Solar collectors are the part of a solar water heating system that capture sunlight and turn it into heat. They are the most visible component, usually mounted on a roof or a ground rack, and their design directly affects how much hot water your household can get from the sun. The Florida Solar Energy Center (FSEC) manual from 2006 describes a solar collector’s job as collecting the sun’s electromagnetic energy and converting it to heat energy. While most active systems in the United States use flat-plate collectors, other designs such as evacuated-tube and integral collector storage units are also available, each with different strengths and limits.
How a solar collector is built
Although collectors look different from one another, most share a few basic parts. The FSEC manual breaks a typical flat-plate collector into these layers:
- Enclosure: A weather-resistant box or frame that holds everything together.
- Glazing: A transparent cover, usually glass, that lets short-wave solar energy in but blocks long-wave heat from radiating back out. It also stops wind from cooling the absorber. Multiple glazing layers increase insulation but reduce the light that reaches the absorber.
- Absorber: A flat metal surface treated with a black or spectrally selective coating that soaks up solar energy and transfers it to the fluid tubes.
- Flow tubes: Metal tubes bonded to the absorber. Water or a heat-transfer fluid flows through them, picking up heat and carrying it to the storage tank.
- Insulation: Material behind the absorber that cuts heat loss through the back of the enclosure. The FSEC manual notes that insulation must withstand very high temperatures, so ordinary Styrofoam is not suitable.
These same principles, capturing sunlight while limiting heat loss, appear in every collector type, though the materials and shapes change.
Flat-plate collectors
Flat-plate collectors are the most common design for residential solar water heating in the United States. The North Carolina Solar Center describes them as insulated, weatherproof metal boxes containing a dark absorber plate under one or more transparent covers. The FSEC manual states that flat-plate collectors are designed to heat water to medium temperatures, giving a figure of approximately 140°F as a guideline at the time of publication.
Inside the box, metal flow tubes carry the heat-transfer fluid. The glazing traps heat, and the insulation behind the absorber reduces losses out the back. According to the North Carolina Solar Center, in typical domestic hot water systems, particularly in warmer regions of the country, flat-plate collectors meet or exceed the annual efficiency of evacuated-tube collectors.
Evacuated-tube collectors
Evacuated-tube collectors use a different approach to insulation. Each tube consists of a highly tempered glass vacuum tube that acts as both glazing and insulation, surrounding an absorber surface. The FSEC manual explains that the vacuum greatly reduces heat losses, which allows these collectors to reach higher temperatures, giving a figure of approximately 300°F as a guideline in its 2006 manual.
There are two common internal designs. In a flooded evacuated-tube collector, the absorber itself forms a tube through which the heat-collection fluid is pumped. In a heat-pipe design, a flat absorber plate covers a pipe filled with a fluid that evaporates at relatively low temperatures. The vapor rises to the top of the tube, transfers heat to water in a manifold, condenses, and returns by gravity to the bottom. The North Carolina Solar Center notes that evacuated-tube collectors are the most common type internationally and can operate efficiently at much higher temperatures than flat-plate collectors, though in warmer regions the annual efficiency difference may favor flat-plate models.
Integral collector storage and batch collectors
Some collectors combine the absorber and the storage tank into a single unit. The FSEC manual calls these integral collector storage (ICS) systems. An ICS collector typically uses several horizontal metal tanks, about four inches or larger in diameter, connected in series. The tanks are coated with a selective or moderately selective absorber finish and enclosed in a highly insulated box with multiple glazing layers. Cold water enters at the bottom, warms up during the day, and exits at the top.
A batch collector is the simplest version of this idea. The FSEC manual describes it as usually one black-painted metal tank inside an insulated, glazed enclosure. It has no pumps or controls. The ENERGY STAR program advises homeowners to steer clear of batch systems if they live in a climate with distinct winters, because these collectors are vulnerable to freezing.
Unglazed collectors for swimming pools
Swimming pool heating is a different task from domestic water heating. The FSEC manual explains that desired pool temperatures in Florida range from about 72°F to 85°F, only slightly above the winter daytime mean temperature. Because the temperature difference between the water and the air is small, expensive insulated boxes and glazing are often not needed.
Unglazed collectors for pools are typically made of plastic or EPDM rubber. They have large-diameter headers and many small fluid passageways. The FSEC manual notes that cool winds above 5 mph can substantially reduce the efficiency of unglazed collectors. High water flow rates help keep the collector-to-air temperature difference low, which improves efficiency. Many manufacturers recommend a flow rate as high as one gallon per minute for each 10 square feet of collector area, though the FSEC manual points out that such high rates are not needed to keep the temperature rise below 10°F for efficient operation.
Comparing collector types
The table below summarizes the main collector types, their typical strengths, and cautions drawn from the source documents. All performance descriptions reflect what the FSEC manual and North Carolina Solar Center state about these designs.
| Collector type | Strengths per source | Cautions per source |
|---|---|---|
| Flat-plate | Proven design for medium temperatures; meets or exceeds annual efficiency of evacuated tubes in warmer regions (NC Solar Center). | Requires freeze protection in cold climates (FSEC). |
| Evacuated-tube | Reaches higher temperatures; vacuum insulation cuts heat loss; common internationally (FSEC, NC Solar Center). | May not outperform flat-plate in typical domestic systems in warmer regions (NC Solar Center). |
| ICS / Batch | Simple design with no pumps; collector and storage are one unit (FSEC). | Heavy when filled; vulnerable to freezing; ENERGY STAR says to avoid batch systems in climates with distinct winters. |
| Unglazed (pool) | Simple plastic or rubber collectors; work for the small temperature lift a pool needs (FSEC). | Cool winds above 5 mph substantially reduce efficiency (FSEC). |
Certification and ratings
When comparing collectors, independent certification helps you verify performance and safety claims. The Solar Rating and Certification Corporation (ICC-SRCC) describes itself as the leading certifier for solar heating and cooling products in North America. Its certifications cover glazed, unglazed, tubular, ICS, and other collector types. ICC-SRCC states that its certifications are designed to help manufacturers comply with building code, regulatory, and incentive program requirements in the United States, Canada, Mexico, and the Caribbean. Most U.S. building codes require compliance with ICC-SRCC solar standards for safety, and the certifications are used to qualify for federal and local incentives.
The ENERGY STAR program also certifies complete solar water heating systems. An ENERGY STAR certified solar water heating system, the program says, is generally designed for use with an electric or gas back-up water heater. You can view certified models on the ENERGY STAR product list and check for available rebates through its rebate finder.
Questions to ask an installer about collector type and freeze protection
Here are key topics to cover with a contractor before you choose a collector and system type:
- Which collector type suits my roof and climate? Ask the contractor to explain why a flat-plate, evacuated-tube, or other collector is recommended for your site’s sun exposure, roof orientation, and local weather.
- How is freeze protection handled? The FSEC manual lists five basic freeze-protection methods: indirect antifreeze fluid circulation, draining, water flow, thermal mass, and auxiliary heating. If you live where temperatures drop below 42°F, ENERGY STAR recommends choosing either a closed-loop system with antifreeze or a system that automatically drains back. Ask the installer to describe exactly how the proposed system protects the collector and external pipes.
- What roof work is needed? The ENERGY STAR program advises you to review your roof warranty and make sure the contract requires the contractor to repair any leaks or damage caused by the installation. Also ask about the roof’s ability to carry the weight of the filled collectors.
- Will the system need a tempering valve? Solar-heated water can reach temperatures high enough to scald. A tempering valve mixes cold water with the hot outlet to deliver safe temperatures at the tap. Confirm that one is included or can be added.
- What maintenance does this collector type need? ENERGY STAR recommends periodic checks of the collector’s cleanliness, glazing, fasteners, piping, and insulation, as well as the pressure relief valve and pump operation. In areas with hard water, you may need to run a descaling solution through the collector loop every few years.
Installation of a solar water heating system connected to a home’s pressurized drinking-water supply must follow local plumbing and building codes and usually requires a permit and a qualified, licensed installer. Always verify that your contractor holds the appropriate licenses and, if possible, a certification from the North American Board of Certified Energy Practitioners (NABCEP), as ENERGY STAR suggests.
Frequently asked questions
Which collector type suits a cold climate?
In areas with freezing winters, evacuated-tube collectors and indirect flat-plate systems with antifreeze are common choices. The FSEC manual explains that evacuated tubes use a vacuum to greatly reduce heat loss, while indirect systems circulate a freeze-proof fluid through the collector and a heat exchanger. ENERGY STAR recommends a closed-loop system with antifreeze or a drainback system if temperatures are likely to fall below 42°F.
Can I use a pool collector to heat my household water?
Unglazed pool collectors are designed for small temperature lifts, typically raising water from ambient air temperature to comfortable swimming temperatures. The FSEC manual describes them as low-temperature collectors and explains that pools can skip insulated boxes and glazing only because the temperature lift is small. Domestic hot water systems in the sources use glazed flat-plate, evacuated-tube or ICS collectors.
How do I know a collector meets safety and performance standards?
Look for ICC-SRCC certification. ICC-SRCC states that its certifications are used to comply with U.S. building codes and qualify for incentives. For complete systems, the ENERGY STAR label indicates that the system meets energy-efficiency criteria and is designed for use with a backup water heater.
For more detail on how collectors fit into a full solar water heating setup, see our solar collectors category page, the system components overview, and the article on solar water heater components explained.
