Solar Heat

Solar Collectors Can Heat Water and Living Spaces

UPDATED 9/29/2011

Bird's eye view

Solar space heating systems require a lot of equipment

Solar hot-water systems use pumps to circulate fluid through a series of solar hot water collectors. Heat is stored in a large insulated tank or series of tanks and can be used for hot water supply, space heating, or both. Solar space heating can be incorporated into radiant or forced air systems.

See below for:

System Types

Solar collectors for both warm and cold climates

Active Solar Space Heating Systems consist of two types: solar hot water systems and solar hot air systems. Solar hot water systems are far more common.

Solar hot water systems are either active or passive.

  • Active solar hot water systems pump water or a solution of water and antifreeze through a collector and transfer the heat to a storage tank. In some systems, the pump is powered by a small photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. panel. To prevent the water from freezing, the system either includes glycol antifreeze in the solar loop, or includes a drainback mechanism that drains water out of the collector when temperatures get dangerously low.
  • Passive solar hot water systems are used to heat domestic hot water in warm-weather regions. They don’t have electric pumps to move water around. The collectors heat the water, and when a tap is opened, the hot water is piped directly to the point of use or to a storage tank. More sophisticated systems use the principle of thermosiphoning to move water from the collector to a storage tank.
  • There are three types of solar thermal collectors. Flat-plate and evacuated-tube collectors are used for heating water, and hot-air collectors are used for heating air.

  • Flat-plate collectors are glazed, insulated boxes containing dark-colored tubing (usually copper) attached to a dark, heat-absorbing metal plate.
  • Evacuated-tube collectors are made up of parallel rows of glass tubes. Inside each tube, a second tube contains a heat absorber. The air between the two tubes is removed (evacuated) to reduce heat loss. Evacuated-tube collectors are more expensive than flat-plate collectors, but are more effective in cloudy weather.
  • Solar hot-air collectors are glazed insulated boxes housing a black-painted metal baffle with air passages on both sides of the baffle. These collectors are used to heat air for a solar hot air system.

  • Design Notes

    Placing solar collectors and storage tanks

    First, implement passive solar design principles. Passive features should always be included before considering an active solar thermal system. For more information, see Cost-Effective Passive Solar Design.

    Design the home with solar panels in mind. If solar panels will be mounted on the house, this should be integral in planning the size and orientation of the roof. Choice of roofing materials is important too. For instance, solar panels can often be clamped to standing-seam metal roofs, reducing the number of roof penetrations that could compromise the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials.. Combined plans for active and passive solar heat can make glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. and roof layout a challenge. Mounting panels on structures below or away from the southern windows could solve this dilemma.

    Solar space-heating collectors are installed at a steeper angle than solar domestic hot water collectors to optimize heat collection during winter months. Most design guides recommend that collectors for space-heating systems be installed at an angle equal to the latitude plus 15 degrees.

    Since a well designed active solar space-heating system requires extensive collector area, such a system is best installed on a house with an extensive (and steep) south-facing roof.

    Have a plan to handle summer overheating of active solar thermal heating systems. Strategies include the use of a swimming pool as a dump load, using deciduous plants for seasonal shade or installing the solar collectors vertically to minimize solar exposure during summer months.

    Heat storage is as important as heat collection. An active solar thermal space heating system usually requires between 240 and 5,000 gallons of thermal storage capacity. The largest residential solar storage tank available has a capacity of 120 gallons, since larger tanks don't fit through a 36-inch-wide door. Designers of solar space heating systems have four choices:

  • Specify several 120-gallon tanks (an expensive but simple solution).
  • Specify a large, thoroughly insulated tank that can be buried near the house (for example, a recycled stainless-steel dairy tank).
  • Specify a collapsible EPDM-lined tank manufactured by STSS, Mechanicsburg, PA (these tanks fit through standard door openings but expand to much larger sizes).
  • Specify a site-built concrete tank (research and careful planning are important to get a leak-proof installation).
  • Even with plenty of storage capacity, most solar heating systems should have some sort of backup. This is especially true of higher temperature domestic hot water systems.

    Builder Tips

    Size of collectors varies

    The size of the collector required for an active solar space-heating system depends on climate, the building's heat load, and the intended percentage of that heat load that a designer aims to meet with solar energy. One rule of thumb suggests that the solar collector area should equal 12% to 30% of the home's floor area.

    Although recommendations vary by climate, designs usually call for 1.5 gallons of storage for each square foot of collector area.

    Code Issues

    Make sure the roof can carry the load

    The 2006 International Residential Code requirements for solar heating equipment can be found in Chapter 23, Section 2301. The section requires that collectors, pumps and controls be accessible for inspection, maintenance and repair (2301.2.1) and that the roof be structured to handle the weight and wind loads associated roof-mounted equipment (2301.2.2). Thermal systems must have a pressure and temperature relief valve and a vacuum relief valve if the system is subject to pressure drops. Winter design temperatures can be found in Table 301.2 (1).

    Section M2301.2.9 requires solar hot-water systems to be equipped with a means to limit the maximum temperature of solar collector fluid in pressurized heat-exchanger vessels located inside the dwelling to 180° F.
    Collectors and storage units must be labeled with the manufacturer’s name and include relevant specifications like minimum/maximum operating temperatures, compatible heat-transfer fluids and weight (2301.3).


    PV versus solar thermal

    A photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. system can be more cost-effective than a solar thermal system. Although a solar thermal system may collect more energy than a similarly priced PV system, the solar thermal energy is less usable. The bulk of the energy collected by a solar thermal system is collected in June and July, when it is least wanted. In contrast, any excess PV power can be absorbed by the grid and credited to a homeowner's account.


    Most systems not cost effective

    Solar hot-air systems use blowers to pull air through a series of solar hot air collectors. Heat is usually stored in a bin full of rocks. In the 1970s and 1980s, many solar experimenters built solar hot-air systems with leaky ductwork and inefficient blower motors. Although newer systems that pay close attention to duct sealing and blower motor efficiency may perform better, they are still unlikely to be cost-effective.


    LEED for HomesLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. Solar water heating is wrapped into the energy modeling, and the score is correspondingly reflected in EA1.2 (Energy & Atmosphere) — HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. Index everywhere but CA; % better than Title 24 in CA. Using the prescriptive pathway through the EA credits (i.e., not modeling), it shows up in EA7.3 (Efficient DHW Equipment), with up to 3 points available.

    NGBSNational Green Building Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions. Under Ch. 7 — Energy Efficiency: up to 20 pts. for solar water heating (704.3.2.1).


    Solar potential varies around the country

    In many U.S. climates, a solar hot water system is a sensible way to heat water for showers and laundry. However, it usually makes less sense to use a solar thermal system for space heating.

    The performance of an active solar space heating system depends on the equipment installed, the climate, the solar potential of the site, how well the house is insulated and air-sealed, and the type of heat distribution system in the house.

    The National Renewable Energy Laboratory says homeowners can expect to meet up to 40% to 60% of their winter space-heating needs with solar collectors; in many climates, however, performance is likely to be lower.

    An active solar thermal space heating system must be larger than the typical solar domestic hot water system, which typically requires two collectors and a 120-gallon storage tank. That means that a solar heating system requires between 4 and 20 solar collectors, and between 240 and 5,000 gallons of thermal storage capacity. Larger systems can provide a larger percentage of a home's heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load..

    The typical cold-climate solar thermal system circulates a solution of glycol (antifreeze) and water through a series of solar collectors. Most solar collectors measure 4 ft. by 8 ft. or 4 ft. by 10 ft. When the sun is shining, a pump circulates the solar fluid from the collectors to a heat-exchange coil immersed in the solar storage tank. To heat the house, the water in the solar storage tank can be circulated through hydronic tubing to baseboard radiators, to a radiant floor, or to a fan-coil unit and ductwork. A wide variety of heating appliances can be used to supply backup heat on cloudy days.

    Solar hot-air collectors are simpler than solar hot-water collectors. A solar hot-air collector consists of an insulated box, painted black on the interior, with a glazed top. Most solar hot-air collectors include a black-painted metal baffle designed to allow air to flow on both sides of the baffle.

    Domestic hot water. If the solar thermal system is used only for domestic hot water, a smaller storage tank — usually 80 to 120 gallons — suffices. The solar tank usually acts as a preheater, installed in line with a conventional water heater — a tank-type electric or gas heater, or an on-demand heater — that provides backup on cloudy days.


    Install passive features first

    Before recommending an active solar thermal heating system, a designer should be sure that a home includes all the basic passive solar features: an impeccable building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials., a plan with the long axis oriented east/west, well designed roof overhangs, window placement that favors south-facing windows, orientation-specific glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill., and extra thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. if necessary. (For more information on passive solar design principles, see Cost-Effective Passive Solar Design.)

    There are two basic ways to store heat from an active solar thermal system. Solar hydronic systems store heat in an insulated water tank. The largest commonly available tank size is 120 gallons, since larger tanks won't fit through a 36-inch-wide door. Although a 120-gallon tank is usually adequate for a solar domestic hot water system, it's too small for a space heating system.

    Solar hot-air systems usually store heat in a bin full of rocks. Solar hot-air systems are simple and robust, and can work well, although they are rarely cost-effective. For best performance, solar hot-air systems must have perfectly sealed ductwork and energy-efficient blowers.

    A radiant-floor system can be an ideal way to distribute solar heated water. Radiant floors use lower water temperatures (100°F to 120°F) than conventional hydronic systems with baseboard radiators (160°F-180°F), so a solar heating system with radiant floors can distribute useful heat even when the temperature of the stored water is too cool for baseboards. Another option is to install a hydronic coil in an air handler or in the plenum of a conventional furnace.

    Active solar space heating systems range in price from about $15,000 to $30,000.


    Hot water for showers and baths

    Many people in the U.S. can produce up to half their domestic hot water needs needs with a solar hot-water system. Solar hot-water systems can be either active or passive. Most solar hot-water systems use a pump to move fluid through one or two collectors. Thermosiphon systems, suitable for simple domestic hot water systems in frost-free climates, rely on natural convection to move water.

    A solar hot-water system is cheaper than a solar space heating system but still costs considerably more than a than conventional water heater — typically between $6,000 and $9,000. Homeowners should budget for regular maintenance, including periodic checks on the pH of the system’s glycol solution.

    An electric resistance or gas water heater should never be used as the only tank in a solar hot-water system. In a solar storage tank, water stratifies by temperature; cool water from the bottom of the tank is pulled up to the collectors to be heated and is returned to the top of the tank. The colder the incoming water, the higher the solar collector's efficiency. In a single-tank system, the electric element or gas flame keeps the stored water hot, leaving no cold water available to send to the solar collectors.

    In some cases, credits or rebates that defray some of the cost of installing a solar water heater are available from local utilities or state and federal governments. Solar systems offer stability and predictability when it comes to energy costs and supply. That’s not something that any system running on fossil fuels can match.


    Visit for estimating costs and paybacks.

    Visit for specifics on solar collectors and systems by manufacturer.

    Visit the Florida Solar Energy Center ( for examples of different solar systems.

    Image Credits:

    1. John Hartman / Fine Homebuilding
    2. Charles Miller/Fine Homebuilding
    3. Eric Doub
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