WATER HEATING BASICS
solar water heating
Take the load off your water heater.
Edited by Diana Young, Liz Merry and Seth Masia
The first efficient solar water-heating system-
was patented in 1891 by Clarence Kemp of
Baltimore. His Climax heater consisted simply of a
network of black-painted pipes in an insulated box,
behind a sheet of glass. It was adopted nationwide
— by 1900, 1,600 Southern California rooftops
were so equipped.
Today, sunny jurisdictions like Israel, Spain and
Hawaii have laws requiring domestic solar hot water
(DSHW) systems on new homes. Almost 75 percent
of homes in China have solar water heating.
A complete DSHW system can be installed for $3,000
to $10,000, depending on its size and complexity. At
today’s energy prices, over the life of the system, the
cost is about 20 percent lower than a conventional
gas water heater and 40 percent lower than an elec-
tric one. As gas and electricity prices rise, DSHW will
look like a better and better deal. The Wall Street
Journal reported last spring that the price of natural
gas had risen 93 percent since the previous August.
The price of a system depends on two main factors:
its size and the climate. A large family uses a lot of
hot water for bathing, laundry and dishwashing,
while a couple with no kids living at home can use a
much smaller system.
and hot tubs
One of the most common uses for solar
energy is to heat pool water. Pool solar
collectors are lighter in weight and less
expensive than domestic solar water
-heating systems. The size should be 50
to 100 percent of the surface area of
the pool. The more solar collector area,
the warmer the pool will be into cool
weather. The pool serves as the storage
tank, and the filtration pump circulates
the pool water through the collectors.
A solar collector can provide all the
heating necessary for a swimming pool,
but hot tubs and spas need a backup or
In warm climates, a simple passive system can pro-
vide plenty of hot water.
In freezing climates, the rooftop part of the system
must be protected either by draining down when the
temperature dips, or by running an antifreeze solu-
tion. Both cold-weather systems require the use of
temperature sensors, electric pumps and automatic
control systems. These add complexity and cost to
Domestic solar water heating can also
be used for space heating. The most
efficient method is an in-floor radiant
heating system that sends hot water
through pipes embedded in the floor.
A gas backup water heater is typically
used because a house is coldest at night
and in winter months. The solar collector area needed is generally 10 to
30 percent of the home’s floor area,
depending on climate.
When a flat plate collector can sit close to the
roof, wind loads are minimized and pipes are
Bu TLER SuN SOLu TIONS
Solar water-heating systems come in two flavors:
passive and active.
Passive solar water-heating systems
Passive systems are installed in areas where freeze
protection is not an issue. The most common types
are integral collector storage (ICS) and thermosiphon.
Recent years have seen the widespread adoption of
evacuated tube systems.
In an ICS (or breadbox) system, cold city water flows
into a rooftop collector. The collector holds 30 to 50
gallons of water in a serpentine pipe with a heat-
capturing coating. Hot water from the collector flows
directly to a conventional water heater; in effect the
sun does most of the work usually performed by the
water heater’s burner. As hot water is withdrawn
from the water heater, cold water is drawn into the
collector, driven by pressure in the city water pipes.
A thermosiphon takes advantage of the fact that
water rises as it’s heated. It uses a flat-plate collec-
tor. Solar-heated water in the collector rises through
tubes and flows into the top of an insulated storage
tank. Colder water at the bottom of this tank is drawn
into the lower entry of the solar collector. Water thus
flows in a continuous loop, continually reheated dur-
ing daylight hours. When a hot-water tap is opened
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