The 6 Commandments of Passive Solar Water Heaters

The 6 Commandments of
Passive Solar Water Heaters
If your heater’s specifications and installation
follow these six commandments, it will work
admirably and will supply inexpensive solar-
warmed water, no matter which specific
design you choose.
1. Locate your heater for maximum solar
exposure. Find a sunny, south-facing
location, preferably close to the backup heater
to minimize piping distance. You might have
to do some calculating to be sure your solar
collector will be exposed to the sun when you
want hot water, but placement and orientation
are the single most important considerations.
Remember the sun is high in summer, low in
winter. Specific optimum angles for your
location during any month of the year can be
found here . (Click “Data Services,” then select
“Altitude and Azimuth of the Sun or Moon
During One Day.”)
Next, determine where your heater will be
installed — roof, platform, wall or ground —
keeping in mind that a filled three-tank 90-
gallon system can weigh more than 800
pounds when full of water. For most
applications, it’s best to keep a large solar
heater on the ground or on a specially built
platform. A rooftop installation may be placed
above a load-bearing wall or reinforced
section of roof. Ground mounting is easiest,
and eliminates the weight problem.
2. Make the collector and storage tank(s) as
efficient as you can. First, decide on the type
of tank(s) you’ll use for your heater. Tanks
come in a wide variety of shapes and sizes,
but long, thin cylinders are the most efficient
(they have the greatest ratio of surface area
to water volume). The cheapest and easiest of
these to obtain are used electric water heater
cores. Examine the “sacrificial anode,” a rod
made of metal with a low resistance to
corrosion that is inserted into the tank from
the top to attract any corrosive elements in
the water. Replace it if substantial corrosion
and/or consumption is evident. A new one
costs only a few bucks, and it will add
measurably to the longevity of your system.
Check carefully for leaks, and never use a
tank that you have doubts about. Wire-brush
it and paint it with rust-resistant flat black
paint, or apply a selective surface coating if
you can afford it.
If you prefer new tanks, you can order glass-
lined electrical water heater cores — minus
the heating element, outer insulation and
sheet metal cover. Stainless steel tanks also
may be available in some areas. (See “Solar
Sources” below.)
3. Ensure that your system will retain heat.
There are a number of options for glazing the
top and the south-facing wall of your unit,
including single- and double-paned glass or
fiberglass and plastics designed for extended
solar exposure. In most cases, you should use
two layers of your chosen glazing material
with an air space between to provide
maximum heat retention. Glass is generally a
suitable glazing for owner-built heaters,
unless you get severe hail. Twin-wall
polycarbonate is a tough alternative. Be sure
to flash the glazing carefully to avoid leaks
and to caulk and seal the panes to avoid
condensation, which can limit energy capture.
4. Size your heater appropriately. To
determine the size you need, allow 30 gallons
of hot water per person in your household (a
conservation-minded family might get by with
only 10 gallons per person). Depending on
your environment and glazing, you’ll want to
plan for 1 to 2.5 gallons of water per square
foot of glazing as a general ratio for good
heating. A smaller water-to-glazing ratio
speeds up heat gain considerably but can
increase the freezing risk. If you can’t meet
the ideal, don’t lose heart. A smaller system
will still provide economical solar water pre-
warming and conserve nonrenewable energy
and cash. Batch systems with relatively small
tanks and simple enclosures are common, and
many are still doing well after 30 years, long
after most of the more complex systems have
disappeared.
5. Make an efficient, freeze-resistant
connection to the backup system. Minimize
pipe runs and insulate the pipe carefully
using foam or fiberglass insulation with
aluminum jacketing. Build it to last — if you
just use foam it will break down within a few
years. It can take up to 72 hours at 12 degrees
to freeze an exposed water heater tank, but
pipes are much more vulnerable. In a brief
freeze, you can leave the hot water on slightly
to keep the pipes from freezing. In very cold
winter climates, drain the collector tank and
pipes in the fall.
Set the system up so you can turn off the
backup heater and run solar hot water
directly to users, and so you can bypass the
solar water system if you want to drain it and
shut it down in winter. Make sure the
connections are building code approved. After
finishing the plumbing system, bleed the air
out of the tank through the screw plug at the
top of the tank until the system is full of
water.
6. Build your system to last. Use the best
materials you can afford or scrounge, and
take proper care in the construction of your
unit. It should work for 20 to 30 years, so it’s
worth doing it right. Be safe: Make sure the
tank supports are strong enough to bear the
load they’ll carry, and get some help with
moving tanks safely. If you build a high
performance heater, consider adding a
tempering valve near the backup heater so no
one gets scalded in a shower.
If you use galvanized tanks and fittings with
copper tubing, make sure the two metals are
separated by appropriate non-conducting,
dielectric fittings to prevent accelerated
corrosion. I’ve had good luck using copper
tubing, plastic dielectric connections and
galvanized fittings on my tanks.