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If you have ever been in an uninsulated attic on a cold, sunny, winter
afternoon, you were probably amazed at how warm it was up there. Wouldn't
it be nice if there was some way to use this available heat? There is-with
a black-roof collector. This system pulls the available heat from
the attic and blows it into an insulated crawl space. According to builder
Roy Moore of Las Cruces, New Mexico, there are over 300 installations in
southern New Mexico that operate this way. They use an unglazed,
dark-colored roof as the collector and are used for both winter heating
and summer cooling. As it turns out, these systems work even better for
cooling than they do for heating, and many black¬ roof houses in southern
New Mexico meet nearly 100 percent of their cooling needs with them. There
have also been several units installed in the San Luis Valley that are for
heating only (see chapter
12). Most of these systems are retrofits and deliver enough heat to be
good sources of supplemental heat.
If your house has a large south-facing roof area and the wind at your site
doesn't blow too hard on cold, sunny winter days, a black-roof system may
be a good retrofit for you. Since the "collector" is already in place, you
can complete the installation during a weekend. The major components
required are some ductwork, a fairly large blower and a differential
thermostat. The cost of the installation should be under $200 (1980).
There are several factors that affect the overall performance of a
black-roof system. The south-facing roof must be large. These collectors
operate very inefficiently (5 to 10 percent versus a collector operating
at 35 to 45 percent) so what they lack in efficiency must be made up in
area. Roofs smaller than 300 square feet won't deliver much heat. The roof
rafters must be uninsulated so that heat is transferred into the attic
space. This usually isn't a problem since most homes have insulation in
between the ceiling joists rather than on the rafters. Roofs built from
rafters rather than from trusses are easier to retrofit.
Black-painted tin roofs are the best roofing for this type of collector,
but asphalt shingles are also suitable. The color of the roof will make
little difference as long as it isn't white or a light color.
Light-colored shingles can be painted with a dark roof paint.
Since these systems deliver very low-grade heat (60 to 80°F), the airflow
from a black-roof system must be isolated from the living area. The heat
can't be blown directly into the house, and the delivery temperatures are
seldom high enough, even in summer, to use in a water-heating system.
These systems have their best application in heating an insulated crawl
space to about 55 to 65°F. These temperatures may seem low, but many crawl
spaces are 40°F or colder during much of the winter. If you can heat your
floor to even 60°F on every sunny day, it can make a sizeable dent in your
fuel bill.
You can collect a good deal of heat from your attic by simply running two
ducts from the attic to the crawl space at opposite ends of your house and
moving the air in a loop. A few simple procedures, however, will
help increase the efficiency of your system without greatly increasing its
cost or complexity.
Baffling and Manifolding
On a sunny winter day the north-facing roof will often be 25~F cooler than
the south facing roof, so it is important to isolate the rafters of the
south roof. Do this by stapling a sheet of 6-mil plastic at the top and
bottom of the rafters to create a large bag. Nail three or four horizontal
rows of 1 x 2 furring strips to the rafters over this bag to create
smaller, 12-inch horizontal bags for the air to flow through. This will
hold the airflow close to the south roof and cause more air turbulence for
better heat transfer. At both ends of this arrangement staple the plastic
along the length of one rafter to form a large manifold bag to feed the
baffle spaces. Sheet-metal ductwork can be attached to this plastic
manifold with duct tape. It will be impossible to make this bag completely
airtight, but do the best you can.
Airflow Requirements
We have done tests on black-roof retrofits, and it appears that a 40- to
5O foot-long flow path is optimal. Most roofs can use horizontal baffling
to achieve this flow length. If your flow will be less than 30 feet,
consider using up-and-down serpentine baffles for the plastic bag.
Most black-roof systems will present a static pressure close to 0.4 inch
WG. Size the blower to move about one-third the amount of air required for
a regular active air-heating collector mounted in place on the roof, or
2/3 cfm per square foot of roof at sea level (see the static pressure
section in chapter 9). This rate of airflow will produce about a 10°F
temperature rise from the inlet to the outlet. The blower for most
installations will need to be large, but remember it is much more
economical to move heated air than it is to heat the air. Lowering the
airflow rate won't increase the temperature differential that much. Using
a very large blower will increase efficiency but will also increase
operating costs.
Controls and Insulation
A differential thermostat with a l5F to-5F differential will be suitable
for controlling the blower, but an ideal thermostat will have adjustable
high and low set-points. This will allow fine tuning of the blower control
and keep the system from running too late into the afternoon.
Dampers will be required on both duct lines to prevent heat loss from the
crawl space at night. The return line to the roof can often be framed in a
race in the back of a closet that has a home-built backdraft damper
mounted horizontally inside it. Reverse airflow down the hot duct can be
prevented by blowing the solar-heated air into a short plastic tube which
deflates at night. Using a motorized damper in these low-cost
installations can nearly double their cost, so their use isn't justified.
It may be a good idea to place 4-foot¬wide pieces of I-inch duct
insulation over the plastic bag at the hot end of the "collector." This
would add only $40 to $5O to the cost of the installation and should help
raise the delivery temperatures a few degrees. We haven't done enough
tests to know if this is really cost-effective. Insulation can always be
added later if you feel your system could benefit from it
Summer Cooling
Black-roof collectors work very well for summer cooling, perhaps better
than they do for wintertime space heating. Since there is no glazing to
slow radiant heat loss from the roof on a clear summer night, a
dark-colored roof will be considerably cooler than the outside air
temperature and cooler than a collector would be that was mounted in its
place. The blower and thermostat can be arranged so that cool, nighttime
air is pulled from the attic and blown under the house. Summertime cooling
works best if there is a slab-on grade or other type of storage under the
house to store the coolth (see the super crawl-space design described in
chapter 11). Evaporative coolers or ceiling fans operating at low speed
during the day can move air within the house and bring it in contact with
the cool floor.
A Final Word
Don't expect miracles from a retrofitted black-roof collector. It is a
quick, inexpensive project that can get you started in solar heating and
'provide you with 10 to 15 percent of your heating needs, depending upon
its size. It won't provide more than this amount, so keep the installation
inexpensive. When you get ready to build a more elaborate system to
provide for more of your needs, you can use the blower, the differential
thermostat and the ductwork and races in the new system.
Sources for Information
If you are considering installing a black roof system on a new house, you
would do well to consult Roy Moore, who has been building these systems
for over eight years and has helped to design systems throughout the
country. For a reasonable cost Roy can revise your blueprints to
accommodate a black-roof system. His systems have qualified for tax
credits.
Roy Moore
5400 Jornada Rd. South Las Cruces, NM 88001 Phone: (505) 522-0055
Note that this is a 1980's writeup, so I'm
not sure if Roy is still doing this, but his business is still listed.
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