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Solar air collectors are similar in operation to liquid collectors with one major exception. Air is circulated through the collector and is the medium of heat transfer. Since air will not freeze under any normal circumstances on the planet, all of the techniques and equipment needed to protect liquid systems can be ignored. Air collectors are being used successfully today for such diverse applications as composting toilets, heating water and benzene removal from the earth. Their main functions are for solar space heating of homes and buildings.
The Collectors
In the late 1970’s, most collectors were of questionable
construction, with efficiencies of 40 to 50%. By 1985, efficiencies had
risen to 60 to 70% and collectors were built with life spans up to fifty
years. All high quality air collectors today have three things in common: an
enclosure made of aluminum or steel, insulation on the back and sides and
low iron, tempered glass glazing on the front. Collectors normally have a
single glazing with a dead air space between the glass and the absorber
plate. Absorber plates are made of aluminum, steel or copper. The plate is
coated on the side facing the sun with black paint or a selective surface
process. Selective surfaces (black chrome) increase efficiency by limiting
reradiated energy back through the glazing. Air collector absorber plates
should be dimpled or corrugated to create air turbulence that helps “wash”
the heat from the plate.
Blowers and Controls
When installed properly a 4 foot x 8 foot air collector can provide
25 to 50% of the energy needed to heat a 400 to 800 square foot space
depending on climate and building construction. This makes air collector
systems ideal for zone type heating systems. We normally use a separate
blower, control and ducting system for every 32 to 64 square feet of
collector area. The blower pulls air from the cold air inlet of the space
through the collector and blows the heated air back to the hot air outlet.
Optimum performance is achieved with a squirrel cage blower of approximately
200 to 400 cubic feet per minute of air flow. Smaller blowers can be used
and performance is slightly effected, but they must be of a squirrel cage
configuration.
The system is controlled by a simple, reliable bi-metal switch in the
collector which turns the blower on at 110° and off at 90°. This gives
automatic operation with each sun cycle. An interior thermostat or manual
switch wired in series with the bi-metal switch allows the occupant to “call
for heat” or shut the system off when heat is not required. The system is
completed with the addition of a backdraft damper in the duct system to
prevent nighttime cooling from thermosyphoning.
Air Movement
Hot air rises and cold air falls. This concept of air movement,
called natural convection, is important in any space heating project no
matter what fuel is used. Wood stoves are superior to fireplaces in heating
mainly because the stove causes a convection current of air throughout the
room or building. Most fireplaces only radiate heat into the immediate area.
Although this concept is ignored in designing many modern fossil fueled
heating systems, it is an integral part of all successful solar heating
installations. If a solar heating system works with natural convection, hot
air rising and cold air falling, then the result in comfort level can exceed
expectations.
Almost everyone today only lives in half their house - the bottom half. We need heat the most when the weather is cold and we are sedentary (sitting or lying down). In any building with little air movement the air tends to stagnate or stratify. Air temperature rises approximately 1° F. per foot. The ceiling of an A-frame house can be 20 to 25° hotter than the floor. Any good space heating system is designed around the fact that we live near the floor and the heat is at the ceiling.
The cold air inlet to the heating system is the key to efficiency and comfort. To heat any given space, the cold air must be removed and circulated through the heater; therefore, the inlet must be ducted so it is on or near the floor. If the inlet is placed on the ceiling, the heater will tend to recalculate the hot air and leave the floor cold. The hot air supply may be placed at the ceiling or on the floor with little difference in performance. If placed on the floor, the hot air rises to the ceiling within a short distance. Since collectors are normally installed on the roof of the building being heated, it is best to place the hot air outlet in the ceiling to shorten the duct run. A system installed in this manner decertifies, or mixes, the air in a building like a ceiling fan, in addition to supplying solar heat.
Orientation
The orientation of air collectors for heating in the winter is the same as given on page 18. The collector should face within 30° of true south and tilt at an angle of latitude plus 15°.
Energy Storage
The main niche for air collectors in the solar industry is supplying 25 to 50% of required energy for space heating. Additional storage is not required in most cases. A normal, well insulated house has enough interior mass alone to store enough heat for three to four hours after sundown, if the temperature is raised 10°F. (i.e., 68° starting, 78° ending). More massive construction, such as adobe or brick, can store even more thermal energy. Due to their complexity, large rock and water storage systems are rarely cost-effective. They often require extraordinary maintenance.
The best features of air collector systems are simplicity and reliability. The collectors are relatively simple devices. A well-made blower can be expected to have a 10 to 20 year life span if properly maintained, and the controls are extremely reliable. Since air will not freeze, no heat exchanger is required. By using packaged insulated flex duct, the installation can be accomplished by anyone with a little mechanical ability. All this adds up to a quick payback on the cost of the equipment and gives you years of warmth with safe clean solar heated air.
Air collectors may be used to heat domestic hot water using an air to liquid heat exchanger which is
much like a car radiator. A small blower flows air through the collector and passes it over the fins of
the heat exchanger and returns the now cooled air to the collector. A 1/100th horsepower pump draws
water from the bottom of the storage tank and flows it through the copper tubing in the heat exchanger
and back to the top of the tank. The heat from the solar heated air is transferred to the water. Although
air collectors are not quite as efficient as liquid collectors in heating water, the systems are simpler and
require less maintenance than glycol water heaters. An air collector solar water heater uses the same
type of differential control as a liquid system, but the differential is usually higher to account for the
lower heat capacity of air. The system must have a backdraft damper to prevent freezing the heat
exchanger tubing in cold weather. See the diagram
*Special thanks to our vendor AAA Solar Supply for use of their diagrams.
*Special thanks to our vendor AAA Solar Supply for use of their diagrams.
ROOF MOUNT AIR COLLECTORS
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SUN
AIRE COLLECTORS
* These items require additional
freight charges
PACKAGED AIR COLLECTOR SYSTEMS SPACE
HEATING SYSTEMS
System B
* These items will require additional
freight charges COMPLETE KITS
* These items will require additional
freight charges |
COLLECTOR MOUNTING
KITS
INSTALLATION KIT FOR AIR SYSTEMS
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BACKDRAFT
DAMPER FOR ROOF MOUNT COLLECTORAttaches to blower, slides into flex-duct. Opens when system is on, closes to keep warm house air from escaping through duct.
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SNAP
DISC
 A single pole, single throw thermally actuated switch that turns on (closes) at 110°F and turns off (opens) at 90°F. Snap discs are used to control air systems for space heating. Normally wired in series with a line voltage thermostat, the snap disc comes with high temperature leads for safe installation in the collector.
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BLOWERS Blowers are designed to have enough power to circulate air through air ducts. Blowers used in both conventional and solar heating systems are called centrifugal and resemble a treadmill in a squirrel cage. Although ducting systems are used in many solar heating systems, the blowers main purpose is to circulate air through the collector. Small highly efficient, permanent split capacitor motors are used on these blowers. Systems powered by photovoltaics normally use a 12 volt
D.C. blower. |
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DAYTON -
PERMANENT SPLIT CAPACITOR BLOWERS Provides economical air delivery. Driven by Dayton motors rated for continuous duty with all position mount sleeve bearings. Motors recognized by UL.
Since around 2002 Dayton stopped selling individual replacement motors and only sells the entire blower unit. Dayton's reason for this is that they have to balance the blower unit to the motor. |
REGISTERS AND GRILLES
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THERMOSTATS Line voltage, heat only thermostats are used on air collector space heating systems. They may be set by the user to regulate indoor temperatures and turning the thermostat all the way down effectively shuts the system off when not in use. |
COOLING
LINE VOLTAGE THERMOSTATS These thermostats are used to control blowers that distribute excess heat from sunrooms.
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LOW VOLTAGE THERMOSTATS 24 volt thermostats are used to regulate indoor temperatures and are mainly integrated into liquid collector space heating options. For these applications they are normally wired in series with either an aquastat or differential controller.
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AQUASTATS Aquastats utilize a remote bulb filled with freon to sense water temperatures in solar storage tanks. When the water is hot enough (80° to 100°F), the aquastat will turn on a pump to deliver the hot water to its desired location. When the tank temperature falls below the selected temperature the pump shuts off.
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GALVANIZED WEATHERPROOFING Used to weatherproof flex duct when exposed to outside environment. (5' straight lengths snap together; adjustable ells for change in direction.)
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INSULATED
FLEXIBLE AIR DUCT Flex duct is constructed of a high temperature (250º) CPE liner bonded to a spring steel wire helix, insulated with fiberglass and jacketed with a polyethelene vapor barrier. It has been used on thousands of air collector type systems and is packaged in compressed boxes approx. 4' long. 25' when extended.
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GALVANIZED BOOTS Boots are used for transition from flex duct to registers or wall openings.
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ADJUSTABLE ROOF JACKS Used for water-tight roof penetrations, for piping and ducting.
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PASCO
THERMOMETER 30° to 250°F; includes 1/2" M.I.P. brass well (thermometer is removable from well).
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PRESSURE
GAUGES 2" dial; 1/4" M.I.P. bottom connection; 0 to100 P.S.I.
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Custom Absorber Plates
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 Low iron tempered glass is used
in all well made collectors. The LOW IRON TEMPERED GLASS
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40 MIL FILON 48" wide; fiberglass reinforced polyester suitable for outer glazing of collectors.
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10 MIL LEXAN 48" wide; polycarbonate suitable for inner glazing.
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