A passive geothermal system employs simple pipes that are buried underground, typically at 6 feet deep or more. They may be arrayed in a looping design for a horizontal system. If space is scarce, pipes can be installed vertically, although that may add fairly significant cost to the project.
Once pipes are buried, fans or circulators are often added to move air around within the greenhouse. Sometimes a heat pump is added if the climate is particularly cold or tender plants require a much warmer environment.
