Solar Water Heaters
Solar water heaters—also called solar domestic hot
water systems—can be a cost-effective way to
generate hot water for your home. They can be used
in any climate, and the fuel they use—sunshine—is
free.
How They Work
Solar water heating systems include storage tanks
and solar collectors. There are two types of solar
water heating systems: active, which have
circulating pumps and controls, and passive, which
don't.
Most solar water heaters require a well-insulated
storage tank. Solar storage tanks have an additional
outlet and inlet connected to and from the
collector. In two-tank systems, the solar water
heater preheats water before it enters the
conventional water heater. In one-tank systems, the
back-up heater is combined with the solar storage in
one tank.
Three types of solar collectors are used for
residential applications:
•Flat-plate collector
Glazed flat-plate collectors are insulated,
weatherproofed boxes that contain a dark absorber
plate under one or more glass or plastic (polymer)
covers. Unglazed flat-plate collectors—typically
used for solar pool heating—have a dark absorber
plate, made of metal or polymer, without a cover or
enclosure.
•Integral collector-storage systems
Also known as ICS or batch systems, they feature one
or more black tanks or tubes in an insulated, glazed
box. Cold water first passes through the solar
collector, which preheats the water. The water then
continues on to the conventional backup water
heater, providing a reliable source of hot water.
They should be installed only in mild-freeze
climates because the outdoor pipes could freeze in
severe, cold weather.
•Evacuated-tube solar collectors
They feature parallel rows of transparent glass
tubes. Each tube contains a glass outer tube and
metal absorber tube attached to a fin. The fin's
coating absorbs solar energy but inhibits radiative
heat loss. These collectors are used more frequently
for U.S. commercial applications.
There are two types of active solar water heating
systems:
•Direct circulation systems
Pumps circulate household water through the
collectors and into the home. They work well in
climates where it rarely freezes.
•Indirect circulation systems
Pumps circulate a non-freezing, heat-transfer fluid
through the collectors and a heat exchanger. This
heats the water that then flows into the home. They
are popular in climates prone to freezing
temperatures.
Passive solar water heating
systems are typically less expensive than active
systems, but they're usually not as efficient.
However, passive systems can be more reliable and
may last longer. There are two basic types of
passive systems:
•Integral collector-storage passive systems
These work best in areas where temperatures rarely
fall below freezing. They also work well in
households with significant daytime and evening
hot-water needs.
•Thermosyphon systems
Water flows through the system when warm water rises
as cooler water sinks. The collector must be
installed below the storage tank so that warm water
will rise into the tank. These systems are reliable,
but contractors must pay careful attention to the
roof design because of the heavy storage tank. They
are usually more expensive than integral
collector-storage passive systems.
Solar water heating systems
almost always require a backup system for cloudy
days and times of increased demand. Conventional
storage water heaters usually provide backup and may
already be part of the solar system package. A
backup system may also be part of the solar
collector, such as rooftop tanks with thermosyphon
systems. Since an integral-collector storage system
already stores hot water in addition to collecting
solar heat, it may be packaged with a demand (tankless
or instantaneous) water heater for backup.
For more information about solar water heating
system components, see the following information:
•Heat Exchangers for Solar Water Heating Systems
•Heat-Transfer Fluids for Solar Water Heating
Systems
Selecting a Solar Water Heater
Before you purchase and install a solar water
heating system, you want to do the following:
•Consider the economics of a solar water heating
system
•Evaluate your site's solar resource
•Determine the correct system size
•Determine the system's energy efficiency
•Estimate and compare system costs
•Investigate local codes, covenants, and
regulations.
For information about specific solar water heater
models and systems, see the Product Information
resources listed on the right side of this page (or
below if you've printed the page).
Installing and Maintaining the System
The proper installation of solar water heaters
depends on many factors. These factors include solar
resource, climate, local building code requirements,
and safety issues; therefore, it's best to have a
qualified, solar thermal systems contractor install
your system.
After installation, properly maintaining your system
will keep it running smoothly. Passive systems don't
require much maintenance. For active systems,
discuss the maintenance requirements with your
system provider, and consult the system's owner's
manual. Plumbing and other conventional water
heating components require the same maintenance as
conventional systems. Glazing may need to be cleaned
in dry climates where rainwater doesn't provide a
natural rinse.
Regular maintenance on simple systems can be as
infrequent as every 3–5 years, preferably by a solar
contractor. Systems with electrical components
usually require a replacement part after or two
after 10 years. For more information about system
maintenance, see the following:
•Solar Water Heating System Maintenance and Repair
•Solar Water Heating System Freeze Protection
•Scaling and Corrosion in Solar Water Heating
Systems
When screening potential contractors for
installation and/or maintenance, ask the following
questions:
•Does your company have experience installing and
maintaining solar water heating systems?
Choose a company that has experience installing the
type of system you want and servicing the
applications you select.
•How many years of experience does your company have
with solar heating installation and maintenance?
The more experience the better. Request a list of
past customers who can provide references.
•Is your company licensed or certified?
Having a valid plumber's and/or solar contractor's
license is required in some states. Contact your
city and county for more information. Confirm
licensing with your state's contractor licensing
board. The licensing board can also tell you about
any complaints against state-licensed contractors.
For contractor information, see the Professional
Services resources listed on the right side of this
page (or below if you've printed it out).
Improving Energy Efficiency After your water heater is properly installed
and maintained, try some additional energy-saving
strategies to help lower your water heating bills,
especially if you require a back-up system. Some
energy-saving devices and systems are more
cost-effective to install with the water heater.
Other Water Heater Options
•Conventional storage water heaters
•Demand water heaters
•Heat pump water heaters
•Tankless coil and indirect water heaters
