Solar Hot Water 101 - Continued
Since they don't involve any pumps or controllers, thermosiphon
systems are simple and extremely reliable. But, because the
tank is outside, they have low flow rates and high storage
losses, making them less efficient than pumped systems. Also,
the tank in these systems is typically mounted on the roof,
which means there are aesthetic and structural issues to deal
with, too (Figure 7).
Because it's located in a warm climate,
this thermosiphon unit contains potable water. Closed-loop
thermosiphon systems containing glycol are also available for
areas where freezing temperatures are common.
In an open-loop recirculation system, pressurized potable water
is actively pumped between the collectors mounted on the roof
and a storage tank installed inside the house (Figure 8). Heat
sensors wired to an electronic controller activate the electric
recirculating pump — typically whenever the collectors
are 5°F warmer than the tank. This "differential" control
causes the pump to run continuously as long as the sun is out
Figure 8.In an open-loop recirculating system, a
sensor-activated pump moves water between the collector and the
storage tank whenever the collector's temperature is warmer
than the tank's. When the temperature drops, the sensor
activates the pump to bring warm water from the tank back into
the collector to protect against freezing.
Figure 9.An the installer inserts a heat sensor
into a flat-plate collector (top left); this sensor and another
one on the storage tank connect to an electronic controller
(top right) that activates the pump (bottom) whenever the
collector is 5°F hotter than the tank.
If the weather gets cold enough, the collector could freeze and
burst, so when the controller senses an imminent freeze the
pump comes on and brings warm water up from the indoor tank. It
shuts off once the collectors reach 40°F. While this is a
simple method of freeze protection, it's not particularly
energy-efficient, and there are several ways it might fail:
Power may go out, the pump can stop working, or a sensor or
controller might malfunction. So, again, we always install a
freeze drip valve just in case.
Although more expensive than such passive systems as
thermosiphon and ICS, open-loop recirculation costs less than
other types of pumped systems.
Closed-Loop Antifreeze System
A closed-loop antifreeze system is designed for areas with
moderate to frequent freezing. These systems resemble pumped
open-loop systems, except they have additional components like
a heat exchanger, two independent sets of pipes, and sometimes
a second circulating pump. One pump circulates antifreeze
between the collectors and a heat exchanger, while the other
circulates potable water between the heat exchanger and the
storage tank (Figure 10).
Figure 10.Designed for cold climates, closed-loop
antifreeze systems use glycol to protect the collector. This
requires a heat exchanger to transfer heat to the potable
water, and a second pump to circulate domestic water between
the heat exchanger and storage tank.
A typical heat exchanger consists of a pair of concentric
copper pipes; liquid from the collectors flows through one pipe
and potable water flows through the other. The liquids don't
mix, but heat transfers easily though the conductive wall of
the inner pipe. It's also possible to exchange heat by running
heated fluid through a coil inside the storage tank or backup
heater, but an external heat exchanger is usually less
expensive and easier to repair.
Because the liquid in the collectors contains a mixture of
propylene glycol and water, it won't freeze. Unlike the
ethylene glycol used in automobile radiators, this antifreeze
is a nontoxic food-grade additive, so if a leak in the heat
exchanger did occur, the worst that would happen to the
homeowner is that the water might taste sweet. Good-quality
antifreeze in a well-designed system should last at least 10
years. But because antifreeze can degrade and become acidic
enough to damage the system, it should be periodically
This type of system is virtually immune to freezing, but the
heat exchanger, additional pump, and antifreeze increase the
cost of the system.
A drain-back system is a closed-loop system that relies on a
pump to lift distilled water from a nonpressurized indoor
reservoir and move it through the collector. When the outdoor
temperature is high enough and the collector is warmer than the
reservoir, the pump comes on and circulates water between the
reservoir and collector. When the pump is off, gravity causes
the water to drain out of the collectors and into the reservoir
below. The controller won't activate the pump when the outdoor
temperature is close to freezing; this keeps water out of the
collector, which protects the system (Figure 11).
Figure 11.A drain-back system uses distilled water
as the collector fluid, pumped from a nonpressurized indoor
holding tank. The circulator pump runs continuously while
heating conditions are good, then shuts off when the
temperature drops, allowing the water in the collector to drain
back to the tank, thereby preventing freeze
Solar-heated water is stored in the reservoir and transferred
to the potable water with an internal or external heat
exchanger. In some designs, a second pump moves water between
the heat exchanger and the storage tank. In others, the
reservoir is the tank, so there's no need for a second
Drain-back systems provide trouble-free, reliable freeze
protection because the closed side of the loop contains
distilled water, which, unlike glycol, doesn't require periodic
replacement. On the other hand, drain-back systems require
greater pump power to lift fluid to the collectors.
Designing the System
Because there are bound to be periods when the sun doesn't
shine for several days in a row, there's no point in trying to
design a solar hot-water heating system that provides 100
percent of the total yearly hot-water demand. We typically aim
for 60 percent to 80 percent capacity, with the backup heater
providing the rest.
As a rule of thumb, we assume that each person in a household
uses 20 gallons of hot water per day, so a family of four would
need an 80-gallon storage tank. In our mild San Francisco
Bay-area climate, 1 square foot of collector will produce about
1.5 gallons of hot water per day, so a system with an 80-gallon
tank requires 53 square feet of collector. Since collectors
aren't available in that size, we would install two 4-by-8-foot
collectors (Figure 12).
Figure 12.A pumped system typically contains more
than one flat-plate collector (left). To allow for easy
installation and repair, the author joins the collectors with
The relationship between collector and tank varies by climate.
In the Sun Belt, the rule of thumb is 1 square foot of
collector per 2 gallons of tank capacity (daily use). In the
Southeast and Mountain states, this ratio is 1-to-1.5, in the
Midwest and Atlantic states it's 1-to-1, and in the Northeast
and Northwest it's 1-to-.75
Orientation. It's generally best to face the
collectors due south, though in some cases it's wise to account
for local weather patterns. For example, in the San Francisco
Bay area there are a lot of overcast mornings, so we prefer to
orient collectors slightly more to the west.
For optimal annual collection, collectors should not face
straight up, but should be tilted above horizontal to an angle
5 to 10 degrees higher than the latitude at which they are
located. Our latitude is 38 degrees, so ideally the collectors
would be tilted 43 to 48 degrees. The steeper angle makes for
better wintertime solar collection, when the sun is lower in
the sky. In cases where aesthetic concerns trump efficiency,
we'll install the collectors at the same pitch as the
Temperature rise. When an actively pumped system
has been properly sized, each exchange of water will increase
the temperature in the storage tank 10°F. On an average
day, there might be eight exchanges, creating a total
temperature rise of 80°F; in hot, sunny weather it could be
more. Our systems routinely reach 180°F in the summer,
especially when water usage is low. This water would be too hot
to use safely, so to prevent scalding we install a tempering
valve downstream from the backup heater.
Excessive pressure can build up in the collectors if they get
too hot, so as a matter of course we install a pressure-relief
valve on the pipe where fluid exits the collector or group of
collectors. A closed-loop system will have a pressure-relief
valve on the roof and, if the loop contains glycol, an
expansion tank in the building (Figure 13).
Figure 13.When this system is up and running, the
gauges (top) will show how much heat the water gains as it
passes through the collectors. Because the water may become too
hot to safely use, the author always installs a tempering valve
to prevent scalding (bottom left). A pressure-relief valve
(bottom right) opens if the collector itself gets too hot; the
cylindrical valve at the top automatically bleeds air from the
In the early 1980s, hefty tax credits and high
energy prices led to a boom in the installation of
solar water heaters. A lot of people entered the
business and installed all kinds of equipment, then
went under after the tax credits expired and energy
prices fell in 1986.
Whereas some of these systems were quite good,
others were experimental, and with so many solar
companies out of business, there were few qualified
people around to maintain and repair them. As a
result, many of the older systems failed and gave a
black eye to a legitimate technology. Our company
runs into these orphaned systems all the time; some
are still going strong while others have been
"broken" for many years.
Bad advice. When
homeowners move into a house with a nonfunctioning
system, they're almost always advised to tear it
out. Unfortunately, most of the people giving this
advice — plumbers, roofers, and GCs —
don't know anything about solar water
Old solar systems, like the one on this original
wood roof, may no longer be operable but can often
be put back into service for a reasonable
An experienced solar hot-water installer can tell
you which systems should be torn out and which can
be repaired. If the system was built with
high-quality components and the collectors have
never frozen, there's a reasonable chance it can be
Inexpensive repairs. Our
repair crews have revived any number of systems by
making a few inexpensive repairs. Sometimes it's a
matter of spending $450 (including labor) to
replace a pump. A leaking storage tank can be
replaced for just over $1,000, which may seem like
a lot, but it's a small price to pay to repair a
system that would cost $6,000 new.
The most common problem with a pumped system is a
failed sensor or loose wire. These repairs may cost
only $100, but most plumbers don't know how to make
Sometimes the problem is simply that the homeowner
doesn't know how to turn on the system.
The lines to the roof are usually 3/4-inch copper. We don't use
PEX because in California it's illegal to use it for potable
water — plus the high temperatures found in the closed
loop of a glycol system could easily be too hot for it.
On new work, we run the lines up through the house. Because we
work in a mild climate, on retrofits we usually run pipes down
the exterior of the house. We insulate all the pipes that carry
hot or recirculated liquid with 3/4-inch neoprene, which
handles high temperatures better than plastic foam insulation
does. Without UV protection, the sun will destroy this
insulation in less than five years, so we jacket it with
aluminum (Figure 14).
Figure 14.The author's crew insulates every pipe
that contains hot or warm water. Here, an installer protects
the neoprene insulation with an aluminum jacket.
Another option is to protect the insulation with a painted
coating, but a metal jacket looks better.
Structural issues. To install the
collectors, we use the same mounting hardware we use to install
the roof-mounted portions of a photovoltaic system (see
"Installing Solar Electric Power," 3/05). The best approach is
to install post mounts before the roofing material goes on, but
it's also possible to retrofit various mounting brackets over
Weight is rarely a concern with flat-plate collectors, the
largest of which weigh less than 175 pounds even when full of
water. But a full ICS unit might weigh 500 pounds, and the
system might require more than one unit. In such a case, it's
important to find out if the roof can carry the load.
Power needs. Most pumps will run on less than one amp of
electricity, so inspectors often allow us to tie into an
existing circuit or share a circuit with another load in new
construction. A few inspectors require us to install a separate
circuit. In some jurisdictions, it's legal to plug pumps and
controllers into wall receptacles, which we do whenever
possible to reduce wiring costs.
Gerberis the owner of Sun Light
& Power in Berkeley, Calif. He has been in the solar
business since 1975.