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Residential Solar Heating Retrofits

Adding (retrofitting) a solar space heating system to your home is one way to combat increasing energy costs and to raise your home's market value. The two major types of solar retrofits are active systems (requiring mechanical energy and hardware such as pumps and fans to distribute heat) and passive systems (which depend on the natural circulation of a fluid for heat movement). A study by the National Renewable Energy Laboratory (NREL) indicates that up to 60% of the nation's homes receive enough sunlight for a solar heating retrofit. The following factors affect the feasibility of any solar retrofit.


Climate plays a major role in system design and cost. Active space heating systems are most economical in climates with long sunny heating seasons and high utility rates, such as Colorado and parts of the Southwest. They may not be economical in cloudy climates, such as the coastal Northwest, or in sunny climates with short heating seasons, such as Southern California and Florida.

Passive 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.


Because solar energy systems depend on sunlight, shading of the solar collection area between 9 am and 3 pm during the heating season reduces system performance and lengthens the system's payback period. (The payback period is how long the system takes to save more money, in energy, than it initially cost.) 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?


Taking steps to ensure that your home is energy-efficient increases the effectiveness of any solar 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, weatherstripping around windows and doors, adding storm doors and double- or triple-glazed windows, and insulating the hot water tank and pipes.

Active Systems

Active space heating systems often use flat, rectangular solar collectors bolted onto the roof. These collectors use either a liquid or air to provide heat. 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. Collectors can also be mounted on ground racks, vertically on a south-facing wall, or on an adjacent structure such as a garage.

Heat Storage and Transfer

Liquid solar space heating systems usually use a large, well-insulated water tank to store solar heat for nighttime use or during cloudy periods. Air heating systems use rock bins. Rock bins require a space from 16 to 36 square feet (1.5 to 3.3 square meters) and 5 to 7 feet (1.5 to 2.1 meters) in height. A rock bin can be constructed in a basement, using readily available materials. Filling the bins with rocks, however, may be physically difficult.

In milder climates, a space heating water tank can be located outside and above the ground (preferably in a shed). You must insulate the tank and pipes to withstand the cold, however.

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 function well with baseboard and radiator systems, which operate at 160°F to 180°F (71°C to 82°C). 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. Check Table 1 below to determine which collectors are compatible with your present heating system.

The following chart lists the common heat distribution systems and the types of solar systems that are compatible with each.

Table 1: Heating System Compatibility

Heat Distribution Systems Compatible Solar Collectors
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


An active system designed to provide all of your space heating requirements will be very expensive. Active space heating systems, however, can provide 30% to 80% of your heat, depending on your geographical location, the system type, and its size. Systems are usually designed to provide about 50% of a home's heating load.

To achieve a 50% heating load, you will need a collector area equal to 10% to 50% of your floor area, depending on your climate and the insulation level in your house. Current costs for parts and professional installation are $40 and up per square foot of collector area, depending on the particular system, the number and type of controls, the size of the storage system, and whether the conventional system's distribution system can be used.

There are also air space heaters without a heat storage system, which act as a supplemental heat source for one room or a small area of the house. These systems can be active or passive.

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.

Passive Systems

The three primary passive solar retrofitting techniques are using south-facing windows to admit sunlight; converting a south-facing masonry wall to an effective solar collector or "Trombe Wall"; or adding a sunspace. All three methods collect solar radiation through south-facing glazing. Absorption of the incoming solar radiation occurs in dense materials (thermal mass) such as water, brick, and concrete that store heat for long periods. The thermal mass is most effective when placed directly in the sun's path.

Most wood frame houses cannot support the weight of an additional masonry wall. Before adding a mass 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.

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, check local ordinances and codes, and-if necessary-obtain a building permit.

South-Facing Windows

The simplest passive retrofit uses one or more south-facing windows. You can either use existing windows as they are or enlarge them. Another option is to add new windows. Sunlight passes through the windows, immediately warming the adjacent space. Thermal mass stores some of the heat for use at night. Movable insulating devices: thermal shades, shutters, or curtains, for example, reduce heat loss through the glazing at night and during heavily overcast periods. 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. 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.

Trombe Wall

A home that has uninsulated, massive brick, stone, concrete, or adobe walls already has a good solar collector. Adding glazing to the front of the south wall will trap incoming solar radiation, charging the wall with heat. If the adjoining room is used primarily during the day, vents can be cut through the wall to provide some heat immediately. If the adjoining room is used primarily at night, vents may not be necessary. Warming of the space occurs as the stored heat slowly migrates through the wall.

An insulating curtain drawn at night in the space between the glazing and the wall will help control heat loss from a Trombe Wall. Using "backdraft dampers" on vents prevents warm room air from exiting through the vents at night.


Sunspaces or solar greenhouses are the most 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 be used to store heat. Storage can also be added in the form of masonry walls, floors, or water-filled containers. Again, movable insulation reduces heat loss during sunless periods.


The middle-range cost of a sunspace is from $8,000 to $12,000. There are do-it-yourself projects that can be built for much less than $8,000, and custom-designed systems can run much higher than $12,000. Maintenance and operating costs are negligible.

For more information on passive solar retrofits or designs, contact the Energy Efficiency and Renewable Energy Clearinghouse (EREC).

This article appears (with slight modification) courtesy of EREN... the Energy Efficiency and Renewable Energy Network , a division of the U.S. Department of Energy.

If you have comments or questions about this article, you may visit the EREN website at or contact them by mail, email or phone:

Energy Efficiency and Renewable Energy Clearinghouse (EREC)
P.O. Box 3048 Merrifield, VA 22116
Voice (USA only): 800-DOE-EREC (363-3732)

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