What Is It and How Does It Work?

Active solar heating systems take a fluid—such as air, water, or an antifreeze solution—heat it in rooftop collectors, and then distribute it either directly to the rooms in your house or to a storage system for later distribution. These types of heating systems can be integrated with radiant floor heating, boilers with hot water radiators, absorption heat pumps and coolers, and forced air systems.

Factors such as the climate, the type of collectors used, and how energy efficient your house is will determine how large the system must be. It may be cost effective to supply up to 80 percent of your home's heating needs with an active solar heating system. If you are considering this type of system, you should also make sure your home is well sealed and insulated to maximize the effectiveness of the heating system. In most cases, a supplementary mechanical heating system is required, but it can be much smaller and cheaper to install and operate than a full-capacity mechanical heating system.

Active solar heating systems require advanced, complex control systems that monitor the temperatures of the solar collectors and storage unit to determine when the conventional backup heating system must be activated. A differential thermostat monitors the difference in temperature between the collectors and the storage unit, which determines when fluid from the collectors can be distributed. Control systems range in cost, energy use, and complexity. In some cases, photovoltaic panels can be used to generate electricity to operate pumps or blowers in an active solar heating system. This initial investment can help you save on energy use in the long term.

Why Should I Consider It?

Active solar heating can be an effective heat source in cold climates with ample solar resources. The system requires very little maintenance and has a long life cycle. An added benefit and cost savings resource of an active solar heating system is that it can be linked to domestic hot water heating and used year round (in winter for space heating, and for water heating in other seasons). Once installed, the system requires very little electricity to operate and relies on the sun to generate heat. It is cheaper to operate than a conventional heating system and reduces heating bills.

What Are the Options?

Active solar heating systems can use either air or water as the fluid that passes through rooftop collectors. Solar air heating systems heat air that can be circulated directly into the rooms in your home, or preheat air that is further heated in a supplementary air-source heat pump system. Solar air heating systems can generate a surprising amount of heat, and even during sunny, cold winter weather, air that enters a solar collector at 70 degrees can be warmed 70 to 90 degrees more. When you have an air-based system installed on your roof, you will also need to install ductwork to distribute air from the collectors to the rooms in your home. Ducts are also required to return used air back to the collector. Solar air heating systems can also be installed on south-facing walls with a hole cut to provide heated air directly into a room.

Solar air heating systems are not at risk of freezing or causing moisture damage through leaks. Also, air-based systems can produce heat for a longer portion of the day than solar liquid heating systems. However, since air is less efficient at transferring heat than liquid, solar air heating systems generally have lower efficiencies than liquid-based systems.

Solar liquid heating systems use much of the same equipment as solar water heaters. In this type of system, water or an antifreeze liquid is heated in rooftop collectors and circulated to a storage tank or heat exchanger to provide heat to your home. Storage of heated liquid can occur in tanks or can take advantage of the thermal qualities of a masonry mass for radiant heating. Correct sizing of storage tanks is important to prevent excess heat loss. You should plan for about 1 to 2 gallons of storage for each square foot of collector area. The size of the storage tanks has spatial implications that must be taken into account during the planning process. Standard domestic hot water heater tanks are common and effective storage systems, but custom tanks can be made from any number of materials, including stainless steel, fiberglass, plastic, concrete, or wood. The primary common consideration is to ensure adequate insulation of the tank to prevent heat loss.

Active solar liquid heating systems are very versatile in terms of the types of distribution systems with which they can be installed. Effective distribution systems include central forced air, hot water baseboards or radiators, or radiant floors. Liquid-to-air heat exchangers can integrate preheated liquid from an active solar system. In this case, the solar-heated liquid is used to heat air before it is blown into rooms in your home. Hot water baseboards and radiators generally require the liquid to be at a hotter temperature than is generally provided by solar liquid heating systems. However, an active solar system can be used to preheat the liquid, which reduces the load required by a supplementary mechanical heating system. Radiant floor heating, on the other hand, does not require a high-temperature liquid in order to be effective. They are, therefore, an ideal application for integration with active solar heating. A radiant floor system can easily be designed to include supplementary storage and/or heating but include a standard hot water heating tank. Radiant floor systems are made up of a concrete slab with pipes running through it. Heated liquid is circulated through the pipes, transferring heat to the slab through convection. The heat is then released into the space through radiation.

As in the case of solar hot water heating, there are three types of collectors that can be used to heat the liquid in the system. The most common type of collector is the flat-plate collector, which is a thin, insulated box with a translucent glass cover and a dark absorber plate inside. Small tubes pass through the box with either water or an antifreeze solution. The absorber plate gets very hot and then transfers its heat to the fluid in the pipes. Flat-plate collectors can be used with direct, indirect, or thermosyphon systems.

Evacuated tube collectors include an array of parallel transparent glass tubes. The tubes contain an absorptive surface on the inside and a selective coating to promote absorption and reduce emissivity. The tubes also contain an antifreeze fluid that is heated and circulated throughout the rest of the system. This type of collector is only used with indirect circulation systems.

Batch systems (also known as Integral collector storage systems) are solar thermal collectors made up of an insulated box with a large tank or tube inside, essentially combining the roles of collector and tank into one. The interior absorptive surfaces are painted black and the face of the box is glazed. This type of system is not as common as flat-plate collectors or evacuated tubes, but it is an option to use with direct circulation and thermosyphon systems.

How Is It Installed?

Active solar heating systems should be installed by experienced and qualified contractors. In order for these systems to perform properly, they must be accurately sized, oriented, and installed. A small amount of ongoing maintenance is necessary to keep your system operating efficiently for its entire life cycle.

What Are the Potential Benefits?

With a typical, residential active solar heating system (about 64 square feet), you can expect to produce the same amount of peak power as a much larger photovoltaic array of 400 square feet. This imbalance makes active solar systems a more cost-effective option for reducing energy usage than photovoltaics.

How Much Does It Cost?

Installation costs for active solar heating systems depend on many factors, such as the size of the system required and the type of distribution that you will be using in your home. As a general rule, you should plan to spend about $30 to $80 per square foot of collector area, including installation.

What Else Should I Know?

Some local building codes, zoning ordinances, and housing development covenants may have restrictions on rooftop mechanical systems, so you should make sure an active solar heating system is allowed in your neighborhood.

Where Can I Get More Information?

U.S. Department of Energy
Solar Rating and Certification Corporation