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Unformatted text preview: ctricians and plumbers should install
the automatic systems. Location
The greenhouse should be located where it gets maximum sunlight. The first choice of location is the south
or southeast side of a building or shade trees. Sunlight
all day is best, but morning sunlight on the east side is
sufficient for plants. An east side location captures the
most November to February sunlight. The next best
sites are southwest and west of major structures, where
plants receive sunlight later in the day. North of major
structures is the least desirable location and is good only
for plants that require little light. Morning sunlight is
most desirable because it allows the plant’s food production process to begin early; thus, growth is maximized.
Deciduous trees, such as maple and oak, can effectively shade the greenhouse from the intense late afternoon summer sun; however, they should not shade the
greenhouse in the morning. Deciduous trees also allow
maximum exposure to the winter sun because they shed
their leaves in the fall. Evergreen trees that have foliage
year round should not be located where they will shade
the greenhouse because they will block the less intense
winter sun. You should aim to maximize winter sun
exposure, particularly if the greenhouse is used all year.
Remember that the sun is lower in the southern sky in
winter causing long shadows to be cast by buildings and
evergreen trees (Figure 1).
Good drainage is another requirement for the site.
When necessary, build the greenhouse above the sur- Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30,1914, in cooperation with the U.S. Department of Agriculture, University of Maryland at College Park,
and local governments. Thomas A. Fretz, Director of Cooperative Extension Service, University of Maryland at College Park. 8 The University of Maryland is equal opportunity. The University’s policies, programs, and activities are in conformance with pertinent Federal and State laws and regulations on nondiscrimination regarding race, color, religion, age, national origin, sex, and disability. Inquiries regarding compliance with Title VI of the Civil Rights Act of 1964, as amended; Title IX of the
Educational Amendments; Section 504 of the Rehabilitation Act of 1973; and the Americans With Disabilities Act of 1990; or related legal requirements should be directed to the Director
of Personnel/Human Relations, Office of the Dean, College of Agriculture and Natural Resources, Symons Hall, College Park, MD 20742. and a two-stage thermostat are needed to control the
A two-speed motor on low speed delivers about 70
percent of its full capacity. If the two fans have the same
capacity rating, then the low-speed fan supplies about
35 percent of the combined total. This rate of ventilation is reasonable for the winter. In spring, the fan operates on high speed. In summer, both fans operate on
Refer to the earlier example of a small greenhouse. A
16-foot wide by 24-foot long house would need an estimated ft3 per minute (cubic feet per minute; CFM)
total capacity; that is, 16 × 24 × 12 ft3 per minute. For
use all year, select two fans to deliver 2,300 ft3 per
minute each, one fan to have two speeds so that the
low-speed rating is about 1,600 ft3 per minute and the
high speed is 2,300 ft3 per minute. Adding the second
fan, the third ventilation rate is the sum of both fans on
high speed, or 4,600 ft3 per minute.
Some glass greenhouses are sold with a manual ridge
vent, even when a mechanical system is specified. The
manual system can be a backup system, but it does not
take the place of a motorized louver. Do not take shortcuts in developing an automatic control system. Small fans with a cubic-foot-per-minute (ft3/min)
air-moving capacity equal to one quarter of the air volume of the greenhouse are sufficient. For small greenhouses (less than 60 feet long), place the fans in diagonally opposite corners but out from the ends and sides.
The goal is to develop a circular (oval) pattern of air
movement. Operate the fans continuously during the
winter. Turn these fans off during the summer when the
greenhouse will need to be ventilated.
The fan in a forced-air heating system can sometimes be used to provide continuous air circulation.
The fan must be wired to an on/off switch so it can run
continuously, separate from the thermostatically controlled burner. Summer sun Winter sun Figure 1. Select location carefully. Note where the shade line occurs in both the winter and summer. rapidly. The lean-to should face the best direction for
adequate sun exposure. Finally, consider the location of
windows and doors on the supporting structure and
that snow, ice, or heavy rain might slide off the roof of
the house onto the structure.
Even-span. An even-span is a full-size structure that
has one gable end attached to another building (Figure 2). It is usually the largest and most costly option,
but it provides more usable space and can be lengthened. The even-span has a better...
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- Spring '08