estimation of furnace capacity will probably be based on the manufacturers’ BTU figures, which are calculated under ideal laboratory conditions. To make your own estimation of a wood burner’s capacity, use the formulas in the box at right. An expanded heating system can incorporate varying degrees of automation. A simple downstream system can function with a single, manually operated duct damper and a coiled bimetal thermostatic strip. The damper diverts return air to the wood-burning furnace; the bimetal strip reacts to heat above the wood fire and raises or lowers a smaller damper that controls combustion. A more complicated installation would include a thermostat in the heated portion of the house, an electric motor in place of the bimetal strip and a sensor that turns on the blower when the air above the firebox reaches a predetermined temperature. A parallel installation also requires specially balanced duct dampers that open and close automatically, depending on which heating appliance is operating at a given moment. Estimating the Usable--Output of a Wood Furnace Because it’s important to match the size of a wood furnace to the needs of your house, it’s advisable to take a hard look at manufacturers’ claims of BTI output. To calculate how many BTUs a furnace can deliver per hour, begin by measuring the usable volume of the firebox. In doing so, include only the parts of the firebox where logs can reasonably fit. You cannot, for example, usually place logs above the height of the firebox door. Multiply the volume of the firebox by the density of the firewood. If you plan to use medium-sized pieces and pack them in with moderate tightness, use a factor of 15 pounds per cubic foot for softwood, 18 for hardwood. If you use small pieces and pack them very tightly, use a factor as high as 22. Next, multiply the result by 6,800, the average BTU content of 1 pound of seasoned wood. Multiply that figure by an efficiency factor of .55. This will give you a good estimate of the BTUs one charge of wood will deliver to your house. Divide the result by a reasonable estimate of how long a full charge of wood will burn—usually six to eight hours. This final figure represents the usable BTU output of the furnace during one hour of operation. By comparing that calculation to an estimate of the number of BTUs it takes to heat your house for one hour, you can determine whether a furnace you are considering will be the optimum size for your house. [100] Two expanded heating systems. The forced-air heating system shown below, left, has a wood-burning furnace that has been added on downstream from the original furnace. A manually operated damper in the plenum of the old furnace controls the flow of air. When a wood fire is burning, the damper is closed. Thus, return air is forced through the original furnace—which is not running—and is channeled into the wood furnace. There the air is heated and blown into the supply ducts for the house, in this example, the blower in the original furnace is adequate to move air through the entire system. The blower runs continuously during the time that a wood fire burns, but can be switched to an automatic setting that turns it on and off while the old furnace is operating. The heating system shown below, right, is a parallel configuration in which both furnaces have their own blower units. Return air can thus be channeled through either furnace without passing through the other one. Arrows indicate the path of the air while a wood fire is burning. When the wood fire goes out or cannot maintain adequate warmth in the house, a thermostat turns on the original furnace. Two carefully counter balanced dampers control the flow of air through the system. The dampers automatically open or shut, depending on which blower unit is running. Linking Furnaces with Dampers and Ducts 1. Cutting into the plenum. Mark the plenum of the existing furnace for three rectangular holes—two for the ducting that will tie in the new wood burner and the third for the manual damper that you have acquired for your new system. Make the duct holes the same dimensions as the connecting pieces, called take offs; make the damper hole just large enough to permit the damper to pass through. Pierce the metal inside a marked outline by holding the tip of an old screwdriver or a cold chisel against the sheet metal and striking the shank with a hammer. Make a hole large enough to admit the tips of a pair of tin snips, then cut along the marked lines. Cut out the other openings in the same manner. MANUAL DAMPER; FLUE; PLENUM; DOWNSTREAM SYSTEM; PARALLEL SYSTEM; 101 2. Installing the damper. Either buy or make a damper that will fit snugly inside the plenum. The shanks on which the damper will swivel have pronged clamps at one end that fasten them to the edges of the damper plate. On the side of the plenum opposite the damper opening, drill a hole just larger than the damper shanks. Pass the damper into the plenum through its opening, then insert one of the shanks in the drilled hole and fasten it with a wing nut. Cut a rectangular piece of sheet metal slightly larger than the damper opening and, at its center, drill a hole for the other shank of the damper. After fitting that shank into the hole, use sheet-metal screws to fasten the new plate to the plenum walls, covering the damper opening. Slide the damper handle onto the square of the shank and tighten a wing nut on that side. 3. Adding ducts. Connect the add-on furnace to the existing furnace with new ducts consisting of take-offs, elbows and straight sections of sheet-metal ducting. Screw the take-offs in place over the openings cut in Step 1 and over the holes manufactured in the jacket of the wood- burning furnace. Add elbows to the take-offs and straight duct sections between the elbows, joining the pieces with S-slips and drive cleats. When the new ducts are in place, have a professional heating engineer test the system to determine whether the rate of air flow meets the specifications of both furnaces. 4 Attaching the furnace controls. Screw the control box for the combustion damper to the body of the furnace through the opening in the jacket directly above the damper. Attach the chain that comes with the control box to the damper door. A bimetal strip inside the control box expands and contracts with heat, thus raising and lowering the chain and controlling the flow of air into the fire. Install a flue as de scribed opposite. [102 ] Building the Flue 1 Installing the chimney-pipe support. At the first floor through which the flue must pass, install a chimney-pipe support to bear the weight of insulated chimney pipes. To determine where to position the support, mark the basement ceiling—between the joists—at a point either directly over the center of the flue cutout atop the furnace or offset to allow for bends. Cut through the ceiling and floor. Then, working from the floor above, build a frame with lumber the same size as the joists. Nail two frame pieces perpendicular to the joists, and nail two flat against the joists. From the basement, raise the chimney-pipe support into the opening and check it for fit—the cylinder of the support should touch the frame on all four sides. With a helper holding the bottom of the support against the basement ceiling, check the top with a level and then nail through the cylinder into each of the four frame pieces. At each floor above the first, use a plumb bob to mark centers for the openings through which the flue will pass. Cut the openings and install the metal spacers supplied by the flue manufacturer. The spacers ensure the proper clearance between the flue and the wood framing of the house. To mark the location for a hole through the roof, hang a plumb bob over each of the four corners of the opening below. Drill a small hole through the roof at the four corner points. Then, from atop the roof, cut out the shingles and sheathing between the marker holes. 3. Completing the flue. Center the flashing-the metal fixture that supports the flue above the roof-over the roof opening and trace its out line on the shingles. Remove the flashing and draw a second outline 3 inches inside the first, across the top and two thirds of the way down the sides. Cut through the shingles touched by the inner outline and remove the cut pieces around the roof opening. Spread roofing cement on the shingles in the lower third of the area bounded by the large outline, then move the flashing back into place, sliding it up under the edges of the cut shingles. Press the lower edge of the flashing into the roofing cement and nail through the flashing into the roof with galvanized nails at 3-inch intervals. Cover the nail heads with roofing cement. Assemble sections of chimney pipe to build up the flue from the chimney-pipe support to the opening in the roof. Raise the top of the flue through the flashing. If it’s required by your local building codes, enclose the chimney pipe with framing and wallboard. Slip a storm collar over the chimney pipe so that it rests on the flashing. Seal the joint between the collar and the pipe with roofing cement. Add sections of chimney pipe until the pipe rises 3 feet above the high side of the roof where it exits, or 2 feet above any surface within a horizontal distance of 10 feet. Add a chimney cap to finish the installation. [103 ] Electric Controls for a Wood Furnace Electric controls can automate a two- furnace heating system and make it much more convenient to use. However, the degree to which automation is possible will vary from system to system. In the heating system shown below, for example, a wood-burning furnace has been added downstream from the original oil burner; the system functions with a single blower, often called “fan” on controls. This equipment can be improved by the addition of a thermostat that will make the wood burner responsive to temperatures in the living quarters. But even with this addition you will still need to manually close the duct damper to the wood furnace when the fire dies. A system with two blowers, on the other hand, can be fully automated—except, of course, for adding firewood. The two- blower system will have counterbalanced dampers that are opened and closed by the force of the blowers and thus save trips to the basement. The controls for the heating system will have similar components, no matter what the configuration of the furnaces. Fan switches, linked to bimetal sensors in plenums, turn the blowers on and off at predetermined high and low temperatures. Limit switches, usually connected to the same sensors as the fan switches, are safety devices used to prevent abnormally high plenum-air temperatures. If the temperature rises past a preset point, the limit switch cuts off power to the gas- or oil-furnace burner. The thermostat for an add-on wood burner will control a motor that adjusts the combustion damper. But the damper is also adjusted by a heat-sensing switch, similar to the fan and limit switches, that keeps the fire burning at a steady rate. The thermostat is therefore wired through the heat-sensing switch. Also, since the original furnace in your heating system will have a thermostat of its own, the new thermostat will have to be set 40 higher than the old one to keep the old furnace inactive while a wood fire is producing sufficient heat. The wiring for these controls involves both low- and high-voltage circuitry. Thus the electrical system must incorporate transformers that reduce the in coming 120 volts to 24 volts to supply the low-voltage circuits. You should be certain your wood-burner combustion damper will close automatically in the event of a power failure. All of the wiring must be done in accordance with local codes and is subject to inspection once the job is completed. The wiring techniques depicted here will be applicable to most add-on wood furnaces. But the particular heating system shown here is merely an example, and the configuration of the various electrical components may be different from the ones needed for your system. Follow the manufacturer’s wiring diagrams to install controls on your own equipment. 1. Installing the hardware. Slip the rods that contain the bimetal sensors for the damper control, the fan switch and the limit switch into the holes already provided in the jacket of the wood-burning furnace. Attach the body of the control assembly with screws. Then run the wires that are connected to the control assembly to the damper motor by attaching the mounting brackets on the wires to the jacket of the furnace. Run low-voltage wires to the new thermostat lo cation, adjacent to the thermostat of the existing furnace. Mount the base of the new thermostat on the wall, over the ends of the low voltage wires. Use a small level to make certain that the thermostat base is absolutely level before tightening the mounting screws. [104] 2. Adding flexible conduit. Cut a length of flexible conduit—sometimes called Greenfield— to extend between the control assembly on the wood furnace and the junction box of the original furnace. Connect the conduit to the two boxes with connectors and lock nuts. Mapping Out the Circuits Wiring the controls. After feeding No. 14 or 12 wire through the conduit installed in Step 2, use wire caps to make the connections shown in this schematic diagram. Wires of the original furnace that are not affected by new circuitry are not shown. The objects of the new wiring are threefold: to activate the motor on the combustion damper; to allow the oil-burner motor to operate only when both limit switches are closed; and to run the blower mo tor when either of its fan switches closes. FAN CONTROL OIL BURNER FAN SWITCH JUNCTION BOX BLOWER MOTOR MOTOR 105 |