Can I switch to solar-heated water in my radiant heat system?

Connie McCullah is a principal in Odin's Hammer, a residential design-build company in Berkeley, California. She is also a national speaker on green remodeling.

Question: I heat my home with radiant heat connected to a water heater that uses propane. Is it possible to retrofit to an integrated system of solar-heated water and/or photovoltaic (PV) panels? I have good southern exposure, but winter temperatures get below 32 degrees.

– Lynn Freeman, Sandy, OR

Answer: Radiant heating is a comfortable and efficient way to heat a home, and doing so with solar power is all the better. The switch to solar thermal water heating makes the most sense if you are planning to make the transition for your home’s hot water anyway. For your radiant heating system alone, solar thermal heating could offset some of your winter energy needs, but you would still have to use the propane water heater as a backup.

Solar thermal panels would be best for your needs. (It is unlikely that you would use PV panels to heat the water, given that this type of system would be less efficient than propane. But if you wanted to switch to all-renewable power you certainly could, using the electricity from the PV panels to power an electric water heater.)

There are two types of solar thermal systems: direct or open loop, and indirect or closed loop.

• In a direct or open-loop system, cold water is piped from the home to solar thermal collectors on the roof. The thermal panel is a thin box with a glass face covering a series of dark tubes that rest against a metal backing. As sunlight passes through the glass, the heat is trapped inside the box. The water inside the dark tubes absorbs the heat, and the heated water flows back into the house to be stored in the water tank.
• If you live in a cold climate with winter freezes, it is better to use an indirect system. This type of system is similar to a direct system, but it uses food-grade antifreeze instead of water. The heated liquid is brought from the solar collector to the water tank, where the pipes loop around the water heater to transfer their heat to the tank.

Movement of the liquid through either type of system can be either active or passive. An active system uses a pump. A passive system is based on the thermosiphon principle that water rises as it heats. As the solar thermal collector pipes heat up, they draw new liquid into the system. The pipes are laid at such an angle that if there is not enough heat to draw the liquid up, then it drains back down into the house. This prevents liquid from being in the pipes if they freeze.

You will want to keep your propane heating system as a backup during mornings, evenings, and the coldest parts of winter. You can use a two-tank or one-tank system. In a two-tank system, the solar-heated water is first sent to the solar tank, then sent through the conventional water heater. In a one-tank system, the solar storage and backup heater are combined in one tank. The propane heater should modulate so it heats the water only to the degree that is needed.

For a radiant heating system you typically need to reserve 10 to 30 percent of the house's roof for solar thermal collectors. The collectors should face south and be tilted at the angle of your latitude.

Solar water heating systems generate more energy than it takes to set them up, so they are good for the environment—but are they good for your pocketbook? I'll give you some ballpark figures to help in your decision. You'll also find a link to a cost calculator below.

A solar thermal collector that provides 75 percent of the hot water needs for a family of four would have an installed cost of roughly $9,500. There is a federal tax credit for 30 percent of a system’s cost up to $2,000. A system like this would save around $288 per year, so it would take around 25 years for the system to pay for itself.

That number may not make you happy, but this one will: Comparing a solar hot water system to an electric water heater, the solar option will avoid more than 60 tons of carbon dioxide emissions over that time period.

For more information:

A calculator for estimating the cost and savings of your system

Solar water heating fact sheet (PDF) from the Energy Efficiency and Renewable Energy Clearinghouse

Solar water heating basics from the Department of Energy's Energy Efficiency and Renewable Energy program

Northern California Solar Energy Association's page on solar water heating

Sun Light and Power's page on solar hot water

Thanks to Daniel Bell, Dennis McCullah, and Dave Yarnell for their assistance with this question.

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