How Solar Hot Water Heaters Work
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How Solar Hot Water Heaters Work

How solar hot water heaters work, their basic components, and finacial incentives.

Most people think of electricity when they hear the words “solar energy.” However a more practical and economical way of harnessing the sun's energy for home use is for domestic hot water heating. Water-heating is the third-largest energy expense in most households, after space heating and air-conditioning, according to the U.S. Department of Energy. Hawaii has begun mandating solar water heaters in most new homes in 2010, but even colder locations such as New York and Colorado have seen an increase in solar hot water heating and are leading the nation for installations.

Besides passive solar where you use sunlight to warm up dense materials in the home, hot water heating is the most efficient way to use solar.

Basic Elements

Most systems consist of a set of solar collectors mounted on the roof of the home, a pump, heat exchanger, and a storage tank. The collectors need a portion of south-facing roof or other location on grade with minimal shading from 11 AM to 3 PM. Two to three collectors are usually enough for most single-family homes. You should use a tool, such as the Solar Pathfinder, ( ) to measure the amount of daily energy your collectors will get from the sun year-round. This will help you size your system as well.

Most of the pre-built collectors look like dark skylights and are not unattractive.

The storage tank is typically 80 gallons, and the water temperature can be as high as 180 degrees. For this reason the needs to be a tempering valve installed on the outlet to reduce the temperature to 120 degrees for safety concerns.

Many companies also supply solar powered pumps to circulate the hot water through the collector and heat exchanger. The heat exchanger is a copper coil that keeps the collector water, or glycol, separate from the domestic water. The storage tank will have a temperature sensor on the will tell the pump to turn off and on.

In most regions, a backup heat source is needed for overcast days, such as a gas boiler that is used to provide home heating or an electric or gas heating element integrated directly into the solar tank or a separate on-demand water heater. Some larger homes with more space and greater hot water needs use two tanks—one as a solar preheat tank that feeds water into a second tank warmed by gas, oil or electricity. Since the temperature entering the gas or electric heater is warmed, the fuel usage is greatly reduced.

Even in very cold climates, on sunny days, solar water heaters can heat water stored up to 100 degrees. In the spring and summer, temperatures of 120 to 140 degrees are common.

Financial Incentives

Most systems can qualify for a 30% federal tax credit, so long as the collectors are certified for performance and durability by the not-for-profit Solar Rating and Certification Corp., . There also are state tax and rebate incentives, which may have additional requirements. Check the Database of State Incentives for Renewables & Efficiency at .


A directory of installers can be found at and at run by the American Solar Energy Society.


Fafco Inc.

Heliodyne Inc.

Velux USA


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Comments (2)

We use them over in Cyprus, they are used by every householder. I will impress DH now with what I have just learned here, thanks.

I haven't heard of solar hot water heaters before. We have been checking in on one of them waterless hot water heaters that heat just when we need it too. Things are changing so fast I can hardly keep up with them. Vote up