Solar water heaters explained

A solar water heater, sometimes called a solar domestic hot water system, uses the sun's heat to warm water for the house. Each design has a collector and a storage tank. It works in any climate. An ENERGY STAR certified system can cut a household's yearly hot water costs about in half. On average, a home's water-heating electricity drops by at least half, and by more in a sunny place such as the Southwest.

A solar water heater suits a house that heats water with electricity, pays high electric rates and has an unshaded, south-facing spot such as a roof. The collector works best facing true south.

Active and passive systems

• Active systems use electric pumps, valves and controllers to move water from the collectors to the tank. They're the common kind in the United States.
• Passive systems have no pumps. Natural convection moves the heated water from the collectors to the tank. A thermosiphon system is passive: water heated in a collector, usually on the roof, rises into the storage tank. These simple systems suit warmer coastal climates.

Direct and indirect circulation

• Direct systems run household water through the collectors. The heated water is stored in a tank, sent to a tankless water heater, or used straight away. They suit climates where it rarely freezes, and need freeze protection in cold ones.
• Indirect, or closed-loop, systems heat a fluid that won't freeze in the collectors. It passes through a heat exchanger in the tank, warms the water, and cycles back. They make sense where it freezes.

The collectors

• Batch collectors, also called integrated collector-storage, heat water in dark tanks or tubes inside an insulated box and hold it until it's drawn. When little hot water is used, the water can get very hot, so a tempering valve mixes in cold water to prevent scalding at the tap. They can't work with closed-loop systems, so they're generally not recommended for cold climates.
• Flat-plate collectors are copper tubes on flat absorber plates in an insulated box under tempered glass. One typically holds 40 gallons, and two supply roughly half the hot water a family of four needs.
• Evacuated-tube collectors are the most efficient. Each tube sits inside a larger glass tube with a vacuum between them, like a thermos. They work in overcast weather and down to -40°F, and a failed tube can be replaced on its own. They can cost twice as much per square foot as flat-plate collectors.

Backup and upkeep

A solar water heater usually works with an electric or gas backup water heater, which may come with the system. A system should be certified, and installed to local plumbing and building codes, with a roof mount that keeps water out of the roof. Look for the ENERGY STAR label, or certification by the Solar Rating and Certification Corporation or the Florida Solar Energy Center. A simple system may need servicing only every 3 to 5 years, preferably by a solar contractor.

An annotated example

What to do next

If the house has a solar water heater, find its owner's manual and set a reminder to have a solar contractor service it every 3 to 5 years.