Harnessing sunshine may reduce your water heating bill, but visit the options first to find the best system.
Q: With two teenage daughters who take long showers, our water heating costs are high. Does using solar water heating make sense? What are my solar options, and is there a system I can make myself?
A: For a typical family of four, water heating can account for about 20 percent of annual utility bills. With family members taking long showers, yours may be a bit higher, but don’t expect a solar system to cut your costs to zero. A target savings of 50 percent often provides a good economic payback.
Before considering solar or any other efficient water heating, install low-flow shower heads with shut-off tickle (lathering) valves, and talk with your family about taking shorter showers.
The two basic systems are “active” and “passive.” Active systems need a storage tank and electric pumps and controls to function. Sometimes 12-volt pumps can be powered by a photovoltaic solar panel located near solar water heating collectors on the roof.
In cold climates, the system must have some type of antifreeze fluid and a heat exchanger so it doesn’t freeze in winter. Other systems that circulate actual potable water through the collector need a draining system to empty the collectors at night during winter.
Passive systems rely on the natural upward flow of less-dense warm water to move the water through the solar collector. With these systems, the warm water storage tank is located above the collector—usually on the roof or in the attic, so there are structural considerations. These cost less than more sophisticated active systems, but they tend to be less efficient, especially in cold weather.
There are many types of solar collectors. The best one for your house depends on your climate, hot water needs, and budget. They can be as simple as black copper tubes in an insulated box with a glass top, to those with vacuum tubes, concentrating reflectors and heat pipe technology. Discuss the various types with a solar contractor.
Unless you are an accomplished craftsman, I suggest building a passive system. Trying to build an active system—with collectors on the roof, plumbing and control systems, and storage tanks—is beyond the skill level of most homeowners. I am a design mechanical engineer, and I doubt I could build a system myself from scratch.
If you do choose the active system, get one with an OG-300 rating from the Solar Rating and Certification Corporation (solar-rating.org). A knowledgeable, qualified installer is important, too—look for contractors certified by the North American Board of Certified Energy Practitioners (nabcep.org). Also check the Database of State Incentives for Renewables and Efficiency (dsireusa.org) for local solar installation incentives, in addition to the federal tax credit. Be sure to review the requirements on system types, size, certifications, installers, and other details to make sure yours qualifies.
Otherwise, try building a passive “batch” system, which is a pre-heater for your existing water heater, with the simplest design called a “breadbox.” It uses a horizontal metal water tank inside a box with a clear top. The sun shines through to heat the water. A slightly more efficient option uses a tall box angled toward the sun. This allows the warmer water to be drawn first from the top of the tank.
You can buy a stainless steel water tank that’s designed for this application with inlet and outlet water fittings. If you can find an older water heater that’s not leaky, strip off the metal skin and insulation to use the inner tank. Paint it flat black to absorb more of the sun’s heat.
It also helps to insulate the solid sides and bottom of the box, especially if you plan to use it most of the year. Very heavy insulation is not needed, as the tank will not get extremely warm, especially if you use hot water throughout the day. One-inch-thick foil-faced rigid foam sheets should be fine. Attach them inside the box to reflect the sun’s heat to the tank.
Install water valves and plumbing so the tank can be drained and bypassed during cold weather. Install heavy insulation around any exposed pipes and bury as much as possible underground.
James Dulley is a nationally recognized mechanical engineer writing about home energy issues for the National Rural Electric Cooperaive Association. If you have a question for James, please email firstname.lastname@example.org, or mail to James Dulley, Michigan Country Lines, 2859 W. Jolly Rd., Okemos, MI 48864. Be sure to let us know which electric co-op you receive service from. Visit dulley.com for more home improvement and do-it-yourself tips.