My company has been designing and installing alternative
energy systems since 1975; to date, we've installed more than
1,000 solar water heating systems. Lately, as energy prices
rise at an ever-accelerating rate, we've seen renewed interest
in solar hot water.
A simple solar hot-water system can be installed for as little
as $4,000, while a larger and more complicated freeze-protected
system can cost as much as $10,000. Thanks to the recently
signed energy bill, the customer's cost is about to fall; the
bill grants a 30 percent federal tax credit to property owners
who install solar energy systems in 2006 and 2007 (see
"Incentives Sweeten Energy Bill for Builders and Homeowners,"
In the News, in this issue).
Homeowners can claim a credit of up to $2,000; for business
owners, there is no cap on the credit.
In areas where natural-gas water heaters are the norm, the
payback period for a residential solar water heating system
will be 9 to 12 years if the value of the credit is included.
Because a well-designed system will last 20 to 30 years, it
should pay for itself two to three times over. If the system
saves $250 worth of natural gas the first year and gas prices
escalate 5 percent per year, then it will save almost $17,000
over 30 years. If the homeowner is currently using an electric
water heater, the savings may top $40,000.
On the environmental side, heating water with the sun will
typically reduce a home's greenhouse gas emissions by 18 tons
over the life of the system. With an extra 60 to 120 gallons of
hot water at their disposal, homeowners can take long,
guilt-free showers. We often equip our solar hot-water systems
with valves that allow the customer to completely shut off the
backup heater and use only solar-heated water in the
Solar Water Heating Basics
All solar water heating systems contain collection, storage,
and transfer components; some systems combine all three into a
single element. Most systems are designed to preheat water that
goes to a backup heater — typically a conventional gas or
electric water heater. A tankless heater will also work as a
backup, as long as it is designed to accept hot-water input
(not all of them are).
Although there are some systems in which the preheated water
flows directly to the backup, it's more common for the
preheated water to be stored in a separate storage tank
upstream from the backup. In hot, sunny weather, the backup is
rarely if ever needed, but during cloudy periods it may have to
provide virtually all the domestic hot water.
Collection. The most visible part of any
solar hot-water system is the collector. There are three main
types of collectors, but all basically consist of a black
collecting surface that transfers heat to a fluid. The
collecting surface is typically enclosed in an insulated
aluminum box with clear glazing to trap the heat.
A flat-plate collector — which is about 3 inches thick
— contains a grid of copper tubing attached to an
aluminum or copper plate (see Figure 1). Both components have a
black surface coating; when sunlight hits the plate, heat is
conducted to the fluid inside the tubing. Sensors measure the
temperature in the collector, and when it's hotter than the
fluid in the system's storage tank, an electronic controller
activates a pump to move the heated fluid to the tank. The
uninstalled cost of a 4-by-8-foot flat-plate collector is
Flat-plate collectors range in size from
3 by 6 feet to 4 by 12 feet and are light enough to be carried
by hand (left). Because the tubing inside is of small diameter,
even a large collector might contain only a gallon or two of
An evacuated tube collector is similar to a flat-plate
collector except that the heat-absorbing tubes are housed in a
series of evacuated glass cylinders. The vacuum insulates
against heat loss in the same way that a thermos bottle does.
Evacuated tube collectors are extremely efficient, but cost
about twice as much as conventional flat-plate
In an integral collector storage — or ICS — unit,
water is heated and stored in a series of interconnecting tubes
in a roof-mounted box. Sometimes called batch heaters, ICS
systems are simple and inexpensive because they require no
pumps or controls. However, since they store water in an
exposed location, they are subject to high heat loss and
freezing (Figure 2). A 42-gallon ICS unit costs about $2,100
Figure 2.Integral collector storage — or ICS
— units contain 4-inch-diameter pipes in which water is
heated and stored (top left). Because of their weight, they are
typically craned into place (top right). Potable water enters
through a pipe at the lower end, is heated by the sun, and
exits through the upper end (bottom) when a hot-water tap is
Storage. Solar energy is available
only for the six to 10 hours that the sun is out, so heated
water must be stored for later use. While ICS systems store hot
water right in the collector, most other systems keep it in a
separate storage tank located upstream from the backup heater.
Because the tank has to hold an entire day's worth of hot
water, it is larger than a conventional heater.
There are some systems that send solar-heated water directly to
the backup, but I'm not a fan of doing this with a conventional
backup heater. Because it's too small to hold an entire day's
worth of water, this kind of heater will short-cycle and heat
the water before the sun has a chance to do its job.
In pumped systems, storage takes place in a pressurized steel
tank that resembles an electric hot-water heater. Usually
located near the backup heater, this storage tank connects to
the collectors with copper pipes (Figure 3).
Figure 3.An installer plumbs an 80-gallon storage
tank for a pumped system (left). The tank has taps for supply
and return lines to and from the collector, plus a cold-water
supply inlet and a hot-water outlet to the home
ICS and thermosiphon systems are pumpless. Water moves through
the ICS unit but does not circulate within it. Within
thermosiphon systems, which rely on the principle that hot
water rises, water circulates between the collector and a tank
Transfer. Solar hot-water systems can
be categorized according to their method of heat transfer and
freeze protection. In open-loop — or direct —
systems, potable water flows through the collector and is
heated there. In closed-loop — or indirect —
systems, the liquid in the collector is isolated from the
potable water and transfers heat to it with a heat exchanger
next to or inside the storage tank.
Closed-loop systems provide the best freeze protection because
the liquid in the collector is chemically or mechanically
protected from freezing. Open-loop systems, on the other hand,
are subject to freezing because the collector contains potable
water. While it's possible to provide some freeze-protection to
open-loop systems, I don't recommend installing them in climate
zones where there are hard freezes more than once every five
On the following pages, I'll describe the most common system
designs, ranging from simpler passive systems to more complex
Integral Collector Storage
In an ICS system, potable cold water is piped into a
roof-mounted unit and preheated by the sun on its way to the
backup heater. Water moves through this system only when a
hot-water tap is opened (Figure 4). An ICS system is simple and
relatively inexpensive, but a lot of heat can be lost through
the glass, so the backup has to run if the client wants hot
water first thing in the morning.
Figure 4.Since ICS heaters both
heat and store water in the rooftop collector, they can be
quite heavy, requiring reinforced roof framing. They are not
well-suited for cold climates.
Early manufacturers of ICS systems simply placed a single bulk
storage tank within a glass-covered insulated enclosure aimed
at the sun. Newer designs typically consist of an
interconnected series of 4-inch-diameter copper tubes in an
8-inch-deep insulated box with glazing on top.
With a capacity ranging from 20 to 50 gallons, these collectors
can be quite heavy. They are also subject to freezing, because
the water is stored on the roof.
Like an ICS system, a thermosiphon system has no pump, but it's
more efficient because it separates heating and storage
functions. When sunlight hits the collector, the liquid inside
heats up and becomes buoyant, then flows up to the storage
tank, which is located above the collector. It's replaced by
cooler liquid that flows down from a separate line on the
bottom of the storage tank (Figure 5). While a pump would
certainly speed up the recirculation process, the convective
flow is more than adequate to move the entire contents of the
tank through the collector several times per day in sunny
Figure 5.Thermosiphon units rely on convection to
move hot water from the collector to the storage tank, which is
mounted right above the collector. As hot water rises into the
storage tank, cool replacement water enters at the bottom of
Thermosiphon systems are available in both open-loop and
closed-loop configurations. In the open-loop version, the
collector contains potable water, whereas the closed-loop
version contains a glycol mix that flows to a heat exchanger
surrounding the tank.
Two mechanisms provide freeze protection in an open-loop
thermosiphon system. Water gets lighter just before it turns to
ice, creating a "reverse thermosiphon" that pulls warm water
down from the tank. I don't rely on this phenomenon alone,
however; we also install a freeze drip valve, which opens when
the collector temperature reaches 35°F (Figure 6). This
bleeds water from the collector and brings warm replacement
water from the tank. Normally, the freeze valve won't open
unless the primary protection fails.
Figure 6.The author's crew always installs a
freeze drip valve on the outlet side of the collectors on
open-loop systems. If the temperature drops below 35°F, the
valve drains enough water from the collectors to bring warm
replacement water up from the house.