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Solar Hot Water

System Options

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Solar hot water systems can be designed for a variety of climates. Most systems include a solar collector, pump, controller, piping, and a back-up source. Typically, the existing conventional water heater is used as the back-up. How does it work? Collectors absorb the sun’s energy to heat either water or antifreeze fluid. In non-freezing climates, collector-heated water can be used directly. In cold climates, collector-heated fluid goes to a storage tank where its heat is transferred (given up) to the potable tank water. Once the heat transfers, the cooled fluid is pumped back to the collector to be reheated by the sun. There are several system types that prevent collector fluids from freezing, and they work well in cold climate states such as Montana and Wyoming. The system options discussed here apply to small systems such as those used for homes. Consult with a qualified solar hot water system installer regarding larger systems.


Flat plate and evacuated tube collectors are the two types commonly used for cold climate solar hot water systems. There are other collector types than can be used for warm climates and larger commercial and industrial applications.

Flat plate

Flat plate collectors are made of copper flow tubes connected to a dark absorber plate within an insulated, weatherproof box covered with hail-resistant tempered glass or plastic. Water or antifreeze can be heated by these collectors.

Evacuated tube

Evacuated tube collectors are composed of rows of parallel, clear glass tubes. Each tube has an inner tube that absorbs solar energy. Air is removed (evacuated) from the space inside the glass tubes forming a vacuum that reduces heat loss. Water or antifreeze can be heated by these collectors. They work well in cloudy conditions, but are not as hail-resistant as flat plate collectors.

Complete System

Two system types that work well in Montana and Wyoming are called “active systems” because they use pumps and other electronic equipment to move fluids and operate the system. “Passive systems” have no moving parts and rely on gravity or convection to move fluids and are typically used in warmer climates. Active systems can be Open Loop or Closed Loop.

Open Loop = Direct

Collectors directly heat the water.
– Hard or acidic water can cause scale and corrosion in tubes and pipes.
– A recirculation system pumps warm storage tank water to the collector during freezing weather, but best for mild climates.

Closed Loop = Indirect

Collectors indirectly heat water with heat-transfer fluids and a heat exchanger.

Two active, closed loop systems that work well in cold climates are:


Heat-transfer fluid = Propylene-Glycol


Heat-transfer fluid = Water

Back-Up Water Heater Systems

Solar hot water systems can generate a large portion of your hot water (especially in warmer months), but typically not 100% in colder climates. Back-up systems turn on when there are prolonged cloudy days or excessive hot water demands cannot be met by the solar system. For an existing home or building, the existing conventional hot water system can be used as the back-up for the new solar hot water system. For a new home or building, a back-up system is installed along with the solar hot water system.

An increasing number of home and business owners are considering tankless water heaters as a primary water heating system. Tankless heaters also serve as excellent back-ups for solar hot water systems. Tankless water heaters are also called on-demand or instantaneous water heaters.

Tankless/On-Demand Water Heaters

The most commonly used water heater systems entail storage tanks that keep water heated continuously whether the water is needed or not. Keeping water heated at all times can account for 15 percent or more of a water heating bill. Tankless (no storage tanks) water heaters only use energy to heat water when it is needed. This is how they conserve energy and reduce your utility bill.

How do they work?

A flow sensor detects when the hot water faucet is turned on or the warm/hot water selection is made for an appliance. For a gas-powered tankless system, the gas valve opens and the burner fires-up. The system measures the incoming water temperature and calculates how quickly the water should flow past the burner through to the faucet or appliance.

While these systems can supply a limitless amount of heated water, there are flow rate limits; thus, they must be sized for your hot water needs. An average flow rate is two to five gallons per minute.

Gas-powered units

Gas-powered units provide higher flow rates than electric powered units. If considering a gas-powered model, ask the manufacturer how much gas the pilot light uses. If you purchase one with a standing pilot light, it can be turned off when not in use to conserve energy. Another energy saving option is an intermittent ignition device. If a lot of hot water is used at the same time each day in your home or building, you might need more than one unit.

Conventional tank water heaters

While conventional storage tank water heaters are less expensive to buy (first price tag), they are more expensive to fuel and maintain (the second price tag). Storage tank heaters vary in efficiency and can last about 15 years; whereas, tankless systems are very efficient (up to 96 percent), and can last 20 or more years.

Note: Not all tankless water heaters will work with a solar hot water system, so be sure to inquire. Some manufacturers have special models that work with solar hot water systems. Also, if considering a gas-powered tankless water heater, determine if your existing gas line can handle the larger natural gas demand (based on distance to the gas meter, other gas appliances, etc.)

SRCC Certification

How can you be sure you are buying a reliable, high performance solar hot water system? Make sure to ask for and buy Solar Rating and Certification Corporation (SRCC) certified collectors and complete systems. SRCC tests performance and certifies almost every solar hot water heater on the market. SRCC is an independent, non-profit organization that determines system performance in accordance with national ratings standards. See www.solarrating.org for complete and up-to-date information.


National Renewable Energy Laboratory (produced) for the U.S. Department of Energy. (1996, March). Residential Solar Heating Collectors. DOE/GO-10096-051.

National Renewable Energy Laboratory (produced) for the U.S. Department of Energy. (1996, March). Solar Water Heating. DOE/GO-10096-050.

National Renewable Energy Laboratory (produced) for the U.S. Department of Energy. (1999, September). Solar Water Heating: Using the Sun to Heat Domestic Water Makes Sense in Almost Any Climate. DOE/GO-10099-726.

Patterson, John. (n.d.). Solar Hot Water Basics. Home Power Magazine on-line. Retrieved February 22, 2011, from: http://homepower.com/basics/hotwater/

U.S. Department of Energy. (2010, October). Demand (Tankless or Instantaneous) Water Heaters. Retrieved January 24, 2011, from: http://www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=12820

U.S. Department of Energy. (2010, October). Solar Water Heaters. Retrieved January 19, 2011, from: http://www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=12850