Residential Solar Heating Retrofits

Heating Potential, Costs, and Benefits
A solar heating system designed to provide all of your space heating requirements will be very expensive, and in many cases impractical. Solar space heating systems are usually designed to provide 30 percent to 80 percent of heating, depending on your geographical location, the system type, and its size. Costs vary widely depending on the type and size of the system and whether the solar system can be efficiently integrated into the existing heating system.

Solar air space heaters, which act as a supplemental heat source for one room or a small area of the house, are the least expensive and simplest to install. Since collector sizes vary, so do prices.

Solar space heating systems may be cost-effective, but you must carefully evaluate the claimed cost savings of the system during the heating season only, against the price and anticipated longevity of the system.

Climate plays a major role in system design and cost. Active solar space heating systems are most economical in climates that have extended heating seasons with many sunny days and/or with high utility rates. They are less cost-effective areas with cloudy conditions during the winter, such as the coastal Northwest, in areas with short heating seasons, such as Southern California and Florida, or in any area with low prices for electricity and other heating fuels.

Passive solar heating systems work best in areas where there is a fairly large difference between daytime and nighttime temperatures. However, whether or not you should proceed with a specific passive retrofit depends more on your home’s structure than the climate.

Besides heating energy cost savings, a solar heating system will reduce the amount of pollution and greenhouse gases that are emitted to the atmosphere to heat your home.

Efficiency First
Taking steps to ensure that your home is energy-efficient increases the effectiveness of any heating system. Inadequate insulation is a leading cause of energy waste in most homes. Insulating your home to recommended levels may allow you to reduce the size of both your heating and cooling systems, thereby reducing heating and cooling costs; or reduce the cost of operating your existing systems. Other energy saving measures include caulking the building joints and plumbing penetrations into the attic, weather stripping around windows and doors, adding storm doors and double- or triple-glazed windows, and insulating the hot water tank and pipes.

Legal Issues
Local covenants, zoning ordinances, and building codes may restrict or not even allow the installation of a solar system on your property, especially in areas with homeowners associations or designated historical districts. Research these issues before you invest any additional time and money in a solar system.

Because solar energy systems depend on sunlight, shading of the solar collection area between 9 a.m. and 3 p.m. during the heating season reduces system performance and cost effectiveness. Orientation of the collectors must be within 30° of true south (which varies up to 22° from magnetic south in the United States). Analyze your home’s site. Does it have a good southern exposure? Will trees, buildings or other landscape features shade the collector area in the winter? Could these features do so in the future?

Collector Installation
Active solar space heating systems use solar collectors to heat a liquid or air to provide heat. They are usually installed on the roof. They may be flush with the roof, or installed on racks that elevate the collectors above the slope of the roof to maximize the amount of solar radiation they receive. Although most roofs can support the added weight, before adding solar collectors-especially liquid-based collectors-to an older home, you should check the condition of the rafters to make sure they can support the added weight and the potential wind loading that the collector(s) may place on the roof structure. The condition of the roofing surface material should also be checked. Resurfacing the roof will most likely require that the collectors be removed, so if the roof is due for resurfacing soon, consider having it resurfaced before installing solar collectors. Collectors can also be mounted on ground racks, vertically on a south-facing wall, or on an adjacent structure such as a garage. Pipes or ducts for transferring heat from the collector(s) to the interior may require a roof or wall penetration.

Solar America
Heat Storage and Transfer
Storing solar heat collected during the day for use at night increases the effectiveness of solar heating systems. Liquid solar space heating systems usually use a large, well-insulated water tank to store solar heat for nighttime use or during cloudy periods. Solar air heating systems may use large bins filled with rocks for heat storage (though this method is now rare due to problems associated with controlling moisture and mold growth in rock storage media). Whatever storage system is used, they require sufficient space and adequate support for the weight of the storage material. The size of the storage system depends on numerous factors, but basically the larger the heating requirement, the larger the storage volume required. It may be possible to locate the storage system outside of the house, but this requires that the system and plumbing or ductwork to and from the system be very well insulated.

Since piping or ductwork usually run inside walls, it may be necessary to remove small sections of the wall. In other cases, the pipes and ducts can be placed in a room corner or in a closet. Generally, liquid systems are easier to retrofit. Water tanks store more heat in a given volume than rock bins, and piping is easier to install than ductwork.

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 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.

Credit: U.S. Department of Energy