Heating your home

Heating Your Home with an Active Solar Energy System

Active solar systems consist of collectors that collect solar radiation and electric fans or pumps to distribute heat from the collectors. A liquid or air is used as the heat transfer fluid. Most systems also incorporate storage systems to provide heat when the sun is not shining.

If you want to heat your home with solar energy, you will need to decide whether you want an active or a passive system. Although passive systems are popular because of their simplicity, they are sometimes impractical to install (retrofit) in an existing home, particularly if much of the site is shaded. Active systems are often more practical for such applications.

Choosing the proper solar energy system depends on varying conditions such as the site, design, and heating needs of the house. Although active systems are typically installed on the roof, they can be ground or wall-mounted to take advantage of the availability of solar radiation. You may prefer the aesthetics of a passive solar sunspace instead of rooftop collectors. If you are unsure about what type of solar energy system to install, contact a solar energy specialist or engineer. No matter what system you choose, you should learn about it before making a purchase.

How Much Heat Should Active Systems Provide?

Active solar energy systems are usually designed to provide 40% to 80% of the home's heating needs. Systems providing less than 40% of the heat needed for a home are rarely cost-effective except when using air panels for walls, window boxes, and other collectors that heat one room and require no heat storage.

The size of the active system helps determine how much heat it can provide. If you are planning to purchase a system, you must determine how large your system should be. Computer software is often used to properly size active systems. (For information on solar computer software, contact the EREC.)

Back-up Heating-A Supplement for Active Systems

Most building codes and mortgage lenders require a back-up heating system for houses heated with solar energy. Back-up systems supply heat when, for example, there are long periods of cloudy weather. Back-up systems range from a wood stove to a conventional heating system.

Positioning Collectors to Perform Optimally

In general, the optimum collector orientation is true south. True south is the highest apparent point in the sky that the sun reaches during the day. (True south should not be confused with magnetic south as indicated on a compass.) Collector orientation may deviate up to 20° from true south without significantly reducing the performance of the system. Collectors should be tilted at an angle equal to your latitude plus 15°. A collector receives the most solar radiation between 9:00 am and 3:00 pm. Trees, buildings, hills, or other obstructions that shade collectors reduce their ability to collect solar radiation. Even partial shading will affect performance and increase the payback period of the system.

You can position collectors in different locations. Collectors usually receive the most sunlight when placed in rows on the roof. In some cases, however, the roof may be too shady. If the roof does not receive enough sunlight, you may want to mount the collectors on a supporting structure on the ground, or in rows on the south wall of the house, where there is enough sunlight for the collectors to perform satisfactorily. Collectors mounted on the ground or on an exterior wall perform almost as well as those mounted on most roofs.

Types of Active Heating Systems

There are two basic types of active solar heating systems. These are liquid or air systems, based on the type of fluid heated in the collectors. Liquid systems use water or an antifreeze solution to capture, transfer, and store heat produced by "hydronic" collectors. Air systems use air to capture, transfer, store, and distribute heat from the "air" collectors. Both of these systems collect solar radiation, then distribute and store the heat that the collectors produce. If the system storage cannot provide adequate space heating, an auxiliary or back-up system provides the additional heat. Not all systems store the heat that they collect; they immediately distribute the heat for space heating. Liquid systems are more popular than air systems because they cost less to operate and take up less space.

Solar air systems distribute air at slightly lower temperatures than modern heat pumps (around 95°F, 35°C). Solar air systems use larger ducts to distribute the heated air than conventional forced-air heating systems. The larger sized ducts allow the air to move at a slower rate, making it feel warmer. (The air from a solar air system feels cooler than it is, because moving air increases the evaporation rate on your skin. This is why a fan or a breeze feels cool to you.)

Depending upon your needs and location, you may find the advantages of an air system outweigh its disadvantages. Air collectors produce heat earlier and later in the day than liquid systems. Air systems produce more usable energy over a heating season than a liquid system of the same size. Also, unlike liquid systems, air systems do not freeze, and minor leaks will not cause problems. Do not, however, ignore leaks; they will usually affect the overall performance of the system.

Liquid Systems

Liquid systems use water, antifreeze, or a phase-change liquid such as methyl alcohol, as the heat transfer or "working" collector fluid. Liquid system components include hydronic collectors, a storage tank, pumps, pipes, a heat exchanger, and controls. The collectors absorb solar radiation and transfer it to the liquid. At the appropriate time, a controller operates a circulating pump, circulating the working fluid through the collector. The liquid returns either to a storage tank or to a heat exchanger for immediate use. Therefore, when the home does not need to be heated and solar radiation is adequate, liquid systems can store solar energy without heating the living space.

Liquid systems have three operating modes. In the "primary mode," collectors send heated liquid to storage while storage distributes heat to the home. Whereas conventional baseboard hot water heating systems provide heat between 160°F and 180°F (71.1°C and 82.2°C), liquid systems heat water to between 90°F and 120°F (32.2°C and 48.9°C). The liquid flows rapidly through the collectors, so its temperature only increases 10°F to 20°F. Heating a smaller volume of liquid to a higher temperature would increase heat loss through the collector, decreasing the efficiency of the system. In the "second operating mode" the liquid system stores excess heat in tanks of water. If the system cannot collect enough solar radiation to heat the home, it goes into its "back-up mode," activating the back-up system.

Hydronic solar collectors for space heating systems are the same as those used in solar domestic hot water (DHW) systems. Please contact EREC for information on solar collectors.

Storing Heat for Liquid Systems

Liquid systems store solar heat in tanks of water or in the masonry mass of a radiant slab system. Most storage tanks require one to two gallons (3.8 to 7.6 Liters) of water for each square foot (0.093 square meter) of collector. The tanks are usually steel, concrete, fiberglass-reinforced plastic (FRP), or wood. Each type of tank has its advantages and disadvantages.

New construction often uses steel tanks because it is easier to attach pipes and fittings. These tanks are easy to construct and usually meet building codes for pressure vessel requirements. Steel tanks, however, corrode from rusting and pitting unless preventative measures are taken. Pitting is a condition where small-diameter holes form in the base metal. Rusting and pitting can be prevented by sealing the tank's surface. You can also add chemicals to inhibit corrosion if the stored water is not used for drinking. Steel tanks often cannot be used for retrofit projects because they are often too large to fit through entrances.

Tanks made of fiberglass-reinforced plastic (FRP) do not corrode. They can be insulated in the factory or on-site. If you own an FRP tank, however, you should never allow operating temperatures of the system to exceed the limits the manufacturer specifies. Like steel tanks, FRP tanks often cannot be used for retrofit projects because they are too large to fit through entrances.

Concrete tanks can be cast-in-place or precast and are suitable for retrofit projects. These tanks usually cost less than others. You must line them, however, to prevent water seepage. Also, connecting pipes without leakage is often difficult. The weight of these tanks could be a disadvantage, depending on whether you want a well-anchored tank on a solid floor in a basement, or whether you are installing the tank on floor joists in the first or second floor.

Wooden tanks with plastic liners are suitable for retrofit applications. They cost less than other tanks. Water should not exceed the manufacturer's recommended temperature for the liner. These types of tanks can only be installed indoors.

Before choosing a storage tank, you should consider several factors. First, you should decide where to place the tank, for example, in the basement or outside. Next, you should choose the size, shape, and material of the tank. You should also note any problems with installing the chosen tank. For example, you may need to construct a tank on-site if a tank of the needed size will not fit through the doorway. Tanks also have limits for temperature and pressure, and must be designed to meet codes stipulated by the local government and the American Society of Mechanical Engineers (ASME). You should also note how much insulation is required to prevent excessive heat loss, and what kind of protective coating the tank needs to prevent corrosion.

Distributing Heat for Liquid Systems

Heat can be distributed using radiant slab heaters, where the heat is stored in the slab, a central forced-air system, or by using hot water baseboards or radiators. Radiant slab systems use plastic or rubber pipes embedded in a concrete floor. Solar-heated water circulates through the pipes and heats the floor, which then radiates heat to the room. Radiant slab heating is most compatible with liquid systems because it performs well at relatively low temperatures. If you choose this type of system for distributing heat, however, you cannot have carpeting, and are limited to how much of the floor you can cover with throw rugs. Radiant slab systems also take longer to heat the home if the heat had been lowered the previous evening than other types of heat distribution systems.

Hot-water baseboards and radiators require water between 160°F and 180°F (71.1°C and 82.2°C). If you use baseboards or radiators with solar heating, you should either significantly increase the surface area of the piping, or preheat the water with the solar system and then use a conventional water system to raise the temperature of the water to 160°F to 180°F (71.1°C to 82.2°C).

It is possible to incorporate a liquid system into a home with an existing forced-air heating system. There are many different designs for distributing heat from liquid systems to a forced air system. A two-coiled heat exchanger arrangement is a popular way to heat forced air. Air returning from the living space is heated as it passes over a solar hot water coil (heat exchanger). The air then passes over a second coil connected to the back-up heater and, if necessary, receives more heat. The solar heat exchanger must be large enough to transfer sufficient heat to the air when using water at 90°F (32.2°C).

Another design options available for forced-air distribution systems includes one that uses a boiler or water heater as a back-up source.

Air Systems with Storage

An air system uses air as the working fluid for collecting solar energy. This type of system is composed of collectors, a rock storage bin, fans, ductwork, and controls. It operates in three modes, depending on how much heat is available. In the "simplest mode," heated air moves directly from the collectors to the house. If it collects excess heat, the "second mode" charges the rock bin. Finally, the "third mode" allows the storage bin or a back-up system to heat the home, even when the system is not collecting heat (e.g., at night).

Some systems use an air handler to distribute the heated air. An air handler is a sheet metal box containing one or more fans and several motor-driven or spring-loaded dampers. It directs air through the ducts, collectors, and storage bin of the solar system. The air handler opens the appropriate dampers, thereby regulating the modes of operation.

How Air Systems Store and Deliver Heat to the Home

To store heat, an air system delivers hot air from the collectors to the storage bin. The air first enters a plenum, which is an empty mixing space at the top of the bin. It passes down through the bin where the rocks absorb most of the heat. The air then returns to the collectors from a lower plenum for reheating. When the system uses rock bins to heat the home, it draws house air from the lower plenum up through the rocks. Warm air is then drawn from the top of the bin and distributed to the house. Thus, the rock bin serves as storage and as a heat exchanger. When storing heat, the top of the bin is usually about 140°F (60°C) and the bottom of the bin is about 70°F (21.1°C). If the air in the bin is too cool, a back-up system heats the air leaving the top of the bin to the desired temperature before distributing it.

Rock bins can be installed indoors, outdoors, or underground. Most rock bins are installed in crawl spaces and basements because warm air naturally rises to the living space. Because of this, bins in these locations distribute the air more efficiently. If you choose to bury the rock bin, it should be thoroughly waterproofed.

Rock bins can be made from cinderblock, concrete, or wood. They should be tightly constructed and sealed to prevent air leaks and moisture intrusion. Air leaks and moisture drastically reduce the efficiency of the system. If you use treated plywood, you should line it with sheetrock and a vapor retarder to protect the rocks and the entire system from gases released by the plywood. Rock bins should be insulated to R-11 (1.93m2°C/W) heated spaces or R-30 (5.3m2°C/W) in unheated spaces.

A rock bin should provide 1/2 to 1 cubic foot (0.014 to 0.028 cubic meters) of storage for every square foot of collector, and should be from 5 to 7 feet (1.5 to 2.1 meters) deep. Rock bins require 2 1/2 to 3 times more space for storage materials than liquid system tanks. Dense rock, such as river rock (which is predominantly quartz), performs best. The rocks should be uniform and about 3/4 inches to 1 1/2 inches (19 to 38 millimeters) in diameter. Before placing rocks in the bin, they should be washed to remove dirt and insect eggs and thoroughly dried. The rocks should also be kept dry inside the bin to prevent problems with mold, mildew, and insects.

The air system delivers warm air from the rock bin to the house through ducts. Ducts should be designed to minimize the noise and maximize the energy efficiency of the air blowers. Solar air systems need larger ducts than conventional furnaces because solar-heated air is cooler than the air that furnaces deliver. They must deliver more solar-heated air to compensate for the lower temperature. Most active systems deliver air at a velocity of 5 to 10 feet per second (1.5 to 3 meters per second). The ductwork should be insulated to R-16 (2.8m2°C/W) to prevent heat losses. Leaky ductwork can significantly impair the efficiency of the system.

Active Systems for Heating One or Two Rooms

Not everybody wants to invest in a large system that supplies much of the heat for the home. Small systems, such as wall air panels and window box collectors, are a simpler and less expensive option for those who only want to heat one or two rooms. These systems are easier to install than a larger system. Active air panels for walls and window box collectors only provide heat during the day. These systems do not have a storage system or elaborate ductwork.

In a small system, air panels are placed directly in or on a south-facing wall. The collector has an air-tight metal or wood frame and a black metal plate for absorbing heat with glazing in front of it. Solar radiation heats the plate that, in turn, heats the air behind it. An electrically powered fan delivers the air to the room or rooms. Active air panels for walls are practical if you have an unshaded, unused wall on the south side of the home. Factory-built panels for on-site installation are available. Also, if you are a do-it-yourselfer, you may choose to build and install your own air panel.

Passive window box collectors are simpler to install than air panels because they fit in an existing window. Therefore you do not need to cut a large hole in your south-facing wall. Air enters the bottom of the collector, rises as it is heated, and enters the room. A baffle keeps the room air from flowing back into the panel (reverse thermosiphoning) when the sun is not shining. These systems only provide a small amount of heat, since the collector area is relatively small. You may find plans for building one of these in your local library.

Controls

Controls for solar heating systems may be more complex than those of a conventional heating system. This is because they may have to analyze more signals and control more devices (including the conventional heating system). Solar controls use sensors, switches, and/or motors to operate the system and to provide back-up heating. The system uses other controls to prevent freezing or extremely high temperatures.

The heart of the control system is a differential thermostat, which measures the difference in temperature between the collectors and storage unit. When the collectors are 10°F to 20°F (-12.2°C to -6.7°C) warmer than the storage unit, the thermostat turns on a pump or fan to circulate water or air through the collector to heat the storage medium or the house.

The operation, performance, and cost of these controls vary. A basic control system should operate the solar system in three or four different modes. Some control systems monitor the temperature in different parts of the system to help determine how it is operating. The most sophisticated controls are microprocessors, which control heat delivery to where it is needed or desired.

The Performance and Maintenance of Active Systems

How well an active solar energy system performs depends on its quality, durability, system design, and installation. Active systems are generally more reliable and efficient than they were a decade ago. If you are planning to purchase an active system, you may want to compare the relative performance of solar collectors. The Solar Rating and Certification Corporation (SRCC; see below) compares the relative energy output of various collectors and publishes this information in the Directory of SRCC Certified Solar Collector Ratings ($33.00, 1998). Certified collectors must meet minimum standards for quality and durability. All collectors that the SRCC certifies carry the SRCC label. (The SRCC also certifies solar energy systems for heating domestic hot water.)

SRCC test data will help you compare the relative performance of different units. Remember that solar collector performance is only one element effecting the total performance of a system. Other components and the system design are also important. Other factors, such as quality of installation and proper siting, also effect performance. When choosing a system, you should also consider the cost and expected life of the collector and the availability of service and parts.

To keep your active system from breaking down and to optimize its performance see that it is regularly maintained. Most systems require 8 to 16 hours of maintenance annually. You should set up a calendar with a list of system maintenance tasks. Different systems require different types of maintenance. Some suggestions include checking pipes and ductwork for leaks and collectors for damage. You may need to clean the filters in the air systems and check the glazing on the collector. You should also lubricate pumps and fans. Manufacturers can provide information about maintaining their specific systems.

During the summer, you may need to protect liquid collectors from boiling by draining the collectors, or dumping excess collected heat through an air-cooled fan coil unit. Whether this is necessary depends on how much hot water you use and the design of your system. The manufacturers of the collectors can recommend the best procedure for their model.

The Cost of Active Space Heating Systems

The cost of active solar heating systems varies. Systems usually cost between $30 and $80 per square foot of collector area, installed. Usually, the larger the system, the less it costs per square foot. When evaluating the cost of a system, consider its size, the type of collector, the total collector area, the costs for designing and engineering, the costs for equipment and installation, and the location of the site. Most systems offer the greatest return on investment when displacing electric heating. The economics of an active space heating system can be improved if space heating is combined with water heating. A dual-purpose system is usually more cost-effective, because an otherwise idle collector can heat water in the summer.

Some states offer sales tax exemptions, income tax credits or deductions, and property tax exemptions or deductions for solar energy systems. If you are not sure what benefits your state offers, contact your state energy office. (Check your local phone directory or contact the Energy Efficiency and Renewable Energy Clearinghouse [EREC] for the address and phone number of your state energy office.)

Sales of active solar heating systems declined after Federal tax credits for residential solar energy systems were eliminated at the end of 1985. The retail price of active systems, however, has only slightly increased, because retailers lowered their prices to compensate for the elimination of the tax credits.

Heating your home with an active solar energy system can significantly reduce your fuel bills in the winter and reduce the need for fossil fuels, such as coal, oil, and natural gas. If your system heats the domestic water supply as well, savings on your fuel bills will continue all year. If you are planning to purchase an active system, however, you should learn as much as you can about the technology. In this way, you will know how to size and install your system properly, make a wise purchase, and maintain the components, and learn how to avoid problems or pitfalls that can occur.