Spring and fall ventilation are in between the summer and winter ventilation requirements, because it
has some cloudy and cold days as well as some sunny and warm days.
2.3.1.6.1 Ventilation system options
Ventilation systems can be natural, with windows, or mechanical, using fans or other systems. When
using the natural air ventilation, physics plays a big role. According to Hooper, “The concept of thermal
buoyancy is based on the physical properties of air; as air is heated it has the natural tendency to rise.”
(Hopper, 2012) When hot air rises and leaks by ridge vents in the roof of the greenhouse, it allows fresh
air to come in by the windows placed on the walls, closer to the ground. It is a process that in theory
would work very well, though the air circulation would depend on the conditions of the outside
weather.
The mechanical system uses fans to produce good air circulation, allowing more control over the air
circulation. The mechanical system would also allow for a more enclosed environment, preventing
unwanted pests from going in, and a better temperature regulation.
When choosing the mechanical system it is important to know the right dimensions of the greenhouse.
To determine the fan size needed for a greenhouse it is necessary to figure the greenhouse’s volume in
cubic feet and the air exchange per minute wanted for the greenhouse. Multiply the length by the width
by the height of the greenhouse to determine the greenhouse’s volume. (Hopper, 2012)
Even if the greenhouse is not constructed as it is said in the blueprints, an approximation of the volume
is easily calculated. Breaking the greenhouse into two separate pieces, a prism and a rectangle, the
formulas are the following,
푉푝푟푖푠푚 =
ℎ∗푏∗푙
2
Equation 1. Volume of a Trianglular Prism
푉푟푒푐푡푎푛푔푙푒 =푙∗푏∗ℎ
Equation 2. Volume of a Rectangular Prism
Where, h is the high, b is the base, and l is the length. After calculating each individual volume, it is
necessary to add them together for the total volume of the greenhouse.
Having the right volume of the greenhouse, it is easy to find the right fan. Researchers found that, “For
ventilation in most greenhouses, the fans should move the desired air volume rate against a static
pressure of 1/8-inch water.” (Dennis E. Buffington, n.d.).
Besides the right fan for the volume of the greenhouse, the placement of the fan is also important.
Normally a horizontally-oriented fan where the air circulates in a circular pattern is best, horizontally
positioning also allows the hot air to rise without air resistance from the fan.
Independent of mechanical systems, the roof vents should always stay open during warm weather, and
it is suggested to close the vents during the winter, or at least be more careful with it, to avoid heat loss.