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Integrating Systems
Solar space heating systems perform most efficiently at collector temperatures between 90°F and 140°F (32.2°C and 60°C). For this reason, solar heating systems work well with central or forced air distribution systems or radiant slab heating systems. Most flat plate liquid collectors do not heat water enough to directly heat a home with baseboard and radiator systems, which operate at 160°F to 180°F (71°C to 82°C), though they can preheat water before it enters a conventional boiler. Medium-temperature, concentrating solar collectors, which achieve higher temperatures, are alternatives to flat-plate collectors for such systems. Electric resistance heaters do not use the ductwork solar systems require. Solar energy systems can be used along with electric resistance, radiator, or baseboard heating if you also install ductwork.
The following chart lists the common heat distribution systems and the types of solar systems that are compatible with each.
| Heat Distribution System | | Heat Distribution System | | Forced Hot Air | | All Collectors | | Hot Water Radiators | | Liquid Flat Plate, Concentrators | | Steam Radiators | | Concentrators, Evacuated Tubes | | Electric Baseboard | | *None | | Radiant Floor (water) | | Liquid Flat Plates | | Radiant Ceiling (electric) | | *None |
* Requires a separate heat distribution system to be compatible with solar collectors.
Solar air heaters that directly heat interior air do not require a heat storage component, and can complement any existing heating system.
Passive Systems
The two primary passive solar retrofit options are increasing south-facing window area to admit more sunlight into the interior and adding a sunspace to the exterior of the house. It may also be possible to convert an existing, south-facing masonry wall to an effective solar collector or "Trombe Wall." All three methods allow solar radiation through south-facing "glazing," where it is absorbed in dense materials (thermal mass) such as water, masonry, or concrete to store the heat for long periods. The thermal mass is most effective when placed directly in the sun's path.
Passive solar energy systems rely primarily on natural methods of moving heat: convection, conduction and radiation. Homes with an open design encourage natural air movement. A fan or blower can usually improve air circulation around room dividers or between rooms.
Before undertaking any retrofit to the building, check local ordinances and codes, and-if necessary-obtain a building permit.
South-Facing Windows
The simplest passive retrofit is to increase the window area on the south-facing wall of your home. You can either enlarge existing windows or add more. Sunlight passes through the windows, immediately warming the adjacent space. Energy-efficient, low-e glazing should be used. Bear in mind that adding a large amount of window area may require new framing members to secure the glazing and carry the roof load. Existing thermal mass in the floor or walls will store some of the heat for use at night, but it may be desirable to add more mass, such as dark tile. Movable insulating devices: thermal shades, shutters, or curtains, for example, reduce heat loss through the glazing at night and during heavily overcast periods.
Careful consideration must be given to avoid overheating in the summer. Shading the window area with an overhang, awning, or deciduous trees (trees that shed their leaves in the fall), helps to prevent excessive summer heat gain.
Sunspaces
Sunspaces or solar greenhouses are a popular passive solar retrofit. A sunspace is a partially or totally glazed room. Besides providing heat, a sunspace separates the adjoining room from the outside air, reducing that room's heat loss. Ideally, the sunspace is situated adjacent to south-facing rooms. If shading, solar access, or other considerations prevent this, the southeast or southwest corner can be used instead, although such an orientation may increase summer overheating problems. In any case, the orientation should not deviate more than 45° from true south. Thermal storage in the sunspace reduces temperature swings, helps prevent nighttime freezing, and increase the heat available at night. An uninsulated masonry wall between the sunspace and house can also be used to store heat. Storage can also be added in the form of masonry walls, floors, or water-filled containers. Again, low-e glazing and movable insulation reduces heat loss during sunless periods.
Trombe Wall
A Trombe Wall is essentially a high-mass brick, stone, concrete, or adobe wall that has glazing on the exterior/front (south facing) side. It is not insulated. It absorbs sun during the day and slowly radiates the collected heat into the interior at night. An existing south-facing high-mass wall could be converted to a Trombe wall by adding glazing to the exterior surface with a gap of 3 to 8 inches (8 to 20 cm) between the glazing and the wall surface. Low-e glazing and/or an insulating curtain drawn at night in the space between the glazing and the wall will help control heat loss from a Trombe Wall to the exterior.
Most wood frame houses cannot support the weight of an additional masonry wall. Before adding a Trombe wall, you or your building contractor should calculate what the new load will be and make sure that the house will meet or surpass load guidelines set by local building codes.
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© 2004 U.S. Deptartment of Energy
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