Unformatted text preview: urn 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 shape than a lean-to
for air circulation to maintain uniform temperatures
during the winter heating season. An even-span can
accommodate two to three benches for growing crops.
Window-mounted. A window-mounted greenhouse can be attached on the south or east side of a
house. This glass enclosure gives space for conveniently
growing a few plants at relatively low cost (Figure 2).
The special window extends outward from the house a
foot or so and can contain two or three shelves. rounding ground so rainwater and irrigation water will
drain away. Other site considerations include the light
requirements of the plants to be grown; locations of
sources of heat, water, and electricity; and shelter from
winter wind. Access to the greenhouse should be convenient for both people and utilities. A workplace for potting plants and a storage area for supplies should be
nearby. Types of Greenhouses
A home greenhouse can be attached to a house or
garage, or it can be a freestanding structure. The chosen
site and personal preference can dictate the choices to
be considered. An attached greenhouse can be a half
greenhouse, a full-size structure, or an extended window structure. There are advantages and disadvantages
to each type. Attached Greenhouses
Lean-to. A lean-to greenhouse is a half greenhouse,
split along the peak of the roof, or ridge line (Figure 2).
Lean-to’s are useful where space is limited to a width of
approximately 7 to 12 feet, and they are the least expensive structures. The ridge of the lean-to is attached to a
building using one side and an existing doorway, if
available. Lean-to’s are close to available electricity,
water, and heat. The disadvantages include some limitations on space, sunlight, ventilation, and temperature
control. The height of the supporting wall limits the
potential size of the lean-to. The wider the lean-to, the
higher the supporting wall must be. Temperature control is more difficult because the wall that the greenhouse is built on may collect the sun’s heat while the
translucent cover of the greenhouse may lose heat Freestanding Structures
Freestanding greenhouses are separate structures;
they can be set apart from other buildings to get more
sun and can be made as large or small as desired
(Figure 2). A separate heating system is needed, and
electricity and water must be installed.
The lowest cost per square foot of growing space is
generally available in a freestanding or even-span greenhouse that is 17 to 18 feet wide. It can house a central
bench, two side benches, and two walkways. The ratio
of cost to the usable growing space is good.
When deciding on the type of structure, be sure to
plan for adequate bench space, storage space, and room
for future expansion. Large greenhouses are easier to 2 Ventilation
Ventilation is the exchange of inside air for outside
air to control temperature, remove moisture, or replenish carbon dioxide (CO2). Several ventilation systems
can be used. Be careful when mixing parts of two
Natural ventilation uses roof vents on the ridge line
with side inlet vents (louvers). Warm air rises on convective currents to escape through the top, drawing cool
air in through the sides.
Mechanical ventilation uses an exhaust fan to move
air out one end of the greenhouse while outside air
enters the other end through motorized inlet louvers.
Exhaust fans should be sized to exchange the total volume of air in the greenhouse each minute.
The total volume of air in a medium to large greenhouse can be estimated by multiplying the floor area
times 8.0 (the average height of a greenhouse). A small
greenhouse (less than 5,000 ft3 in air volume) should
have an exhaust-fan capacity estimated by multiplying
the floor area by 12.
The capacity of the exhaust fan should be selected at
one-eighth of an inch static water pressure.The static
pressure rating accounts for air resistance through the
louvers, fans, and greenhouse and is usually shown in
the fan selection chart.
Ventilation requirements vary with the weather and
season. One must decide how much the green...
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- Spring '08