Solar Water Heaters

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Energy Source	The Sun, with Gas or Electric Backup
Minimum Efficiency Recommended	Not available
Maximum Efficiency Available	Not available
Expected Life	20 years
Approximate Cost To Install	$2,000-$4,000

Using the sun to heat water can be cost-effective and environmentally friendly. Solar water heating is generally more sensible for families that use a lot of hot water. Although it can work anywhere, it is more cost-effective at lower latitudes and in sunnier climates.

A solar water heater needs a backup system. One easy way to provide backup is to add a solar system to an electric or gas storage water heater. A tankless heater can also be used for backup.

There are active and passive solar water heaters. The simplest systems are passive, using nothing but solar energy and gravity to circulate water between the storage tank and the collector, where the water heats up. As water in the collector heats, the hotter water rises into a storage tank placed slightly above the collector, while cooler water runs down to replace it. Active systems are generally more reliable than passive ones, and they can be put in more places. They usually have a pump to move the water from the collector to the storage tank, so the collector can be on the roof, in the yard, or wherever is convenient.

Solar hot water systems can be vulnerable to freezing. If outdoor water lines freeze, their piping can be destroyed. Open-loop systems, which run tap water through the system, need to be protected. But no matter how well protected they are, temperatures below 35°F will keep these systems from functioning. A closed loop, which runs antifreeze or air through the outdoor pipes and then transfers the heat to the tap water, will keep the pipes from freezing.

Special Features

Open-loop solar systems circulate tap water directly through the collector and store it in the tank. This works best with water that is not hard or acidic. Hard or acidic water corrodes the copper pipes of the collector.

Closed-loop systems circulate heat transfer fluid, instead of tap water, through the collector. This fluid may be treated water, air, antifreeze solution, or a special oil. It picks up heat in the collector and transfers it to water. These systems are easier to install, but more expensive to operate and maintain.

One of the best ways to get a solar system installed is through a utility program. Vendors and service people who are selling many units in cooperation with a utility program are likely to survive long enough to repair your system if it ever gives you trouble.

A tempering tank is the simplest solar hot-water system. A tank in a warm or sunny area such as an attic or sunroom is hooked up to a water heater. Before cold water goes to the water heater, the tank prewarms it to room temperature. The water heater then has less work to do on each gallon of water, improving its recovery rate and cutting fuel use. Tempering tanks are usually put in attics in hot climates.

Precautions

Solar hot water boomed in the 1970s. But when federal tax credits ended, the industry faced a bust. Fly-by-night contractors went bankrupt, and thousands of homeowners were left with poorly designed, hard-to-maintain systems. Today's installers are generally better, but you still need to ask for references. Any system may need maintenance or new parts, so you want a contractor who is well established and will be around for a while.

Sizing

Most solar systems are designed to meet one-half to three-quarters of a family's hot-water needs, or all of their summer needs. The remainder is supplied by the backup system.

Your contractor will need to size the collectors, the storage tank and the backup system. A rule of thumb is that an efficient collector in good weather will heat between 1 and 2 gallons of water per square foot per day. So if your family uses 64 gallons of hot water per day, you would need between 16 and 32 square feet of collector to supply half of the household's needs.

Excerpted with permission from No-Regrets Remodeling by Home Energy (1997)