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Passive thermosiphoning solar systems, which rely on the natural circulation that occurs as warm air rises and cool air moves in to replace it, are ideal for heating areas adjoining a south-facing wall. But if the rooms that need heat are on the north end of the house, an active solar heating system, such as the one described at right, provides an economical and versatile solution. Although active and passive collectors are similar in appearance, the two systems differ in several important aspects. The active system, instead of thermosiphoning air from the bottom to the top of the collector, uses a blower to pull air horizontally across the collector panels. To heat the large quantities of air in an active system, a larger collector surface area is required: The minimum recommended size is approximately 100 square feet, the maximum 200 square feet. Beyond 200 square feet, so much heat is generated that a special storage system is necessary to conserve it (box, --- 63). Finally, active systems use a network of ducts to route heated air through the house and return cold air to the collector for reheating. An active system offers several advantages over a passive one. A collector mounted on a south-facing bedroom wall, For example, can channel daytime solar heat to a living room or kitchen at the northern end of the house. If the southern house wall is obstructed, the collector can be installed on a south- facing garage or porch and ductwork ex tended to the living space. And unlike passive systems, active systems can be fully automated, with thermostats that turn the system on when heat is needed and shut it down when collector temperatures drop below the useful range. The materials and techniques necessary to build the collector are similar to those used for the thermosiphoning air panel illustrated. The collector is glazed with standard 34-by-76-inch panels of double-insulated glass. Because the characteristics of glazing panels frequently differ, depending on the type of glass and edging used, be sure to follow the manufacturer’s recommendations on providing adequate support and allowing room for expansion and contraction as the glass heats and cools. The heat absorber is made from sheets of black-painted corrugated aluminum, and the wall and stud spaces directly behind the collector are protected from heat and moisture with foil-faced sheathing of rigid cardboard. The collector is fastened to the south-facing wall with angle brackets at the top and with wood blocking underneath. The only major design difference is the active system’s manifolds—two vertical air passages at either end of the collector. The manifolds are created by removing the sheathing and insulation from the stud spaces at the collector’s extreme right and left. An intake duct connected to one manifold supplies cool air to the entire collector; a single exhaust duct draws the heated air from the manifold at the opposite end. The construction also differs slightly in the size of the lumber used: A larger frame is used to support the weight of the additional glass in the giant collector and to provide slightly wider air spaces on both sides of the corrugated heat absorber. The fan that circulates the air is critical to the efficient operation of an active system. The quietest and most effective fans are centrifugal blowers, also known as squirrel-cage fans because of the cylindrical arrangement of their blades. Blowers are sized and rated according to two criteria: the amount of air they move, in cubic feet per minute (cfm); and the resistance to air flow, or static pressure (sp), of the system in which they are used. Static pressure is primarily deter mined by the depth of the air space be hind the absorber plates. The static pres sure of the system shown at right is approximately 0.5 inch—a measurement obtained on a special air-pressure gauge used by solar engineers. The necessary cfm rating of a blower is calculated by multiplying the square footage of the collector by 2.5. Thus, a 100-square-foot collector designed like the one at right requires a fan rated at 250 cfm and 0.5 sp, that is, one capable of delivering 250 cubic feet of air per minute at a static pres sure of half an inch. The ductwork must be matched to the air-handling ability of the blower. The dealer who supplies the fan will be able to advise you on the appropriate size of the ducts. They should be run in the most direct possible route from the collector to the room that needs heat. In houses built on slabs, ducts can be run through living areas at floor or ceiling height, then boxed in with wood frames covered with paneling or wallboard. In houses with basements, ducts may be brought out of the collector into the adjoining room, then turned down through the floor into the basement, where they can be hung along ceiling joists. Any ducts that pass through unheated space must be insulated; wrap them in fiberglass batts with the foil facing outward, or purchase factory- insulated ductwork. The brain of the air-handling system is a differential thermostat, a special 120- volt monitoring device capable of measuring and comparing temperatures at two remote locations. The thermostat body is mounted on a wall near the blower and wired to the blower motor and to the house current. Low-voltage sensors are in stalled at the collector’s hot-air outlet and in the solar-heated living space, and connected to the thermostat body with bell wire. When the collector tempera ture rises above the room temperature by a preselected number of degrees—16° F. is a common setting for active systems— the thermostat turns the fan on. When the collector cools to a predetermined temperature, the thermostat shuts the fan off, permitting the collector to reheat or to remain off overnight. When its fan shuts off, an active system will operate like a passive, thermosiphoning one unless the ducts are blocked in some fashion. During hot, sunny days superfluous warm air from the collector will be thermosiphoned into the living space, overheating it. At night or on cold, cloudy days, the thermosiphoning will work in reverse: Cold air in the collector will drop down to the low, return duct and flow into the living space, while warmed air from that room flows out of the higher supply duct to the collector, where it’s chilled. This solar backfiring can be readily thwarted by one-way air valves, called backflow dampers (-- 57), or by air registers and grates that open and close manually. 58 How air flows n an active system. The secret of heat generation in an active collector is the horizontal air flow behind the heat-absorber plate; As cool air from the living area enters the collector’s intake manifold, it’s routed to three separate air channels (arrows), spaced 2 feet apart to assure equal distribution of air across the collector. As the air begins its long journey through the channels, it’s forced into a sinuous, rippling flow by the corrugations of the heat absorber, which are at right angles to the direction of flow. The resulting turbulence causes the air to come in contact with the entire surface of the absorber, thereby increasing the ex traction of heat. At the other end, the air, heated as much as 70° during its passage, is pulled out of the exhaust manifold into the supply duct leading to the living areas requiring heat. A centrifugal blower, controlled by a thermostat with sensors located at the collector outlet and in the heated room, is mounted on the supply duct. Its location on the duct supplying air to the room rather than on the return duct is a deliberate design feature. By pulling instead of pushing air through the collector, the blower makes the air pressure in the collector lower than in the surrounding air. This negative pres sure ensures that outside air will be drawn into the collector if any leaks develop. Thus, valuable heated air will be kept from escaping outdoors. Building the collector. A five-panel active collector is framed all around with 2-by-6-inch lumber. The frame is fastened to the wall after the collector’s exterior dimensions have been measured and marked and the house siding removed. Sheathing and insulation are removed between the pairs of studs at the extreme left and right, to create two vertical air spaces—an intake manifold and an exhaust manifold. A mounting frame for the heat absorber is made by nailing 2-by-2s to the outer frame, with the inner surface of the 2-by-2s flush with the inner surface of the 2-by-6s. The sheathing, manifolds and exposed surfaces of the absorber mounting frame are covered with foil-faced reflective backing. Two additional 2-by-2s are then nailed across the foil to create the collector’s three horizontal air channels. In each manifold, a hole is cut through to the interior to accommodate the supply and return ducts, and sheet-metal fittings called take-off collars -- are set into the holes and nailed to the studs on each side of the manifold bays. The absorber plates are screwed to the 2-by-2 mounting frame and the horizontal 2-by-2s, with the edges of adjoining sheets over lapped and caulked. Vertical mullions—1-by-5-inch lumber lengths—are first rip-cut to 4-inch widths and spaced to provide support where the collector’s glazing panels meet. The mullions are then pushed flush against the surface of the absorber plate and toenailed at top and bottom to the outer 2-by-6 frame. Then glazing stops, fashioned from 1-by-4-inch lumber rip-cut to fit between the absorber and the glazing panels, are nailed to each side of the mullions, to the top and bottom inside surfaces of the 2-by-6 frame between the mullions, and along the inside of the two vertical members at each end of the frame. Panels of double-insulated glass are now installed between mullions, against the glazing stops, using setting blocks to cushion the bottom edges of the glass --- 73. Finally, 1-by-3-inch wood battens are fastened to the outer framing and mullions to secure the glazing panels, all joints between glass and battens are caulked, and the top of the collector is flashed to channel off rain and snow. BLOWER; LIVING AREA SUPPLY DUCT; AIR SENSOR ; THERMOSTAT; RETURN DUCT MANIFOLD; INTAKE MANIFOLD; HEAT ABSORBER; EXHAUST MANIFOLD; HEAT ABSORBERS; GLAZING PANELS. ABSORBER; MULLION MOUNTING INTAKE REFLEC11VE FRAME MANIFOLD 2x2 INSULATION 59 Fans and Ducts for Moving Heated Air Hanging ducts from a ceiling. Map out the most direct practical route for the ducts, minimizing turns and bends in order to maintain the maximum air flow. Then, at 3-to 4-foot intervals along the chosen route, fasten flexible metal hanging straps to the ceiling. While a helper sup ports one end, lift a prefabricated length of duct into position, making sure that the crimped end points in the same direction as the air flow. Wrap the flexible straps snugly around the duct and secure them with a nut and bolt threaded through the strap perforations. TAKE-OFF COLLAR Routing ductwork through floors. To turn duct runs down through a floor, you will need three duct fittings matched to the diameter of the main ductwork: a 90 dgr elbow to make the turn; a straight length of duct to pass through the flooring; and a tabbed starter collar to support and secure the other fittings. First, mark the location of the hole by inserting the crimped end of the elbow into the transition fitting protruding from the collector manifold and scribing a circle on the flooring directly underneath the other end of the elbow. Remove the elbow from the transition fitting and use a saber saw to cut a hole 1/ inch larger than the marking. 60 Pry open the seam in the starter collar and bend all the tabs outward at right angles. Wrap the opened collar snugly around the straight length of duct and set the duct into the hole so that the tabs on the collar lie flush with the floor, preventing both fittings from falling through. Adjust the duct within the collar so that its upper end will meet the lower end of the elbow that extends from the manifold. Mark this position on the duct and remove the duct and collar from the hole. Fasten the collar to the duct with a couple of sheet-metal screws. Reinsert the assembly into the hole and nail every third collar tab to the flooring. Attach the elbow to the duct and manifold, and tape all joints to make them air tight. Continue the duct run underneath the floor by attaching appropriate sections to the protruding end of the straight duct. Mounting a fan in the duct. While fans of many shapes and configurations are available for active solar heating systems, one of the most convenient and compact units is a blower that attaches directly to the underside of the duct. To mount such a fan, first cut a hole to the fan manufacturer’s specifications in a 2-foot length of duct, starting by drilling a small hole and enlarging it with tin snips. Bend the fan’s two mounting flanges outward to conform to the shape of the duct and set the fan into the hole. Mark the duct through the two screw holes in each flange, then remove the fan and drill holes at each mark. Reposition the fan and fasten it to the duct with sheet-metal screws driven through the holes in the flange. Attach the duct section to the main ductwork, using hanging straps on either side of the fan to support the extra weight. Circulating Heat without Ducts Circulating air through a network of ducts is not the only way of moving solar heat through the house. A less elaborate method is to use through-the- wall fans. Mounted in a hole between studs, the fans pull air through a grille on one side and blow it into the next room through an opposite grille. This design lends itself to a variety of solar applications. Installed on an outside wall, the fan can pull air into the living space from an exterior passive collector or greenhouse. Installed on an interior wall, it can distribute air from a solar- heated room into an adjoining space. To ensure that cool air flows out as the fan blows warm air in, return registers can be installed near floor level in the same wall as the fan, or a door to the adjoining room can be left open. Both the fans and registers can be adjusted for various wall thicknesses; the model shown has an outer metal frame that slides over the inner frame, changing the unit’s depth from 334 to 6 inches. The fan may be controlled manually with a switch on its exhaust grille, or it may be wired to a differential thermo stat for automatic operation. 61 Wiring the Fan and Thermostat Installing the thermostat. Mount the differential thermostat on a wall near the blower. Then run two-conductor low-voltage cables—one cable for each sensor—from the thermostat to the take-off collar at the collector’s exhaust manifold and to the point where the return duct pulls air from the living space. Fasten the cable wires to the screw terminals on the front of the thermostat. At the collector, drill a screw hole in the top and bottom of the take-off collar. Then drill holes in the copper mounting straps attached to the sensor. Align the holes in the strap and collar and thread a small bolt through each set of holes. Secure the bolts with nuts fastened outside the collar (bottom). Drill another hole in the side of the collar and enlarge it so that it will accept a rubber grommet; pull the sensors wire leads through the grommet and use wire caps to attach them to the thermostat’s low-voltage cable. Mount the second sensor in the return duct; connect its leads to the other thermostat cable. Bringing power to the blower. Turn off all power at the junction box or other power source closest to the fan and thermostat. Then mount a switch box on the wall at a convenient location near the thermostat and fan. Run No. 14 plastic-sheathed cable from the junction box to the switch box, securing the ends with cable clamps. In the junction box, connect the new cable’s black wire to the existing black wires with a wire cap; similarly, join the new cable’s white and bare ground wires to the existing white and ground wires. 62 Run the thermostat’s power cord to the switch box; if the model does not have a power cable attached, use No. 14 plastic-sheathed cable and connect it to the marked terminals under the thermostat’s cover. At the switch box, join the white wires from the power cable and the junction-box cable with a wire cap and loin the ground wires to each other and, with a grounding screw, to the switch box. Connect the black wires to the switch terminals and install the switch and cover plate. Plug in the blower’s power cord to the grounded blower outlet on the thermo stat front; if the blower cord does not have one, attach a three-pronged grounding plug. Turn on the power to the junction box. You can now use the switch to turn off the solar heating system at night or in summer to prevent overheating. Stockpiling Heat in a Rock-filled Bin Solar collectors that are larger than 200 square feet generate so much heat that some form of storage is generally necessary to prevent the house from over heating. On sunny winter days, even the smaller collectors can produce more heat than is immediately needed. If the excess warmth is stored in rocks or other heat-absorbing material, it can then be used during the evening and night hours when the collector is no longer providing heat. A typical forced-air storage system, shown in simplified form above, utilizes an insulated bin containing 1- to 2-inch rocks that hold the daytime heat. While heat can be stored more compactly in water or phase-change material (-- 32), rock storage is usually less expensive, and the containers are less likely to leak heat. Storage bins can take up a great deal of floor space, because they must hold literally tons of rock—50 to 60 pounds for each square foot of collector surface. In new houses, the bin is frequently integrated into the foundation during construction; in existing houses, it would have to be placed in the basement or on a slab that is strong enough to support the additional weight. Storage bins can be readily connected to an existing forced-air heating system, which will eliminate the need for separate ducts to distribute the solar heat through the house. When the sun is shining, a duct- mounted blower draws sun-warmed air through a solar supply duct into the storage bin. After warming the heat- retaining rocks, the air leaves the bin through a solar return duct and is recycled through the collector to pick up more heat. At night, or whenever heat is needed, a return duct from the house brings cool air to the storage bin. Heated by the rocks, the house air flows back into the furnace through a furnace supply duct and from there is pushed by the furnace blower through the regular house supply duct. When the rocks have given up most of their heat, the furnace kicks in automatically to provide backup heating. The operation of the blowers is con trolled by a differential thermostat with sensors mounted in the collector, in the storage bin and in the living space. Dampers are installed in all the ducts leading into and out of the storage bin; in some of the systems, the dampers are opened and closed manually, while in the more sophisticated installations a series of motorized dampers automatically open and close on command from the thermostat. This versatile system can even be adapted to provide summer air conditioning in arid climates where humidity is not a problem. During cool summer nights, cold air from the collector is circulated through the storage bin, chilling the rocks. During the heat of the day, house air is cooled as it passes through the cold storage bin. COLLECTOR; STORAGE BIN. Prev | Next: The Sun Space: A Solar Collector You Can Live In |